Patent Publication Number: US-2013237786-A1

Title: System and methods for processing analyte sensor data

Description:
INCORPORATION BY REFERENCE TO RELATED APPLICATIONS 
     Any and all priority claims identified in the Application Data Sheet, or any correction thereto, are hereby incorporated by reference under 37 CFR 1.57. This application is a division of U.S. application Ser. No. 12/639,746 filed Dec. 16, 2009, which is a continuation of U.S. application Ser. No. 10/633,367 filed Aug. 1, 2003, now U.S. Pat. No. 7,778,680, the contents of which are incorporated by reference herein in their entirety, and are hereby made a part of this specification. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems and methods for analyte sensor data processing. Particularly, the present invention relates to retrospectively and/or prospectively initiating a calibration, converting sensor data, updating the calibration, evaluating received reference and sensor data, and evaluating the calibration for the analyte sensor. 
     BACKGROUND OF THE INVENTION 
     Diabetes mellitus is a disorder in which the pancreas cannot create sufficient insulin (Type I or insulin dependent) and/or in which insulin is not effective (Type 2 or non-insulin dependent). In the diabetic state, the victim suffers from high blood sugar, which may cause an array of physiological derangements (e.g., kidney failure, skin ulcers, or bleeding into the vitreous of the eye) associated with the deterioration of small blood vessels. A hypoglycemic reaction (low blood sugar) may be induced by an inadvertent overdose of insulin, or after a normal dose of insulin or glucose-lowering agent accompanied by extraordinary exercise or insufficient food intake. 
     Conventionally, a diabetic person carries a self-monitoring blood glucose (SMBG) monitor, which typically comprises uncomfortable finger pricking methods. Due to the lack of comfort and convenience, a diabetic will normally only measure his or her glucose level two to four times per day. Unfortunately, these time intervals are so far spread apart that the diabetic will likely find out too late, sometimes incurring dangerous side effects, of a hyper- or hypo-glycemic condition. In fact, it is not only unlikely that a diabetic will take a timely SMBG value, but the diabetic will not know if their blood glucose value is going up (higher) or down (lower) based on conventional methods, inhibiting their ability to make educated insulin therapy decisions. 
     SUMMARY OF THE INVENTION 
     Systems and methods are needed that accurately provide estimated glucose measurements to a diabetic patient continuously and/or in real time so that they may proactively care for their condition to safely avoid hyper- and hypo-glycemic conditions. Real time and retrospective estimated glucose measurements require reliable data processing in order to provide accurate and useful output to a patient and/or doctor. 
     Similarly, systems and methods are needed that accurately provide substantially continuous estimated analyte measurements for a variety of known analytes (e.g., oxygen, salts, protein, and vitamins) to provide prospective and/or retrospective data analysis and output to a user. 
     Accordingly, systems and methods are provided for retrospectively and/or prospectively calibrating a sensor, initializing a sensor, converting sensor data into calibrated data, updating and maintaining a calibration over time, evaluating received reference and sensor data for clinical acceptability, and evaluating the calibration statistical acceptability, to ensure accurate and safe data output to a patient and/or doctor. 
     In a first embodiment a method is provided for initializing a substantially continuous analyte sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; and determining a stability of the continuous analyte sensor. 
     In an aspect of the first embodiment, the step of determining the stability of the substantially continuous analyte sensor includes waiting a predetermined time period between about one minute and about six weeks. 
     In an aspect of the first embodiment, the step of determining the stability of the substantially continuous analyte sensor includes evaluating at least two matched data pairs. 
     In an aspect of the first embodiment, the step of determining the stability of the substantially continuous analyte sensor includes evaluating one of pH, oxygen, hypochlorite, interfering species, correlation of matched pairs, R-value, baseline drift, baseline offset, and amplitude. 
     In an aspect of the first embodiment, the method further includes providing one of an audible, visual, or tactile output to a user based on the stability of the sensor. 
     In an aspect of the first embodiment, the step of providing output based on the stability of the sensor includes indicating at least one of a numeric estimated analyte value, a directional trend of analyte concentration, and a graphical representation of an estimated analyte value. 
     In an aspect of the first embodiment, the step of receiving sensor data includes receiving sensor data from a substantially continuous glucose sensor. 
     In an aspect of the first embodiment, the step of receiving sensor data includes receiving sensor data from an implantable glucose sensor. 
     In an aspect of the first embodiment, the step of receiving sensor data includes receiving sensor data from subcutaneously implantable glucose sensor. 
     In an aspect of the first embodiment, the step of receiving reference data includes receiving reference data from a self-monitoring blood glucose test. 
     In an aspect of the first embodiment, the step of receiving reference data includes downloading reference data via a cabled connection. 
     In an aspect of the first embodiment, the step of receiving reference data includes downloading reference data via a wireless connection. 
     In an aspect of the first embodiment, the step of receiving reference data from a reference analyte monitor includes receiving within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the first embodiment, the step of forming a calibration set includes evaluating at least one matched data pair using inclusion criteria. 
     In an aspect of the first embodiment, the step of receiving sensor data includes receiving sensor data that has been algorithmically smoothed. 
     In an aspect of the first embodiment, the step of receiving sensor data includes algorithmically smoothing the received sensor data. 
     In an aspect of the first embodiment, the step of forming a calibration set includes including in the calibration set between one and six matched data pairs. 
     In an aspect of the first embodiment, the step of forming a calibration set includes including six matched data pairs. 
     In an aspect of the first embodiment, the step of forming a calibration set further includes determining a value for n, where n is greater than one and represents the number of matched data pairs in the calibration set. 
     In an aspect of the first embodiment, the step of determining a value for n is determined as a function of the frequency of the received reference data points and signal strength over time. 
     In a second embodiment, a system is provided for initializing a continuous analyte sensor, including: a sensor data module operatively connected to a continuous analyte sensor that receives a data stream including a plurality of time spaced sensor data points from the analyte sensor; a reference input module adapted to obtain reference data from a reference analyte monitor, including one or more reference data points; a processor module that forms one or more matched data pairs by matching reference data to substantially time corresponding sensor data and subsequently forms a calibration set including the one or more matched data pairs; and a start-up module associated with the processor module programmed to determine the stability of the continuous analyte sensor. 
     In an aspect of the second embodiment, the sensor data module is adapted to wirelessly receive sensor data points from the sensor. 
     In an aspect of the second embodiment, the start-up module is programmed to wait a predetermined time period between six hours and six weeks. 
     In an aspect of the second embodiment, the start-up module is programmed to evaluate at least two matched data pairs. 
     In an aspect of the second embodiment, the start-up module is programmed to evaluate one of pH, oxygen, hypochlorite, interfering species, correlation of matched pairs, R-value, baseline drift, baseline offset, and amplitude. 
     In an aspect of the second embodiment, the system further includes an output control module associated with the processor module and programmed to control output of sensor data. 
     In an aspect of the second embodiment, the output control module indicates at least one of a numeric estimated analyte value, a directional trend of analyte concentration, and a graphical representation of an estimated analyte value. 
     In an aspect of the second embodiment, the sensor data module is configured to receive sensor data from substantially the continuous glucose sensor. 
     In an aspect of the second embodiment, the sensor data module is configured to receive sensor data from an implantable glucose sensor. 
     In an aspect of the second embodiment, the sensor data module is configured to receive sensor data from subcutaneously implantable glucose sensor. 
     In an aspect of the second embodiment, the reference input module is configured to receive reference data from a self-monitoring blood glucose test. 
     In an aspect of the second embodiment, the reference input module is configured to download reference data via a cabled connection. 
     In an aspect of the second embodiment, the reference input module is configured to download reference data via a wireless connection. 
     In an aspect of the second embodiment, the system further includes a reference analyte monitor integral with the system and wherein the reference input module is configured to receive an internal communication from the reference analyte monitor. 
     In an aspect of the second embodiment, the processor module includes programming to evaluate at least one matched data pair using inclusion criteria. 
     In an aspect of the second embodiment, the reference input module is configured to receive sensor data that has been algorithmically smoothed. 
     In an aspect of the second embodiment, the reference input module is configured to algorithmically smooth the received sensor data. 
     In an aspect of the second embodiment, the calibration set includes between one and six matched data pairs. 
     In an aspect of the second embodiment, the calibration set includes six matched data pairs. 
     In an aspect of the second embodiment, the calibration set includes n matched data pairs, where n is greater than one. 
     In an aspect of the second embodiment, n is a function of the frequency of the received reference data points and signal strength over time. 
     In a third embodiment, a computer system is provided for initializing a continuous analyte sensor, the computer system including: a sensor data receiving module that receives sensor data from the substantially continuous analyte sensor via a receiver, including one or more sensor data points; a reference data receiving module that receives reference data from a reference analyte monitor, including one or more reference data points; a data matching module that forms one or more matched data pairs by matching reference data to substantially time corresponding sensor data; a calibration set module that forms a calibration set including at least one matched data pair; and a stability determination module that determines the stability of the continuous analyte sensor. 
     In an aspect of the third embodiment, the stability determination module includes a system for waiting a predetermined time period. 
     In an aspect of the third embodiment, the stability determination module evaluates at least two matched data pairs. 
     In an aspect of the third embodiment, the stability determination module evaluates one of pH, oxygen, hypochlorite, interfering species, correlation of matched pairs, R-value, baseline drift, baseline offset, and amplitude. 
     In an aspect of the third embodiment, the computer system further includes an interface control module that provides output to the user based on the stability of the sensor. 
     In an aspect of the third embodiment, the output from the interface control module includes at least one of a numeric estimated analyte value, an indication of directional trend of analyte concentration, and a graphical representation of an estimated analyte value. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive sensor data from a substantially continuous glucose sensor. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive sensor data from an implantable glucose sensor. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive sensor data from a subcutaneously implantable glucose sensor. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive sensor data from a self-monitoring blood glucose test. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive sensor data from a cabled connection. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to download reference data via a wireless connection. 
     In an aspect of the third embodiment, the reference data receiving module is adapted to receive reference data from an internal reference analyte monitor that is housed integrally the computer system. 
     In an aspect of the third embodiment, the calibration set module evaluates at least one matched data pair using inclusion criteria. 
     In an aspect of the third embodiment, the sensor data receiving module is adapted to receive sensor data that has been algorithmically smoothed. 
     In an aspect of the third embodiment, the computer system further includes a data smoothing module that smoothes the received sensor data. 
     In an aspect of the third embodiment, the calibration set module includes between one and six matched data pairs. 
     In an aspect of the third embodiment, the calibration set module includes six matched data pairs. 
     In an aspect of the third embodiment, the calibration set includes n number of matched data pairs, where n is greater than one. 
     In an aspect of the third embodiment, n is a function of the frequency of the received reference data points and signal strength over time. 
     In a fourth embodiment, method is provided for initializing a substantially continuous analyte sensor, the method including: receiving sensor data from a substantially continuous analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including one or more reference data points; forming one or more matched data pairs by matching reference data to substantially time corresponding sensor data; forming a calibration set including at least one matched data pair; determining stability of continuous analyte sensor; and outputting information reflective of the sensor data once a predetermined level of stability has been determined. 
     In a fifth embodiment, a system is provided for initializing a continuous analyte sensor, including: a sensor data module operatively linked to a continuous analyte sensor and configured to receive one or more sensor data points from the sensor; a reference input module adapted to obtain one or more reference data points; and a processor module associated with the sensor data module and the input module and programmed to match reference data points with time-matched sensor data points to form a calibration set including at least one matched data pair; and a start-up module associated with the processor module programmed to determine the stability of the continuous analyte sensor and output information reflective of the sensor data once a predetermined level of stability has been determined. 
     In a sixth embodiment, a computer system is provided for initializing a continuous analyte sensor, the system including: a sensor data receiving module that receives sensor data including one or more sensor data points from the substantially continuous analyte sensor via a receiver; a reference data receiving module for receiving reference data from a reference analyte monitor, including one or more reference data points; a data matching module for forming one or more matched data pairs by matching reference data to substantially time corresponding sensor data; a calibration set module for forming a calibration set including at least one matched data pair; a stability determination module for evaluating the stability of the continuous analyte sensor; and an interface control module that outputs information reflective of the sensor data once a predetermined level of stability has been determined. 
     In a seventh embodiment, a method for initializing a glucose sensor, the method including: receiving sensor data from the glucose sensor, including one or more sensor data points; receiving reference data from a reference glucose monitor, including one or more reference data points; forming one or more matched data pairs by matching reference data to substantially time corresponding sensor data; determining whether the glucose sensor has reached a predetermined level of stability. 
     In an eighth embodiment, a system is provided for initializing a continuous analyte sensor, including: a sensor data module operatively linked to a continuous analyte sensor and configured to receive one or more sensor data points from the sensor; a reference input module adapted to obtain one or more reference data points; and a processor module associated with the sensor data module and the input module and programmed to match reference data points with time-matched sensor data points to form a calibration set including at least one matched data pair; and a stability module associated with the processor module programmed to determine the stability of the continuous analyte sensor. 
     In a ninth embodiment, a method is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including one or more reference data points; and evaluating the clinical acceptability at least one of the reference and sensor analyte data using substantially time corresponding reference or sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data. 
     In an aspect of the ninth embodiment, the method further includes providing an output through a user interface responsive to the clinical acceptability evaluation. 
     In an aspect of the ninth embodiment, the step of providing an output includes alerting the user based on the clinical acceptability evaluation. 
     In an aspect of the ninth embodiment, the step of providing an output includes altering the user interface based on the clinical acceptability evaluation. 
     In an aspect of the ninth embodiment, the step of altering the user interface includes at least one of providing color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In an aspect of the ninth embodiment, the step of evaluating the clinical acceptability includes using one of a Clarke Error Grid, a mean absolute difference calculation, a rate of change calculation, a consensus grid, and a standard clinical acceptance test. 
     In an aspect of the ninth embodiment, the method further includes requesting additional reference data if the clinical acceptability evaluation determines clinical unacceptability. 
     In an aspect of the ninth embodiment, the method further includes repeating the clinical acceptability evaluation step for the additional reference data. 
     In an aspect of the ninth embodiment, the method further includes a step of matching reference data to substantially time corresponding sensor data to form a matched pair after the clinical acceptability evaluation step. 
     In a tenth embodiment, a system is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the method including: means for receiving a data stream from an analyte sensor, a plurality of time-spaced sensor data points; means for receiving reference data from a reference analyte monitor, including one or more reference data points; and means for evaluating the clinical acceptability of at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data. 
     In an aspect of the tenth embodiment, the system further includes means for providing an output based through a user interface responsive to the clinical acceptability evaluation. 
     In an aspect of the tenth embodiment, the means for providing an output includes means for alerting the user based on the clinical acceptability evaluation. 
     In an aspect of the tenth embodiment, the means for providing an output includes means for altering the user interface based on the clinical acceptability evaluation. 
     In an aspect of the tenth embodiment, the means for altering the user interface includes at least one of providing color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In an aspect of the tenth embodiment, the means for evaluating the clinical acceptability includes using one of a Clarke Error Grid, a mean absolute difference calculation, a rate of change calculation, a consensus grid, and a standard clinical acceptance test. 
     In an aspect of the tenth embodiment, the system further includes means for requesting additional reference data if the clinical acceptability evaluation determines clinical unacceptability. 
     In an aspect of the tenth embodiment, the system further includes means for repeated the clinical acceptability evaluation for the additional reference data. 
     In an aspect of the tenth embodiment, the system further includes means for matching reference data to substantially time corresponding sensor data to form a matched data pair after the clinical acceptability evaluation. 
     In an eleventh embodiment, a computer system is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the computer system including: a sensor data receiving module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference data receiving module that receives reference data from a reference analyte monitor, including one or more reference data points; and a clinical acceptability evaluation module that evaluates at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data. 
     In an aspect of the eleventh embodiment, the computer system further includes an interface control module that controls the user interface based on the clinical acceptability evaluation. 
     In an aspect of the eleventh embodiment, the interface control module alerts the user based on the clinical acceptability evaluation. 
     In an aspect of the eleventh embodiment, the interface control module alters the user interface based on the clinical acceptability evaluation. 
     In an aspect of the eleventh embodiment, the interface control module alters the user interface by providing at least one of providing color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In an aspect of the eleventh embodiment, the clinical acceptability evaluation module uses one of a Clarke Error Grid, a mean absolute difference calculation, a rate of change calculation, a consensus grid, and a standard clinical acceptance test to evaluate clinical acceptability. 
     In an aspect of the eleventh embodiment, the interface control module that requests additional reference data if the clinical acceptability evaluation determines clinical unacceptability. 
     In an aspect of the eleventh embodiment, the interface control module evaluates the additional reference data using clinical acceptability evaluation module. 
     In an aspect of the eleventh embodiment, the computer system further includes a data matching module that matches clinically acceptable reference data to substantially time corresponding clinically acceptable sensor data to form a matched pair. 
     In a twelfth embodiment, a method is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including one or more reference data points; evaluating the clinical acceptability at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data; and providing an output through a user interface responsive to the clinical acceptability evaluation. 
     In an thirteenth embodiment, a method is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including one or more reference data points; and evaluating the clinical acceptability at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data, including using one of a Clarke Error Grid, a mean absolute difference calculation, a rate of change calculation, and a consensus grid. 
     In an fourteenth embodiment, a computer system is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference input module that receives reference data from a reference analyte monitor, including one or more reference data points; a clinical module that evaluates at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data; and an interface control module that controls the user interface based on the clinical acceptability evaluation. 
     In an fifteenth embodiment, a computer system is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference input module that receives reference data from a reference analyte monitor, including one or more reference data points; and a clinical module that evaluates at least one of the reference and sensor analyte data with substantially time corresponding reference and sensor data, wherein the clinical module uses one of a Clarke Error Grid, a mean absolute difference calculation, a rate of change calculation, a consensus grid, and a standard clinical acceptance test to evaluate clinical acceptability. 
     In an sixteenth embodiment, a computer system is provided for evaluating clinical acceptability of at least one of reference and sensor analyte data, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor via a receiver; a reference input module that receives reference data from a reference analyte monitor, including one or more reference data points; and a clinical module that uses a Clarke Error Grid to evaluate the clinical acceptability at least one of the reference and sensor analyte data using substantially time corresponding reference and sensor data; and a fail-safe module that controls the user interface responsive to the clinical module evaluating clinical unacceptability. 
     In an seventeenth embodiment, a method is provided for evaluating clinical acceptability of at least one of reference and sensor glucose data, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference glucose monitor, including one or more reference data points; evaluating the clinical acceptability at least one of the reference and sensor glucose data using substantially time corresponding reference and sensor data, wherein the at least one of the reference and sensor analyte data is evaluated for deviation from its substantially time corresponding reference or sensor data and clinical risk associated with that deviation based on the glucose value indicated by at least one of the sensor and reference data; and a fail-safe module that controls the user interface responsive to the clinical module evaluating clinical unacceptability. 
     In an eighteenth embodiment, a method is provided for maintaining calibration of a substantially continuous analyte sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; creating a conversion function based on the calibration set; converting sensor data into calibrated data using the conversion function; subsequently obtaining one or more additional reference data points and creating one or more new matched data pairs; evaluating the calibration set when the new matched data pair is created, wherein evaluating the calibration set includes at least one of 1) ensuring matched data pairs in the calibration set span a predetermined time range, 2) ensuring matched data pairs in the calibration set are no older than a predetermined value, 3) ensuring the calibration set has substantially distributed high and low matched data pairs over the predetermined time range, and 4) allowing matched data pairs only within a predetermined range of analyte values; and subsequently modifying the calibration set if such modification is required by the evaluation. 
     In an aspect of the eighteenth embodiment, the step of evaluating the calibration set further includes at least one of evaluating a rate of change of the analyte concentration, evaluating a congruence of respective sensor and reference data in the matched data pairs, and evaluating physiological changes. 
     In an aspect of the eighteenth embodiment, the step of evaluating the calibration set includes evaluating only the new matched data pair. 
     In an aspect of the eighteenth embodiment, the step of evaluating the calibration set includes evaluating all of the matched data pairs in the calibration set and the new matched data pair. 
     In an aspect of the eighteenth embodiment, the step of evaluating the calibration set includes evaluating combinations of matched data pairs from the calibration set and the new matched data pair. 
     In an aspect of the eighteenth embodiment, the step of receiving sensor data includes receiving a data stream from a long-term implantable analyte sensor. 
     In an aspect of the eighteenth embodiment, the step of receiving sensor data includes receiving a data stream that has been algorithmically smoothed. 
     In an aspect of the eighteenth embodiment, the step of receiving sensor data stream includes algorithmically smoothing the data stream. 
     In an aspect of the eighteenth embodiment, the step of receiving reference data includes downloading reference data via a cabled connection. 
     In an aspect of the eighteenth embodiment, the step of receiving reference data includes downloading reference data via a wireless connection. 
     In an aspect of the eighteenth embodiment, the step of receiving reference data from a reference analyte monitor includes receiving within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the eighteenth embodiment, the reference analyte monitor includes self-monitoring of blood analyte. 
     In an aspect of the eighteenth embodiment, the step of creating a conversion function includes linear regression. 
     In an aspect of the eighteenth embodiment, the step of creating a conversion function includes non-linear regression. 
     In an aspect of the eighteenth embodiment, the step of forming a calibration set includes including in the calibration set between one and six matched data pairs. 
     In an aspect of the eighteenth embodiment, the step of forming a calibration set includes including six matched data pairs. 
     In an aspect of the eighteenth embodiment, the step of forming a calibration set further includes determining a value for n, where n is greater than one and represents the number of matched data pairs in the calibration set. 
     In an aspect of the eighteenth embodiment, the step of determining a value for n is determined as a function of the frequency of the received reference data points and signal strength over time. 
     In an aspect of the eighteenth embodiment, the method further includes determining a set of matching data pairs from the evaluation of the calibration set and re-forming a calibration set. 
     In an aspect of the eighteenth embodiment, the method further includes repeating the step of re-creating the conversion function using the re-formed calibration set. 
     In an aspect of the eighteenth embodiment, the method further includes converting sensor data into calibrated data using the re-created conversion function. 
     In a nineteenth embodiment, a system is provided for maintaining calibration of a substantially continuous analyte sensor, the system including: means for receiving a data stream from an analyte sensor, a plurality of time-spaced sensor data points; means for receiving reference data from a reference analyte monitor, including two or more reference data points; means for providing two or more matched data pairs by matching reference analyte data to substantially time corresponding sensor data; means for forming a calibration set including at least two matched data pair; means for creating a conversion function based on the calibration set; means for converting sensor data into calibrated data using the conversion function; subsequently obtaining one or more additional reference data points and creating one or more new matched data pairs; means for evaluating the calibration set when the new matched data pair is created, wherein evaluating the calibration set includes at least one of 1) ensuring matched data pairs in the calibration set span a predetermined time range, 2) ensuring matched data pairs in the calibration set are no older than a predetermined value, 3) ensuring the calibration set has substantially distributed high and low matched data pairs over the predetermined time range, and 4) allowing matched data pairs only within a predetermined range of analyte values; and means for modifying the calibration set if such modification is required by the evaluation. 
     In an aspect of the nineteenth embodiment, the means for evaluating the calibration set further includes at least one of means for evaluating a rate of change of the analyte concentration, means for evaluating a congruence of respective sensor and reference data in matched data pairs; and means for evaluating physiological changes. 
     In an aspect of the nineteenth embodiment, the means for evaluating the calibration set includes means for evaluating only the one or more new matched data pairs. 
     In an aspect of the nineteenth embodiment, the means for evaluating the calibration set includes means for evaluating all of the matched data pairs in the calibration set and the one or more new matched data pairs. 
     In an aspect of the nineteenth embodiment, the means for evaluating the calibration set includes means for evaluating combinations of matched data pairs from the calibration set and the one or more new matched data pair. 
     In an aspect of the nineteenth embodiment, the means for receiving sensor data includes means for receiving sensor data from a long-term implantable analyte sensor. 
     In an aspect of the nineteenth embodiment, the means for receiving sensor data includes means for receiving sensor data that has been algorithmically smoothed. 
     In an aspect of the nineteenth embodiment, the means for receiving sensor data includes means for algorithmically smoothing the receiving sensor data. 
     In an aspect of the nineteenth embodiment, the means for receiving reference data includes means for downloading reference data via a cabled connection. 
     In an aspect of the nineteenth embodiment, the means for receiving reference data includes means for downloading reference data via a wireless connection. 
     In an aspect of the nineteenth embodiment, the means for receiving reference data from a reference analyte monitor includes means for receiving within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the nineteenth embodiment, the means for receiving reference data includes means for receiving from a self-monitoring of blood analyte. 
     In an aspect of the nineteenth embodiment, the means for creating a conversion function includes means for performing linear regression. 
     In an aspect of the nineteenth embodiment, the means for creating a conversion function includes means for performing non-linear regression. 
     In an aspect of the nineteenth embodiment, the means for forming a calibration set includes including in the calibration set between one and six matched data pairs. 
     In an aspect of the nineteenth embodiment, the means for forming a calibration set includes including in the calibration set six matched data pairs. 
     In an aspect of the nineteenth embodiment, the means for forming a calibration set further includes determining a value for n, where n is greater than one and represents the number of matched data pairs in the calibration set. 
     In an aspect of the nineteenth embodiment, the means for determining a value for n is determined as a function of the frequency of the received reference data points and signal strength over time. 
     In an aspect of the nineteenth embodiment, the system further includes means for determining a set of matching data pairs from the evaluation of the calibration set and re-forming a calibration set. 
     In an aspect of the nineteenth embodiment, the system further includes the means for repeating the set of creating the conversion function using the re-formed calibration set. 
     In an aspect of the nineteenth embodiment, the system further includes means for converting sensor data into calibrated data using the re-created conversion function. 
     In a twentieth embodiment, a computer system is provided for maintaining calibration of a substantially continuous analyte sensor, the computer system including: a sensor data receiving module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference data receiving module that receives reference data from a reference analyte monitor, including two or more reference data points; a data matching module that forms two or more matched data pairs by matching reference data to substantially time corresponding sensor data; a calibration set module that forms a calibration set including at least two matched data pairs; a conversion function module that creates a conversion function using the calibration set; a sensor data transformation module that converts sensor data into calibrated data using the conversion function; and a calibration evaluation module that evaluates the calibration set when the new matched data pair is provided, wherein evaluating the calibration set includes at least one of 1) ensuring matched data pairs in the calibration set span a predetermined time period, 2) ensuring matched data pairs in the calibration set are no older than a predetermined value, 3) ensuring the calibration set has substantially distributed high and low matched data pairs over a predetermined time range, and 4) allowing matched data pairs only within a predetermined range of analyte values, wherein the conversion function module is programmed to re-create the conversion function of such modification is required by the calibration evaluation module. 
     In an aspect of the twentieth embodiment, the evaluation calibration module further evaluates at least one of a rate of change of the analyte concentration, a congruence of respective sensor and reference data in matched data pairs; and physiological changes. 
     In an aspect of the twentieth embodiment, the evaluation calibration module evaluates only the new matched data pair. 
     In an aspect of the twentieth embodiment, the evaluation calibration module evaluates all of the matched data pairs in the calibration set and the new matched data pair. 
     In an aspect of the twentieth embodiment, the evaluation calibration module evaluates combinations of matched data pairs from the calibration set and the new matched data pair. 
     In an aspect of the twentieth embodiment, the sensor data receiving module receives the data stream from a long-term implantable analyte sensor. 
     In an aspect of the twentieth embodiment, the sensor data receiving module receives an algorithmically smoothed data stream. 
     In an aspect of the twentieth embodiment, the sensor data receiving module includes programming to smooth the data stream. 
     In an aspect of the twentieth embodiment, the reference data receiving module downloads reference data via a cabled connection. 
     In an aspect of the twentieth embodiment, the reference data receiving module downloads reference data via a wireless connection. 
     In an aspect of the twentieth embodiment, the reference data receiving module receives within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the twentieth embodiment, the reference data receiving module receives reference data from a self-monitoring of blood analyte. 
     In an aspect of the twentieth embodiment, the conversion function module includes programming that performs linear regression. 
     In an aspect of the twentieth embodiment, the conversion function module includes programming that performs non-linear regression. 
     In an aspect of the twentieth embodiment, the calibration set module includes in the calibration set between one and six matched data pairs. 
     In an aspect of the twentieth embodiment, the calibration set module includes in the calibration set six matched data pairs. 
     In an aspect of the twentieth embodiment, the calibration set module further includes programming for determining a value for n, where n is greater than one and represents the number of matched data pairs in the calibration set. 
     In an aspect of the twentieth embodiment, the programming for determining a value for n determines n as a function of the frequency of the received reference data points and signal strength over time. 
     In an aspect of the twentieth embodiment, data matching module further includes programming to re-form the calibration set based on the calibration evaluation. 
     In an aspect of the twentieth embodiment, the conversion function module further includes programming to re-create the conversion function based on the re-formed calibration set. 
     In an aspect of the twentieth embodiment, the sensor data transformation module further including programming for converting sensor data into calibrated using the re-created conversion function. 
     In a twenty-first embodiment, a method is provided for maintaining calibration of a glucose sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; creating a conversion function based on the calibration set; subsequently obtaining one or more additional reference data points and creating one or more new matched data pairs; and evaluating the calibration set when the new matched data pair is created, wherein evaluating the calibration set includes at least one of 1) ensuring matched data pairs in the calibration set span a predetermined time range, 2) ensuring matched data pairs in the calibration set are no older than a predetermined value, 3) ensuring the calibration set has substantially distributed high and low matched data pairs over the predetermined time range, and 4) allowing matched data pairs only within a predetermined range of analyte values. 
     In a twenty-second embodiment, a computer system is provided for maintaining calibration of a glucose sensor, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference input module that receives reference data from a reference analyte monitor, including two or more reference data points; a processor module that forms two or more matched data pairs by matching reference data to substantially time corresponding sensor data and subsequently forms a calibration set including the two or more matched data pairs; and a calibration evaluation module that evaluates the calibration set when the new matched data pair is provided, wherein evaluating the calibration set includes at least one of 1) ensuring matched data pairs in the calibration set span a predetermined time period, 2) ensuring matched data pairs in the calibration set are no older than a predetermined value, 3) ensuring the calibration set has substantially distributed high and low matched data pairs over a predetermined time range, and 4) allowing matched data pairs only within a predetermined range of analyte values, wherein the conversion function module is programmed to re-create the conversion function of such modification is required by the calibration evaluation module. 
     In a twenty-third embodiment, a method is provided for evaluating the quality of a calibration of an analyte sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; creating a conversion function based on the calibration set; receiving additional sensor data from the analyte sensor; converting sensor data into calibrated data using the conversion function; and evaluating the quality of the calibration set using a data association function. 
     In an aspect of the twenty-third embodiment, the step of receiving sensor data includes receiving a data stream that has been algorithmically smoothed. 
     In an aspect of the twenty-third embodiment, the step of receiving sensor data includes algorithmically smoothing the data stream. 
     In an aspect of the twenty-third embodiment, the step of receiving sensor data includes receiving sensor data from a substantially continuous glucose sensor. 
     In an aspect of the twenty-third embodiment, the step of receiving sensor data includes receiving sensor data from an implantable glucose sensor. 
     In an aspect of the twenty-third embodiment, the step of receiving sensor data includes receiving sensor data from a subcutaneously implantable glucose sensor. 
     In an aspect of the twenty-third embodiment, the step of receiving reference data includes receiving reference data from a self-monitoring blood glucose test. 
     In an aspect of the twenty-third embodiment, the step of receiving reference data includes downloading reference data via a cabled connection. 
     In an aspect of the twenty-third embodiment, the step of receiving reference data includes downloading reference data via a wireless connection. 
     In an aspect of the twenty-third embodiment, the step of receiving reference data from a reference analyte monitor includes receiving within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the twenty-third embodiment, the step of evaluating the quality of the calibration set based on a data association function includes performing one of linear regression, non-linear regression, rank correlation, least mean square fit, mean absolute deviation, and mean absolute relative difference. 
     In an aspect of the twenty-third embodiment, the step of evaluating the quality of the calibration set based on a data association function includes performing linear least squares regression. 
     In an aspect of the twenty-third embodiment, the step of evaluating the quality of the calibration set based on a data association function includes setting a threshold of data association. 
     In an aspect of the twenty-third embodiment, the step of evaluating the quality of the calibration set based on data association includes performing linear least squares regression and wherein the step of setting a threshold hold includes an R-value threshold of 0.79. 
     In an aspect of the twenty-third embodiment, the method further includes providing an output to a user interface responsive to the quality of the calibration set. 
     In an aspect of the twenty-third embodiment, the step of providing an output includes displaying analyte values to a user dependent upon the quality of the calibration. 
     In an aspect of the twenty-third embodiment, the step of providing an output includes alerting the dependent upon the quality of the calibration. 
     In an aspect of the twenty-third embodiment, the step of providing an output includes altering the user interface dependent upon the quality of the calibration. 
     In an aspect of the twenty-third embodiment, the step of providing an output includes at least one of providing color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In a twenty-fourth embodiment, a system is provided for evaluating the quality of a calibration of an analyte sensor, the system including: means for receiving a data stream from an analyte sensor, a plurality of time-spaced sensor data points; means for receiving reference data from a reference analyte monitor, including two or more reference data points; means for providing two or more matched data pairs by matching reference analyte data to substantially time corresponding sensor data; means for forming a calibration set including at least two matched data pair; means for creating a conversion function based on the calibration set; means for converting sensor data into calibrated data using the conversion function; means for evaluating the quality of the calibration set based on a data association function. 
     In an aspect of the twenty-fourth embodiment, the means for receiving sensor data includes means for receiving sensor data that has been algorithmically smoothed. 
     In an aspect of the twenty-fourth embodiment, the means for receiving sensor data includes means for algorithmically smoothing the receiving sensor data. 
     In an aspect of the twenty-fourth embodiment, the means for receiving sensor data includes means for receiving sensor data from substantially continuous glucose sensor. 
     In an aspect of the twenty-fourth embodiment, the means for receiving sensor data includes means for receiving sensor data from an implantable glucose sensor. 
     In an aspect of the twenty-fourth embodiment, the means for receiving sensor data includes means for receiving sensor data from subcutaneously implantable glucose sensor. 
     In an aspect of the twenty-fourth embodiment, the means for receiving reference data includes means for receiving reference data from a self-monitoring blood glucose test. 
     In an aspect of the twenty-fourth embodiment, the means for receiving reference data includes means for downloading reference data via a cabled connection. 
     In an aspect of the twenty-fourth embodiment, the means for receiving reference data includes means for downloading reference data via a wireless connection. 
     In an aspect of the twenty-fourth embodiment, the means for receiving reference data from a reference analyte monitor includes means for receiving within a receiver internal communication from a reference analyte monitor integral with the receiver. 
     In an aspect of the twenty-fourth embodiment, the means for evaluating the quality of the calibration set includes means for performing one of linear regression, non-linear regression, rank correlation, least mean square fit, mean absolute deviation, and mean absolute relative difference. 
     In an aspect of the twenty-fourth embodiment, the means for evaluating the quality of the calibration set includes means for performing linear least squares regression. 
     In an aspect of the twenty-fourth embodiment, the means for evaluating the quality of the calibration set includes means for setting a threshold of data association. 
     In an aspect of the twenty-fourth embodiment, the means for evaluating the quality of the calibration set includes means for performing linear least squares regression and wherein the means for setting a threshold hold includes an R-value threshold of 0.71. 
     In an aspect of the twenty-fourth embodiment, the system further includes means for providing an output to a user interface responsive to the quality of the calibration set. 
     In an aspect of the twenty-fourth embodiment, the means for providing an output includes means for displaying analyte values to a user dependent upon the quality of the calibration. 
     In an aspect of the twenty-fourth embodiment, the means for providing an output includes means for alerting the dependent upon the quality of the calibration. 
     In an aspect of the twenty-fourth embodiment, the means for providing an output includes means for altering the user interface dependent upon the quality of the calibration. 
     In an aspect of the twenty-fourth embodiment, the means for providing an output includes at least one of providing color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In a twenty-fifth embodiment, a computer system is provided for evaluating the quality of a calibration of an analyte sensor, the computer system including: a sensor data receiving module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference data receiving module that receives reference data from a reference analyte monitor, including two or more reference data points; a data matching module that forms two or more matched data pairs by matching reference data to substantially time corresponding sensor data; a calibration set module that forms a calibration set including at least two matched data pairs; a conversion function module that creates a conversion function using the calibration set; a sensor data transformation module that converts sensor data into calibrated data using the conversion function; and a quality evaluation module that evaluates the quality of the calibration set based on a data association function. 
     In an aspect of the twenty-fifth embodiment, the sensor data receiving module receives sensor data that has been algorithmically smoothed. 
     In an aspect of the twenty-fifth embodiment, the computer system further includes a data smoothing module that algorithmically smoothes sensor data received from the sensor data receiving module. 
     In an aspect of the twenty-fifth embodiment, the sensor data receiving module is adapted to receive sensor data from substantially continuous glucose sensor. 
     In an aspect of the twenty-fifth embodiment, the sensor data receiving module is adapted to receive sensor data from an implantable glucose sensor. 
     In an aspect of the twenty-fifth embodiment, the sensor data receiving module is adapted to receive sensor data from subcutaneously implantable glucose sensor. 
     In an aspect of the twenty-fifth embodiment, the reference data receiving module is adapted to receive reference data from a self-monitoring blood glucose test. 
     In an aspect of the twenty-fifth embodiment, the reference data receiving module is adapted to download reference data via a cabled connection. 
     In an aspect of the twenty-fifth embodiment, the reference data receiving module is adapted to download reference data via a wireless connection. 
     In an aspect of the twenty-fifth embodiment, the reference data receiving module is adapted to receive reference data from a reference analyte monitor integral with the receiver. 
     In an aspect of the twenty-fifth embodiment, the quality evaluation module performs one of linear regression, non-linear regression, rank correlation, least mean square fit, mean absolute deviation, and mean absolute relative difference to evaluate calibration set quality. 
     In an aspect of the twenty-fifth embodiment, the quality evaluation module performs linear least squares regression. 
     In an aspect of the twenty-fifth embodiment, the quality evaluation module sets a threshold for the data association function. 
     In an aspect of the twenty-fifth embodiment, the quality evaluation module performs linear least squares regression and wherein the threshold of the data association function includes an R-value threshold of at least 0.79. 
     In an aspect of the twenty-fifth embodiment, the computer system further includes an interface control module that controls the user interface based on the quality of the calibration set. 
     In an aspect of the twenty-fifth embodiment, the interface control module displays analyte values to a user dependent upon the quality of the calibration set. 
     In an aspect of the twenty-fifth embodiment, the interface control module alerts the user based upon the quality of the calibration set. 
     In an aspect of the twenty-fifth embodiment, the interface control module alters the user interface based upon the quality of the calibration set. 
     In an aspect of the twenty-fifth embodiment, the interface control module provides at least one of color-coded information, trend information, directional information (e.g., arrows or angled lines), and/or fail-safe information. 
     In a twenty-sixth embodiment, a method is provided for evaluating the quality of a calibration of an analyte sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; creating a conversion function based on the calibration set; receiving additional sensor data from the analyte sensor; converting sensor data into calibrated data using the conversion function; and evaluating the quality of the calibration set based on a data association function selected from the group consisting of linear regression, non-linear regression, rank correlation, least mean square fit, mean absolute deviation, and mean absolute relative difference. 
     In a twenty-seventh embodiment, a method is provided for evaluating the quality of a calibration of an analyte sensor, the method including: receiving a data stream from an analyte sensor, including one or more sensor data points; receiving reference data from a reference analyte monitor, including two or more reference data points; providing at least two matched data pairs by matching reference analyte data to substantially time corresponding sensor data; forming a calibration set including the at least two matching data pairs; creating a conversion function based on the calibration set; receiving additional sensor data from the analyte sensor; converting sensor data into calibrated data using the conversion function; evaluating the quality of the calibration set using a data association function; and providing an output to a user interface responsive to the quality of the calibration set. 
     In a twenty-eighth embodiment, a computer system is provided for evaluating the quality of a calibration of an analyte sensor, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference input module that receives reference data from a reference analyte monitor, including two or more reference data points; a processor module that forms two or more matched data pairs by matching reference data to substantially time corresponding sensor data and subsequently forms a calibration set including the two or more matched data pairs; and a conversion function module that creates a conversion function using the calibration set; a sensor data transformation module that converts sensor data into calibrated data using the conversion function; a quality evaluation module that evaluates the quality of the calibration set based on a data association selected from the group consisting of linear regression, non-linear regression, rank correlation, least mean square fit, mean absolute deviation, and mean absolute relative difference. 
     In a twenty-ninth embodiment, a computer system is provided for evaluating the quality of a calibration of an analyte sensor, the computer system including: a sensor data module that receives a data stream including a plurality of time spaced sensor data points from a substantially continuous analyte sensor; a reference input module that receives reference data from a reference analyte monitor, including two or more reference data points; a processor module that forms two or more matched data pairs by matching reference data to substantially time corresponding sensor data and subsequently forms a calibration set including the two or more matched data pairs; and a conversion function module that creates a conversion function using the calibration set; a sensor data transformation module that converts sensor data into calibrated data using the conversion function; a quality evaluation module that evaluates the quality of the calibration set based on data association; and a fail-safe module that controls the user interface based on the quality of the calibration set. 
     In a thirtieth embodiment, a method is provided for evaluating the quality of a calibration of a glucose sensor, the method including: receiving sensor data from a glucose sensor, including one or more sensor data points; receiving reference data from a reference glucose monitor, including one or more reference data points; providing one or more matched data pairs by matched reference glucose data to substantially time corresponding sensor data; forming a calibration set including at least one matched data pair; and evaluating the quality of the calibration set based on data association. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a glucose sensor in one embodiment. 
         FIG. 2  is a block diagram that illustrates the sensor electronics in one embodiment. 
         FIG. 3  is a graph that illustrates data smoothing of a raw data signal in one embodiment. 
         FIGS. 4A to 4D  are schematic views of a receiver in first, second, third, and fourth embodiments, respectively. 
         FIG. 5  is a block diagram of the receiver electronics in one embodiment. 
         FIG. 6  is a flow chart that illustrates the initial calibration and data output of the sensor data in one embodiment. 
         FIG. 7  is a graph that illustrates a regression performed on a calibration set to obtain a conversion function in one exemplary embodiment. 
         FIG. 8  is a flow chart that illustrates the process of evaluating the clinical acceptability of reference and sensor data in one embodiment. 
         FIG. 9  is a graph of two data pairs on a Clarke Error Grid to illustrate the evaluation of clinical acceptability in one exemplary embodiment. 
         FIG. 10  is a flow chart that illustrates the process of evaluation of calibration data for best calibration based on inclusion criteria of matched data pairs in one embodiment. 
         FIG. 11  is a flow chart that illustrates the process of evaluating the quality of the calibration in one embodiment. 
         FIGS. 12A and 12B  are graphs that illustrate an evaluation of the quality of calibration based on data association in one exemplary embodiment using a correlation coefficient. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     The following description and examples illustrate some exemplary embodiments of the disclosed invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present invention. 
     DEFINITIONS 
     In order to facilitate an understanding of the disclosed invention, a number of terms are defined below. 
     The term “analyte,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, to refer to a substance or chemical constituent in a biological fluid (for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid or urine) that can be analyzed. Analytes may include naturally occurring substances, artificial substances, metabolites, and/or reaction products. In some embodiments, the analyte for measurement by the sensor heads, devices, and methods is analyte. However, other analytes are contemplated as well, including but not limited to acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase; adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle), histidine/urocanic acid, homocysteine, phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers; arginase; benzoylecgonine (cocaine); biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase; conjugated 1-β hydroxy-cholic acid; cortisol; creatine kinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine; de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA (acetylator polymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cystic fibrosis, Duchenne/Becker muscular dystrophy, analyte-6-phosphate dehydrogenase, hemoglobinopathies, A,S,C,E,D-Punjab, beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol); desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanus antitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D; fatty acids/acylglycines; free β-human chorionic gonadotropin; free erythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine (FT3); fumarylacetoacetase; galactose/gal-1-phosphate; galactose-1-phosphate uridyltransferase; gentamicin; analyte-6-phosphate dehydrogenase; glutathione; glutathione perioxidase; glycocholic acid; glycosylated hemoglobin; halofantrine; hemoglobin variants; hexosaminidase A; human erythrocyte carbonic anhydrase I; 17 alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase; immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β); lysozyme; mefloquine; netilmicin; phenobarbitone; phenytoin; phytanic/pristanic acid; progesterone; prolactin; prolidase; purine nucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3); selenium; serum pancreatic lipase; sissomicin; somatomedin C; specific antibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody, arbovirus, Aujeszky&#39;s disease virus, dengue virus,  Dracunculus medinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus, Giardia duodenalisa, Helicobacter pylori , hepatitis B virus, herpes virus, HIV-1, IgE (atopic disease), influenza virus,  Leishmania donovani , leptospira, measles/mumps/rubella,  Mycobacterium leprae, Mycoplasma pneumoniae , Myoglobin,  Onchocerca volvulus , parainfluenza virus,  Plasmodium falciparum , poliovirus,  Pseudomonas aeruginosa , respiratory syncytial virus, rickettsia (scrub typhus),  Schistosoma mansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosoma cruzi/rangeli , vesicular stomatis virus,  Wuchereria bancrofti , yellow fever virus); specific antigens (hepatitis B virus, HIV-1); succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine (T4); thyroxine-binding globulin; trace elements; transferrin; UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A; white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat, vitamins and hormones naturally occurring in blood or interstitial fluids may also constitute analytes in certain embodiments. The analyte may be naturally present in the biological fluid, for example, a metabolic product, a hormone, an antigen, an antibody, and the like. Alternatively, the analyte may be introduced into the body, for example, a contrast agent for imaging, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to insulin; ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine); depressants (barbituates, methaqualone, tranquilizers such as Valium, Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics (heroin, codeine, morphine, opium, meperidine, Percocet, Percodan, Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogs of fentanyl, meperidine, amphetamines, methamphetamines, and phencyclidine, for example, Ecstasy); anabolic steroids; and nicotine. The metabolic products of drugs and pharmaceutical compositions are also contemplated analytes. Analytes such as neurochemicals and other chemicals generated within the body may also be analyzed, such as, for example, ascorbic acid, uric acid, dopamine, noradrenaline, 3-methoxytyramine (3MT), 3,4-Dihydroxyphenylacetic acid (DOPAC), Homovanillic acid (HVA), 5-Hydroxytryptamine (5HT), and 5-Hydroxyindoleacetic acid (FHIAA). 
     The terms “operably connected” and “operably linked,” as used herein, are broad terms and are used in their ordinary sense, including, without limitation, one or more components being linked to another component(s) in a manner that allows transmission of signals between the components, e.g., wired or wirelessly. For example, one or more electrodes may be used to detect the amount of analyte in a sample and convert that information into a signal; the signal may then be transmitted to an electronic circuit means. In this case, the electrode is “operably linked” to the electronic circuitry. 
     The term “EEPROM,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, electrically erasable programmable read-only memory, which is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (e.g., written to) repeatedly through the application of higher than normal electrical voltage. 
     The term “SRAM,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, static random access memory (RAM) that retains data bits in its memory as long as power is being supplied. 
     The term “A/D Converter,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, hardware that converts analog signals into digital signals. 
     The term “microprocessor,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation a computer system or processor designed to perform arithmetic and logic operations using logic circuitry that responds to and processes the basic instructions that drive a computer. 
     The term “RF transceiver,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, a radio frequency transmitter and/or receiver for transmitting and/or receiving signals. 
     The term “jitter” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, uncertainty or variability of waveform timing, which may be cause by ubiquitous noise caused by a circuit and/or environmental effects; jitter can be seen in amplitude, phase timing, or the width of the signal pulse. 
     The term “raw data signal,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, an analog or digital signal directly related to the measured analyte from the analyte sensor. In one example, the raw data signal is digital data in “counts” converted by an A/D converter from an analog signal (e.g., voltage or amps) representative of an analyte concentration. 
     The term “counts,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, a unit of measurement of a digital signal. In one example, a raw data signal measured in counts is directly related to a voltage (converted by an A/D converter), which is directly related to current. 
     The term “analyte sensor,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, any mechanism (e.g., enzymatic or non-enzymatic) by which analyte can be quantified. For example, some embodiments utilize a membrane that contains glucose oxidase that catalyzes the conversion of oxygen and glucose to hydrogen peroxide and gluconate: 
       Glucose+O 2 →Gluconate+H 2 O 2  
 
     Because for each glucose molecule metabolized, there is a proportional change in the co-reactant O 2  and the product H 2 O 2 , one can use an electrode to monitor the current change in either the co-reactant or the product to determine glucose concentration. 
     The term “host,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, mammals, particularly humans. 
     The term “matched data pairs”, as used herein, is a broad term and is used in its ordinary sense, including, without limitation, reference data (e.g., one or more reference analyte data points) matched with substantially time corresponding sensor data (e.g., one or more sensor data points). 
     The term “Clarke Error Grid”, as used herein, is a broad term and is used in its ordinary sense, including, without limitation, an error grid analysis, which evaluates the clinical significance of the difference between a reference glucose value and a sensor generated glucose value, taking into account 1) the value of the reference glucose measurement, 2) the value of the sensor glucose measurement, 3) the relative difference between the two values, and 4) the clinical significance of this difference. See Clarke et al., “Evaluating Clinical Accuracy of Systems for Self-Monitoring of Blood Glucose”, Diabetes Care, Volume 10, Number 5, September-October 1987, which is incorporated by reference herein in its entirety. 
     The term “Consensus Error Grid”, as used herein, is a broad term and is used in its ordinary sense, including, without limitation, an error grid analysis that assigns a specific level of clinical risk to any possible error between two time corresponding glucose measurements. The Consensus Error Grid is divided into zones signifying the degree of risk posed by the deviation. See Parkes et al., “A New Consensus Error Grid to Evaluate the Clinical Significance of Inaccuracies in the Measurement of Blood Glucose”, Diabetes Care, Volume 23, Number 8, August 2000, which is incorporated by reference herein in its entirety. 
     The term “clinical acceptability”, as used herein, is a broad term and is used in its ordinary sense, including, without limitation, determination of the risk of inaccuracies to a patient. Clinical acceptability considers a deviation between time corresponding glucose measurements (e.g., data from a glucose sensor and data from a reference glucose monitor) and the risk (e.g., to the decision making of a diabetic patient) associated with that deviation based on the glucose value indicated by the sensor and/or reference data. One example of clinical acceptability may be 85% of a given set of measured analyte values within the “A” and “B” region of a standard Clarke Error Grid when the sensor measurements are compared to a standard reference measurement. 
     The term “R-value,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, one conventional way of summarizing the correlation of data; that is, a statement of what residuals (e.g., root mean square deviations) are to be expected if the data are fitted to a straight line by the a regression. 
     The term “data association” and “data association function,” as used herein, are a broad terms and are used in their ordinary sense, including, without limitation, a statistical analysis of data and particularly its correlation to, or deviation from, from a particular curve. A data association function is used to show data association. For example, the data that forms that calibration set as described herein may be analyzed mathematically to determine its correlation to, or deviation from, a curve (e.g., line or set of lines) that defines the conversion function; this correlation or deviation is the data association. A data association function is used to determine data association. Examples of data association functions include, but are not limited to, linear regression, non-linear mapping/regression, rank (e.g., non-parametric) correlation, least mean square fit, mean absolute deviation (MAD), mean absolute relative difference. In one such example, the correlation coefficient of linear regression is indicative of the amount of data association of the calibration set that forms the conversion function, and thus the quality of the calibration. 
     The term “quality of calibration” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, the statistical association of matched data pairs in the calibration set used to create the conversion function. For example, an R-value may be calculated for a calibration set to determine its statistical data association, wherein an R-value greater than 0.79 determines a statistically acceptable calibration quality, while an R-value less than 0.79 determines statistically unacceptable calibration quality. 
     The term “substantially” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, being largely but not necessarily wholly that which is specified. 
     The term “congruence” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, the quality or state of agreeing, coinciding, or being concordant. In one example, congruence may be determined using rank correlation. 
     The term “concordant” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, being in agreement or harmony, and/or free from discord. 
     The phrase “continuous (or continual) analyte sensing,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, the period in which monitoring of analyte concentration is continuously, continually, and or intermittently (but regularly) performed, for example, about every 5 to 10 minutes. 
     The term “sensor head,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, the region of a monitoring device responsible for the detection of a particular analyte. In one example, a sensor head comprises a non-conductive body, a working electrode (anode), a reference electrode and a counter electrode (cathode) passing through and secured within the body forming an electrochemically reactive surface at one location on the body and an electronic connective means at another location on the body, and a sensing membrane affixed to the body and covering the electrochemically reactive surface. The counter electrode has a greater electrochemically reactive surface area than the working electrode. During general operation of the sensor a biological sample (e.g., blood or interstitial fluid) or a portion thereof contacts (directly or after passage through one or more membranes or domains) an enzyme (e.g., glucose oxidase); the reaction of the biological sample (or portion thereof) results in the formation of reaction products that allow a determination of the analyte (e.g., glucose) level in the biological sample. In some embodiments, the sensing membrane further comprises an enzyme domain (e.g., and enzyme layer), and an electrolyte phase (e.g., a free-flowing liquid phase comprising an electrolyte-containing fluid described further below). 
     The term “electrochemically reactive surface,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, the surface of an electrode where an electrochemical reaction takes place. In the case of the working electrode, the hydrogen peroxide produced by the enzyme catalyzed reaction of the analyte being detected creates a measurable electronic current (e.g., detection of analyte utilizing analyte oxidase produces H 2 O 2  peroxide as a by product, H 2 O 2  reacts with the surface of the working electrode producing two protons (2H + ), two electrons (2e − ) and one molecule of oxygen (O 2 ) which produces the electronic current being detected). In the case of the counter electrode, a reducible species, e.g., O 2  is reduced at the electrode surface in order to balance the current being generated by the working electrode. 
     The term “electronic connection,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, any electronic connection known to those in the art that may be utilized to interface the sensor head electrodes with the electronic circuitry of a device such as mechanical (e.g., pin and socket) or soldered. 
     The term “sensing membrane,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, a permeable or semi-permeable membrane that may be comprised of two or more domains and constructed of materials of a few microns thickness or more, which are permeable to oxygen and may or may not be permeable to an analyte of interest. In one example, the sensing membrane comprises an immobilized glucose oxidase enzyme, which enables an electrochemical reaction to occur to measure a concentration of glucose. 
     The term “biointerface membrane,” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, a permeable membrane that may be comprised of two or more domains and constructed of materials of a few microns thickness or more, which may be placed over the sensor body to keep host cells (e.g., macrophages) from gaining proximity to, and thereby damaging, the sensing membrane or forming a barrier cell layer and interfering with the transport of analyte across the tissue-device interface. 
     In the disclosure which follows, the following abbreviations apply: Eq and Eqs (equivalents); mEq (milliequivalents); M (molar); mM (millimolar) μM (micromolar); N (Normal); mol (moles); mmol (millimoles); μmol (micromoles); nmol (nanomoles); g (grams); mg (milligrams); μg (micrograms); Kg (kilograms); L (liters); mL (milliliters); dL (deciliters); μL (microliters); cm (centimeters); mm (millimeters); μm (micrometers); nm (nanometers); h and hr (hours); min. (minutes); s and sec. (seconds); ° C. (degrees Centigrade). 
     Overview 
     The preferred embodiments relate to the use of an analyte sensor that measures a concentration of analyte of interest or a substance indicative of the concentration or presence of the analyte. In some embodiments, the sensor is a continuous device, for example a subcutaneous, transdermal, or intravascular device. In some embodiments, the device may analyze a plurality of intermittent blood samples. The analyte sensor may use any method of analyte-sensing, including enzymatic, chemical, physical, electrochemical, spectrophotometric, polarimetric, calorimetric, radiometric, or the like. 
     The analyte sensor uses any known method, including invasive, minimally invasive, and non-invasive sensing techniques, to provide an output signal indicative of the concentration of the analyte of interest. The output signal is typically a raw signal that is used to provide a useful value of the analyte of interest to a user, such as a patient or physician, who may be using the device. Accordingly, appropriate smoothing, calibration, and evaluation methods may be applied to the raw signal and/or system as a whole to provide relevant and acceptable estimated analyte data to the user. 
     Sensor 
     The analyte sensor useful with the preferred embodiments may be any device capable of measuring the concentration of an analyte of interest. One exemplary embodiment is described below, which utilizes an implantable glucose sensor. However, it should be understood that the devices and methods described herein may be applied to any device capable of detecting a concentration of analyte of and providing an output signal that represents the concentration of the analyte. 
       FIG. 1  is an exploded perspective view of a glucose sensor in one embodiment. The implantable glucose sensor  10  utilizes amperometric electrochemical sensor technology to measure glucose. In this exemplary embodiment, a body  12  and a head  14  house electrodes  16  and sensor electronics, which are described in more detail with reference to  FIG. 2 . Three electrodes  16  are operably connected to the sensor electronics ( FIG. 2 ) and are covered by a sensing membrane  17  and a biointerface membrane  18 , which are attached by a clip  19 . In alternative embodiments, the number of electrodes may be less than or greater than three. 
     The three electrodes  16 , which protrude through the head  14 , including a platinum working electrode, a platinum counter electrode, and a silver/silver chloride reference electrode. The top ends of the electrodes are in contact with an electrolyte phase (not shown), which is a free-flowing fluid phase disposed between the sensing membrane and the electrodes. The sensing membrane  17  includes an enzyme, e.g., glucose oxidase, which covers the electrolyte phase. In turn, the biointerface membrane  18  covers the sensing membrane  17  and serves, at least in part, to protect the sensor from external forces that may result in environmental stress cracking of the sensing membrane  17 . 
     In the illustrated embodiment, the counter electrode is provided to balance the current generated by the species being measured at the working electrode. In the case of a glucose oxidase based glucose sensor, the species being measured at the working electrode is H 2 O 2 . Glucose oxidase catalyzes the conversion of oxygen and glucose to hydrogen peroxide and gluconate according to the following reaction: 
       Glucose+O 2 →Gluconate+H 2 O 2  
 
     The change in H 2 O 2  can be monitored to determine glucose concentration because for each glucose molecule metabolized, there is a proportional change in the product H 2 O 2 . Oxidation of H 2 O 2  by the working electrode is balanced by reduction of ambient oxygen, enzyme generated H 2 O 2 , or other reducible species at the counter electrode. The H 2 O 2  produced from the glucose oxidase reaction further reacts at the surface of working electrode and produces two protons (2H + ), two electrons (2e − ) and one oxygen molecule (O 2 ) (See, e.g., Fraser, D. M. “An Introduction to In vivo Biosensing: Progress and problems.” In “Biosensors and the Body,” D. M. Fraser, ed., 1997, pp. 1-56 John Wiley and Sons, New York.) 
     In one embodiment, a potentiostat is used to measure the electrochemical reaction(s) at the electrode(s) (see  FIG. 2 ). The potentiostat applies a constant potential between the working and reference electrodes to produce a current value. The current that is produced at the working electrode (and flows through the circuitry to the counter electrode) is proportional to the diffusional flux of H 2 O 2 . Accordingly, a raw signal may be produced that is representative of the concentration of glucose in the users body, and therefore may be utilized to estimate a meaningful glucose value, such as described elsewhere herein. 
     One problem of enzymatic glucose sensors such as described above is the non-glucose reaction rate-limiting phenomenon. For example, if oxygen is deficient, relative to the amount of glucose, then the enzymatic reaction will be limited by oxygen rather than glucose. Consequently, the output signal will be indicative of the oxygen concentration rather than the glucose concentration. 
       FIG. 2  is a block diagram that illustrates the sensor electronics in one embodiment. In this embodiment, the potentiostat  20  is shown, which is operatively connected to electrodes  16  ( FIG. 1 ) to obtain a current value, and includes a resistor (not shown) that translates the current into voltage. An A/D converter  21  digitizes the analog signal into counts for processing. Accordingly, the resulting raw data signal in counts is directly related to the current measured by the potentiostat  20 . 
     A microprocessor  22  is the central control unit that houses EEPROM  23  and SRAM  24 , and controls the processing of the sensor electronics. It may be noted that alternative embodiments utilize a computer system other than a microprocessor to process data as described herein. In some alternative embodiments, an application-specific integrated circuit (ASIC) may be used for some or all the sensor&#39;s central processing. The EEPROM  23  provides semi-permanent storage of data, storing data such as sensor ID and necessary programming to process data signals (e.g., programming for data smoothing such as described below). The SRAM  24  is used for the system&#39;s cache memory, for example for temporarily storing recent sensor data. 
     A battery  25  is operatively connected to the microprocessor  22  and provides the necessary power for the sensor. In one embodiment, the battery is a Lithium Manganese Dioxide battery, however any appropriately sized and powered battery may be used (e.g., AAA, Nickel-cadmium, Zinc-carbon, Alkaline, Lithium, Nickel-metal hydride, Lithium-ion, Zinc-air, Zinc-mercury oxide, Silver-zinc, or hermetically-sealed). In some embodiments, a plurality of batteries may be used to power the system. A Quartz Crystal  26  is operatively connected to the microprocessor  22  and maintains system time for the computer system as a whole. 
     An RF Transceiver  27  is operably connected to the microprocessor  22  and transmits the sensor data from the sensor to a receiver (see  FIGS. 4 and 5 ). Although an RF transceiver is shown here, other embodiments include a wired rather than wireless connection to the receiver. In yet other embodiments, the receiver is transcutaneously powered via an inductive coupling, for example. A quartz crystal  28  provides the system time for synchronizing the data transmissions from the RF transceiver. It may be noted that the transceiver  27  may be substituted for a transmitter in one embodiment. 
     Data Smoothing 
     Typically, an analyte sensor produces a raw data signal that is indicative of the analyte concentration of a user, such as described in more detail with reference to  FIGS. 1 and 2 , above. However, it is well known that the above described glucose sensor is only one example of an abundance of analyte sensors that are able to provide a raw data signal output indicative of the concentration of the analyte of interest. Thus, it should be understood that the devices and methods of the preferred embodiments, including data smoothing, calibration, evaluation, and other data processing, may be applied to raw data obtained from any analyte sensor capable of producing a output signal. 
     It has been found that raw data signals received from an analyte sensor include signal noise, which degrades the quality of the data. Thus, it has been known to use smoothing algorithms help improve the signal-to-noise ratio in the sensor by reducing signal jitter, for example. One example of a conventional data smoothing algorithms include finite impulse response filter (FIR), which is particularly suited for reducing high-frequency noise (see Steil et al. U.S. Pat. No. 6,558,351). Other analyte sensors have utilized heuristic and moving average type algorithms to accomplish data smoothing of signal jitter in data signals, for example. 
     It is advantageous to also reduce signal noise by attenuating transient, low frequency, non-analyte related signal fluctuations (e.g., transient ischemia and/or long transient periods of postural effects that interfere with sensor function due to lack of oxygen and/or other physiological effects). 
     In one embodiment, this attenuation of transient low frequency non-analyte related signal noise is accomplished using a recursive filter. In contrast to conventional non-recursive (e.g., FIR) filters in which each computation uses new input data sets, a recursive filter is an equation that uses moving averages as inputs; that is, a recursive filter includes previous averages as part of the next filtered output. Recursive filters are advantageous at least in part due to their computational efficiency. 
       FIG. 3  is a graph that illustrates data smoothing of a raw data signal in one embodiment. In this embodiment, the recursive filter is implemented as a digital infinite impulse response filter (IIR) filter, wherein the output is computed using 6 additions and 7 multiplies as shown in the following equation: 
     
       
         
           
             
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     This polynomial equation includes coefficients that are dependent on sample rate and frequency behavior of the filter. In this exemplary embodiment, frequency behavior passes low frequencies up to cycle lengths of 40 minutes, and is based on a 30 second sample rate. 
     In some embodiments, data smoothing may be implemented in the sensor and the smoothed data transmitted to a receiver for additional processing. In other embodiments, raw data may be sent from the sensor to a receiver for data smoothing and additional processing therein. In yet other embodiments, the sensor is integral with the receiver and therefore no transmission of data is required. 
     In one exemplary embodiment, wherein the sensor is an implantable glucose sensor, data smoothing is performed in the sensor to ensure a continuous stream of data. In alternative embodiments, data smoothing may be transmitted from the sensor to the receiver, and the data smoothing performed at the receiver; it may be noted however that there may be a risk of transmit-loss in the radio transmission from the sensor to the receiver when the transmission is wireless. For example, in embodiments wherein a sensor is implemented in vivo, the raw sensor signal may be more consistent within the sensor (in vivo) than the raw signal transmitted to a source (e.g., receiver) outside the body (e.g., if a patient were to take the receiver off to shower, communication between the sensor and receiver may be lost and data smoothing in the receiver would halt accordingly.) Consequently, it may be noted that a multiple point data loss in the filter may take, for example, anywhere from 25 to 40 minutes for the smoothed data to recover to where it would have been had there been no data loss. 
     Receiver 
       FIGS. 4A to 4D  are schematic views of a receiver in first, second, third, and fourth embodiments, respectively. A receiver  40  comprises systems necessary to receive, process, and display sensor data from an analyte sensor, such as described elsewhere herein. Particularly, the receiver  40  may be a pager-sized device, for example, and comprise a user interface that has a plurality of buttons  42  and a liquid crystal display (LCD) screen  44 , and which may include a backlight. In some embodiments the user interface may also include a keyboard, a speaker, and a vibrator such as described with reference to  FIG. 5 . 
       FIG. 4A  illustrates a first embodiment wherein the receiver shows a numeric representation of the estimated analyte value on its user interface, which is described in more detail elsewhere herein. 
       FIG. 4B  illustrates a second embodiment wherein the receiver shows an estimated glucose value and one hour of historical trend data on its user interface, which is described in more detail elsewhere herein. 
       FIG. 4C  illustrates a third embodiment wherein the receiver shows an estimated glucose value and three hours of historical trend data on its user interface, which is described in more detail elsewhere herein. 
       FIG. 4D  illustrates a fourth embodiment wherein the receiver shows an estimated glucose value and nine hours of historical trend data on its user interface, which is described in more detail elsewhere herein. 
     In some embodiments a user is able to toggle through some or all of the screens shown in  FIGS. 4A to 4D  using a toggle button on the receiver. In some embodiments, the user is able to interactively select the type of output displayed on their user interface. In some embodiments, the sensor output may have alternative configurations, such as is described with reference to  FIG. 6 , block  69 , for example. 
       FIG. 5  is a block diagram of the receiver electronics in one embodiment. It may be noted that the receiver may comprise a configuration such as described with reference to  FIGS. 4A to 4D , above. Alternatively, the receiver may comprise any configuration, including a desktop computer, laptop computer, a personal digital assistant (PDA), a server (local or remote to the receiver), or the like. In some embodiments, a receiver may be adapted to connect (via wired or wireless connection) to a desktop computer, laptop computer, a PDA, a server (local or remote to the receiver), or the like in order to download data from the receiver. In some alternative embodiments, the receiver is housed within or directly connected to the sensor in a manner that allows sensor and receiver electronics to work directly together and/or share data processing resources. Accordingly, the receiver, including its electronics, may be generally described as a “computer system.” 
     A quartz crystal  50  is operatively connected to an RF transceiver  51  that together function to receive and synchronize data signals (e.g., raw data signals transmitted from the RF transceiver). Once received, the microprocessor  52  processes the signals, such as described below. 
     The microprocessor  52  is the central control unit that provides the necessary processing, such as calibration algorithms stored within an EEPROM  53 . The EEPROM  53  is operatively connected to the microprocessor  52  and provides semi-permanent storage of data, storing data such as receiver ID and necessary programming to process data signals (e.g., programming for performing calibration and other algorithms described elsewhere herein). In some embodiments, an application-specific integrated circuit (ASIC) may be used for some or all the receiver&#39;s central processing. An SRAM  54  is used for the system&#39;s cache memory and is helpful in data processing. 
     The microprocessor  52 , which is operatively connected to EEPROM  53  and SRAM  54 , controls the processing of the receiver electronics including, but not limited to, a sensor data receiving module, a reference data receiving module, a data matching module, a calibration set module, a conversion function module, a sensor data transformation module, a quality evaluation module, a interface control module, and a stability determination module, which are described in more detail below. It may be noted that any of the above processing may be programmed into and performed in the sensor electronics ( FIG. 2 ) in place of, or in complement with, the receiver electronics ( FIG. 5 ). 
     A battery  55  is operatively connected to the microprocessor  52  and provides the necessary power for the receiver. In one embodiment, the battery is a AAA battery, however any appropriately sized and powered battery may be used. In some embodiments, a plurality of batteries may be used to power the system. A quartz crystal  56  is operatively connected to the microprocessor  52  and maintains system time for the computer system as a whole. 
     A user interface  57  comprises a keyboard, speaker, vibrator, backlight, LCD, and a plurality of buttons. The components that comprise the user interface  57  provide the necessary controls to interact with the user. A keyboard may allow, for example, input of user information about himself/herself, such as mealtime, exercise, insulin administration, and reference analyte values. A speaker may provide, for example, audible signals or alerts for conditions such as present and/or predicted hyper- and hypoglycemic conditions. A vibrator may provide, for example, tactile signals or alerts for reasons such as described with reference to the speaker, above. A backlight may be provided, for example, to aid the user in reading the LCD in low light conditions. An LCD may be provided, for example, to provide the user with visual data output such as described in more detail with reference to  FIGS. 4A to 4D  and  FIG. 6 . Buttons may provide toggle, menu selection, option selection, mode selection, and reset, for example. 
     Communication ports, including a personal computer (PC) com port  58  and a reference analyte monitor com port  59  may be provided to enable communication with systems that are separate from, or integral with, the receiver. The PC com port  58  comprises means for communicating with another computer system (e.g., PC, PDA, server, or the like). In one exemplary embodiment, the receiver is able to download historic data to a physician&#39;s PC for retrospective analysis by the physician. The reference analyte monitor com port  59  comprises means for communicating with a reference analyte monitor so that reference analyte values may be automatically downloaded into the receiver. In one embodiment, the reference analyte monitor is integral with the receiver, and the reference analyte com port  59  allows internal communication between the two integral systems. In another embodiment, the reference analyte monitor com port  59  allows a wireless or wired connection to the reference analyte monitor such as a self-monitoring blood glucose monitor (e.g., for measuring finger stick blood samples). 
     Algorithms 
     Reference is now made to  FIG. 6 , which is a flow chart that illustrates the initial calibration and data output of the sensor data in one embodiment. 
     Calibration of an analyte sensor comprises data processing that converts sensor data signal into an estimated analyte measurement that is meaningful to a user. Accordingly, a reference analyte value is used to calibrate the data signal from the analyte sensor. 
     At block  61 , a sensor data receiving module, also referred to as the sensor data module, receives sensor data (e.g., a data stream), including one or more time-spaced sensor data points, from a sensor via the receiver, which may be in wired or wireless communication with the sensor. The sensor data point(s) may be smoothed, such as described with reference to  FIG. 3 , above. It may be noted that during the initialization of the sensor, prior to initial calibration, the receiver (e.g., computer system) receives and stores the sensor data, however may not display any data to the user until initial calibration and possibly stabilization of the sensor has been determined. 
     At block  62 , a reference data receiving module, also referred to as the reference input module, receives reference data from a reference analyte monitor, including one or more reference data points. In one embodiment, the reference analyte points may comprise results from a self-monitored blood analyte test (e.g., from a finger stick test). In one such embodiment, the user may administer a self-monitored blood analyte test to obtain an analyte value (e.g., point) using any known analyte sensor, and then enter the numeric analyte value into the computer system. In another such embodiment, a self-monitored blood analyte test comprises a wired or wireless connection to the receiver (e.g. computer system) so that the user simply initiates a connection between the two devices, and the reference analyte data is passed or downloaded between the self-monitored blood analyte test and the receiver. In yet another such embodiment, the self-monitored analyte test is integral with the receiver so that the user simply provides a blood sample to the receiver, and the receiver runs the analyte test to determine a reference analyte value. 
     It may be noted that certain acceptability parameters may be set for reference values received from the user. For example, in one embodiment, the receiver may only accept reference analyte values between about 40 and about 400 mg/dL. Other examples of determining valid reference analyte values are described in more detail with reference to  FIG. 8 . 
     At block  63 , a data matching module, also referred to as the processor module, matches reference data (e.g., one or more reference analyte data points) with substantially time corresponding sensor data (e.g., one or more sensor data points) to provide one or more matched data pairs. In one embodiment, one reference data point is matched to one time corresponding sensor data point to form a matched data pair. In another embodiment, a plurality of reference data points are averaged (e.g., equally or non-equally weighted average, mean-value, median, or the like) and matched to one time corresponding sensor data point to form a matched data pair. In another embodiment, one reference data point is matched to a plurality of time corresponding sensor data points averaged to form a matched data pair. In yet another embodiment, a plurality of reference data points are averaged and matched to a plurality of time corresponding sensor data points averaged to form a matched data pair. 
     In one embodiment, a time corresponding sensor data comprises one or more sensor data points that occur 15±5 min after the reference analyte data timestamp (e.g., the time that the reference analyte data is obtained). In this embodiment, the 15 minute time delay has been chosen to account for an approximately 10 minute delay introduced by the filter used in data smoothing and an approximately 5 minute physiological time-lag (e.g., the time necessary for the analyte to diffusion through a membrane(s) of an analyte sensor). In alternative embodiments, the time corresponding sensor value may be more or less than the above-described embodiment, for example ±60 minutes. Variability in time correspondence of sensor and reference data may be attributed to, for example a longer or shorter time delay introduced by the data smoothing filter, or if the configuration of the analyte sensor incurs a greater or lesser physiological time lag. 
     It may be noted that in some practical implementations of the sensor, the reference analyte data may be obtained at a time that is different from the time that the data is input into the receiver. Accordingly, it should be noted that the “time stamp” of the reference analyte (e.g., the time at which the reference analyte value was obtained) is not the same as the time at which the reference analyte data was obtained by receiver. Therefore, some embodiments include a time stamp requirement that ensures that the receiver stores the accurate time stamp for each reference analyte value, that is, the time at which the reference value was actually obtained from the user. 
     In some embodiments, tests are used to evaluate the best matched pair using a reference data point against individual sensor values over a predetermined time period (e.g., about 30 minutes). In one such exemplary embodiment, the reference data point is matched with sensor data points at 5-minute intervals and each matched pair is evaluated. The matched pair with the best correlation may be selected as the matched pair for data processing. In some alternative embodiments, matching a reference data point with an average of a plurality of sensor data points over a predetermined time period may be used to form a matched pair. 
     At block  64 , a calibration set module, also referred to as the processor module, forms an initial calibration set from a set of one or more matched data pairs, which are used to determine the relationship between the reference analyte data and the sensor analyte data, such as will be described in more detail with reference to block  67 , below. 
     The matched data pairs, which make up the initial calibration set, may be selected according to predetermined criteria. It may be noted that the criteria for the initial calibration set may be the same as, or different from, the criteria for the update calibration set, which is described in more detail with reference to  FIG. 10 . In some embodiments, the number (n) of data pair(s) selected for the initial calibration set is one. In other embodiments, n data pairs are selected for the initial calibration set wherein n is a function of the frequency of the received reference data points. In one exemplary embodiment, six data pairs make up the initial calibration set. 
     In some embodiments, the data pairs are selected only within a certain analyte value threshold, for example wherein the reference analyte value is between about 40 and about 400 mg/dL. In some embodiments, the data pairs that form the initial calibration set are selected according to their time stamp. In some embodiments, the calibration set is selected such as described with reference to  FIG. 10   
     At block  65 , a stability determination module, also referred to as the start-up module, determines the stability of the analyte sensor over a period of time. It may be noted that some analyte sensors may have an initial instability time period during which the analyte sensor is unstable for environmental, physiological, or other reasons. One example of initial sensor instability is an embodiment wherein the analyte sensor is implanted subcutaneously; in this example embodiment, stabilization of the analyte sensor may be dependent upon the maturity of the tissue ingrowth around and within the sensor. Another example of initial sensor instability is in an embodiment wherein the analyte sensor is implemented transdermally; in this example embodiment, stabilization of the analyte sensor may be dependent upon electrode stabilization and/or sweat, for example. 
     Accordingly, in some embodiments, determination of sensor stability may include waiting a predetermined time period (e.g., an implantable sensor is known to require a time period for tissue, and a transdermal sensor is known to require time to equilibrate the sensor with the user&#39;s skin); in some embodiments, this predetermined waiting period is between about one minute and about six weeks. In some embodiments, the sensitivity (e.g., sensor signal strength with respect to analyte concentration) may be used to determine the stability of the sensor; for example, amplitude and/or variability of sensor sensitivity may be evaluated to determine the stability of the sensor. In alternative embodiments, detection of pH levels, oxygen, hypochlorite, interfering species (e.g., ascorbate, urea, and acetaminophen), correlation between sensor and reference values (e.g., R-value), baseline drift and/or offset, and the like may be used to determine the stability of the sensor. In one exemplary embodiment, wherein the sensor is a glucose sensor, it is known to provide a signal that is associated with interfering species (e.g., ascorbate, urea, acetaminophen), which may be used to evaluate sensor stability. In another exemplary embodiment, wherein the sensor is a glucose sensor such as described with reference to  FIGS. 1 and 2 , the counter electrode can be monitored for oxygen deprivation, which may be used to evaluate sensor stability or functionality. 
     At decision block  66 , the system (e.g., microprocessor) determines whether the analyte sensor is sufficiently stable according to certain criteria, such as described above. In one embodiment wherein the sensor is an implantable glucose sensor, the system waits a predetermined time period believed necessary for sufficient tissue ingrowth and evaluates the sensor sensitivity (e.g., between about one minute and six weeks). In another embodiment, the receiver determines sufficient stability based on oxygen concentration near the sensor head. In yet another embodiment, the sensor determines sufficient stability based on a reassessment of baseline drift and/or offset. In yet another alternative embodiment, the system evaluates stability by monitoring the frequency content of the sensor data stream over a predetermined amount of time (e.g., 24 hours); in this alternative embodiment, a template (or templates) are provided that reflect acceptable levels of glucose physiology and are compared with the actual sensor data, wherein a predetermined amount of agreement between the template and the actual sensor data is indicative of sensor stability. It may be noted that a few examples of determining sufficient stability are given here, however a variety of known tests and parameters may be used to determine sensor stability without departing from the spirit and scope of the preferred embodiments. 
     If the receiver does not assess that the stability of the sensor is sufficient, then the processing returns to block  61 , wherein the receiver receives sensor data such as described in more detail above. The above-described steps are repeated until sufficient stability is determined. 
     If the receiver does assess that the stability of the sensor is sufficient, then processing continues to block  67  and the calibration set is used to calibrate the sensor. 
     At block  67 , the conversion function module uses the calibration set to create a conversion function. The conversion function substantially defines the relationship between the reference analyte data and the analyte sensor data. 
     A variety of known methods may be used with the preferred embodiments to create the conversion function from the calibration set. In one embodiment, wherein a plurality of matched data points form the initial calibration set, a linear least squares regression is performed on the initial calibration set such as described with reference to  FIG. 7 . 
       FIG. 7  is a graph that illustrates a regression performed on a calibration set to create a conversion function in one exemplary embodiment. In this embodiment, a linear least squares regression is performed on the initial calibration set. The x-axis represents reference analyte data; the y-axis represents sensor data. The graph pictorially illustrates regression of the matched pairs  76  in the calibration set. Regression calculates a slope  72  and an offset  74  (y=mx+b), which defines the conversion function. 
     In alternative embodiments other algorithms could be used to determine the conversion function, for example forms of linear and non-linear regression, for example fuzzy logic, neural networks, piece-wise linear regression, polynomial fit, genetic algorithms, and other pattern recognition and signal estimation techniques. 
     In yet other alternative embodiments, the conversion function may comprise two or more different optimal conversions because an optimal conversion at any time is dependent on one or more parameters, such as time of day, calories consumed, exercise, or analyte concentration above or below a set threshold, for example. In one such exemplary embodiment, the conversion function is adapted for the estimated glucose concentration (e.g., high vs. low). For example in an implantable glucose sensor it has been observed that the cells surrounding the implant will consume at least a small amount of glucose as it diffuses toward the glucose sensor. Assuming the cells consume substantially the same amount of glucose whether the glucose concentration is low or high, this phenomenon will have a greater effect on the concentration of glucose during low blood sugar episodes than the effect on the concentration of glucose during relatively higher blood sugar episodes. Accordingly, the conversion function is adapted to compensate for the sensitivity differences in blood sugar level. In one implementation, the conversion function comprises two different regression lines wherein a first regression line is applied when the estimated blood glucose concentration is at or below a certain threshold (e.g., 150 mg/dL) and a second regression line is applied when the estimated blood glucose concentration is at or above a certain threshold (e.g., 150 mg/dL). In one alternative implementation, a predetermined pivot of the regression line that forms the conversion function may be applied when the estimated blood is above or below a set threshold (e.g., 150 mg/dL), wherein the pivot and threshold are determined from a retrospective analysis of the performance of a conversion function and its performance at a range of glucose concentrations. In another implementation, the regression line that forms the conversion function is pivoted about a point in order to comply with clinical acceptability standards (e.g., Clarke Error Grid, Consensus Grid, mean absolute relative difference, or other clinical cost function). Although only a few example implementations are described, the preferred embodiments contemplate numerous implementations wherein the conversion function is adaptively applied based on one or more parameters that may affect the sensitivity of the sensor data over time. 
     Referring again to  FIG. 6 , at block  68 , a sensor data transformation module uses the conversion function to transform sensor data into substantially real-time analyte value estimates, also referred to as calibrated data, as sensor data is continuously (or intermittently) received from the sensor. For example, in the embodiment of  FIG. 7 , the sensor data, which may be provided to the receiver in “counts”, is translated in to estimate analyte value(s) in mg/dL. In other words, the offset value at any given point in time may be subtracted from the raw value (e.g., in counts) and divided by the slope to obtain the estimate analyte value: 
     
       
         
           
             
               mg 
                
               
                 / 
               
                
               dL 
             
             = 
             
               
                 ( 
                 
                   rawvalue 
                   - 
                   offset 
                 
                 ) 
               
               slope 
             
           
         
       
     
     In some alternative embodiments, the sensor and/or reference analyte values are stored in a database for retrospective analysis. 
     At block  69 , an output module provides output to the user via the user interface. The output is representative of the estimated analyte value, which is determined by converting the sensor data into a meaningful analyte value such as described in more detail with reference to block  68 , above. User output may be in the form of a numeric estimated analyte value, an indication of directional trend of analyte concentration, and/or a graphical representation of the estimated analyte data over a period of time, for example. Other representations of the estimated analyte values are also possible, for example audio and tactile. 
     In one exemplary embodiment, such as shown in  FIG. 4A , the estimated analyte value is represented by a numeric value. In other exemplary embodiments, such as shown in  FIGS. 4B to 4D , the user interface graphically represents the estimated analyte data trend over predetermined a time period (e.g., one, three, and nine hours, respectively). In alternative embodiments, other time periods may be represented. 
     In some embodiments, the user interface begins displaying data to the user after the sensor&#39;s stability has been affirmed. In some alternative embodiments however, the user interface displays data that is somewhat unstable (e.g., does not have sufficient stability at block  66 ); in these embodiments, the receiver may also include an indication of instability of the sensor data (e.g., flashing, faded, or another indication of sensor instability displayed on the user interface). In some embodiments, the user interface informs the user of the status of the stability of the sensor data. 
     Accordingly, after initial calibration of the sensor, and possibly determination of stability of the sensor data, real-time continuous analyte information may be displayed on the user interface so that the user may regularly and proactively care for his/her diabetic condition within the bounds set by his/her physician. 
     In alternative embodiments, the conversion function is used to predict analyte values at future points in time. These predicted values may be used to alert the user of upcoming hypoglycemic or hyperglycemic events. Additionally, predicted values may be used to compensate for the time lag (e.g., 15 minute time lag such as described elsewhere herein), so that an estimate analyte value displayed to the user represents the instant time, rather than a time delayed estimated value. 
     In some embodiments, the substantially real time estimated analyte value, a predicted future estimate analyte value, a rate of change, and/or a directional trend of the analyte concentration is used to control the administration of a constituent to the user, including an appropriate amount and time, in order to control an aspect of the user&#39;s biological system. One such example is a closed loop glucose sensor and insulin pump, wherein the analyte data (e.g., estimated glucose value, rate of change, and/or directional trend) from the glucose sensor is used to determine the amount of insulin, and time of administration, that may be given to a diabetic user to evade hyper- and hypoglycemic conditions. 
     Reference is now made to  FIG. 8 , which is a flow chart that illustrates the process of evaluating the clinical acceptability of reference and sensor data in one embodiment. Although some clinical acceptability tests are disclosed here, any known clinical standards and methodologies may be applied to evaluate the clinical acceptability of reference and analyte data herein. 
     It may be noted that the conventional analyte meters (e.g., self-monitored blood analyte tests) are known to have a +−20% error in analyte values. For example, gross errors in analyte readings are known to occur due to patient error in self-administration of the blood analyte test. In one such example, if the user has traces of sugar on his/her finger while obtaining a blood sample for a glucose concentration test, then the measured glucose value will likely be much higher than the actual glucose value in the blood. Additionally, it is known that self-monitored analyte tests (e.g., test strips) are occasionally subject to manufacturing error. 
     Another cause for error includes infrequency and time delay that may occur if a user does not self-test regularly, or if a user self-tests regularly but does not enter the reference value at the appropriate time or with the appropriate time stamp. Therefore, it may be advantageous to validate the acceptability of reference analyte values prior to accepting them as valid entries. Accordingly, the receiver evaluates the clinical acceptability of received reference analyte data prior to their acceptance as a valid reference value. 
     In one embodiment, the reference analyte data (and/or sensor analyte data) is evaluated with respect to substantially time corresponding sensor data (and/or substantially time corresponding reference analyte data) to determine the clinical acceptability of the reference analyte and/or sensor analyte data. Clinical acceptability considers a deviation between time corresponding glucose measurements (e.g., data from a glucose sensor and data from a reference glucose monitor) and the risk (e.g., to the decision making of a diabetic patient) associated with that deviation based on the glucose value indicated by the sensor and/or reference data. Evaluating the clinical acceptability of reference and sensor analyte data, and controlling the user interface dependent thereon, may minimize clinical risk. 
     In one embodiment, the receiver evaluates clinical acceptability each time reference data is obtained. In another embodiment, the receiver evaluates clinical acceptability after the initial calibration and stabilization of the sensor, such as described with reference to  FIG. 6 , above. In some embodiments, the receiver evaluates clinical acceptability as an initial pre-screen of reference analyte data, for example after determining if the reference glucose measurement is between about 40 and 400 mg/dL. In other embodiments, other methods of pre-screening data may be used, for example by determining if a reference analyte data value is physiologically feasible based on previous reference analyte data values (e.g., below a maximum rate of change). 
     After initial calibration such as described in more detail with reference to  FIG. 6 , the sensor data receiving module  61  receives substantially continuous sensor data (e.g., a data stream) via a receiver and converts that data into estimated analyte values. As used herein, “substantially continuous” is broad enough to include a data stream of individual measurements taken at time intervals (e.g., time-spaced) ranging from fractions of a second up to, e.g., 1, 2, or 5 minutes. As sensor data is continuously converted, it may be occasionally recalibrated such as described in more detail with reference  FIG. 10 . Initial calibration and re-calibration of the sensor requires a reference analyte value. Accordingly, the receiver may receive reference analyte data at any time for appropriate processing. These reference analyte values may be evaluated for clinical acceptability such as described below as a fail-safe against reference analyte test errors. 
     At block  81 , the reference data receiving module, also referred to as the reference input module, receives reference analyte data from a reference analyte monitor. In one embodiment, the reference data comprises one analyte value obtained from a reference monitor. In some alternative embodiments however, the reference data includes a set of analyte values entered by a user into the interface and averaged by known methods such as described elsewhere herein. 
     In some embodiments, the reference data is pre-screened according to environmental and physiological issues, such as time of day, oxygen concentration, postural effects, and patient-entered environmental data. In one example embodiment, wherein the sensor comprises an implantable glucose sensor, an oxygen sensor within the glucose sensor is used to determine if sufficient oxygen is being provided to successfully complete the necessary enzyme and electrochemical reactions for glucose sensing. In another example embodiment wherein the sensor comprises an implantable glucose sensor, the counter electrode could be monitored for a “rail-effect”, that is, when insufficient oxygen is provided at the counter electrode causing the counter electrode to reach operational (e.g., circuitry) limits. In yet another example embodiment, the patient is prompted to enter data into the user interface, such as meal times and/or amount of exercise, which could be used to determine likelihood of acceptable reference data. 
     It may be further noted that evaluation data, such as described in the paragraph above, may be used to evaluate an optimum time for reference analyte measurement. Correspondingly, the user interface may then prompt the user to provide a reference data point for calibration within a given time period. Consequently, because the receiver proactively prompts the user during optimum calibration times, the likelihood of error due to environmental and physiological limitations may decrease and consistency and acceptability of the calibration may increase. 
     At block  82 , the clinical acceptability evaluation module, also referred to as clinical module, evaluates the clinical acceptability of newly received reference data and/or time corresponding sensor data. In some embodiments of evaluating clinical acceptability, the rate of change of the reference data as compared to previous data is assessed for clinical acceptability. That is, the rate of change and acceleration (or deceleration) of many analytes has certain physiological limits within the body. Accordingly, a limit may be set to determine if the new matched pair is within a physiologically feasible range, indicated by a rate of change from the previous data that is within known physiological and/or statistical limits. Similarly, in some embodiments any algorithm that predicts a future value of an analyte may be used to predict and then compare an actual value to a time corresponding predicted value to determine if the actual value falls within a clinically acceptable range based on the predictive algorithm, for example. 
     In one exemplary embodiment, the clinical acceptability evaluation module  82  matches the reference data with a substantially time corresponding converted sensor value such as described with reference to  FIG. 6  above, and plots the matched data on a Clarke Error Grid such as described in more detail with reference to  FIG. 9 . 
       FIG. 9  is a graph of two data pairs on a Clarke Error Grid to illustrate the evaluation of clinical acceptability in one exemplary embodiment. The Clarke Error Grid may be used by the clinical acceptability evaluation module to evaluate the clinical acceptability of the disparity between a reference glucose value and a sensor glucose (e.g., estimated glucose) value, if any, in an embodiment wherein the sensor is a glucose sensor. The x-axis represents glucose reference glucose data and the y-axis represents estimated glucose sensor data. Matched data pairs are plotted accordingly to their reference and sensor values, respectively. In this embodiment, matched pairs that fall within the A and B regions of the Clarke Error Grid are considered clinically acceptable, while matched pairs that fall within the C, D, and E regions of the Clarke Error Grid are not considered clinically acceptable. Particularly,  FIG. 9  shows a first matched pair  92  is shown which falls within the A region of the Clarke Error Grid, therefore is it considered clinically acceptable. A second matched pair  94  is shown which falls within the C region of the Clarke Error Grid, therefore it is not considered clinically acceptable. 
     It may be noted that a variety of other known methods of evaluation of clinical acceptability may be utilized. In one alternative embodiment, the Consensus Grid is used to evaluate the clinical acceptability of reference and sensor data. In another alternative embodiment, a mean absolute difference calculation may be used to evaluate the clinical acceptability of the reference data. In another alternative embodiment, the clinical acceptability may be evaluated using any relevant clinical acceptability test, such as a known grid (e.g., Clarke Error or Consensus), and including additional parameters such as time of day and/or the increase or decreasing trend of the analyte concentration. In another alternative embodiment, a rate of change calculation may be used to evaluate clinical acceptability. In yet another alternative embodiment, wherein the received reference data is in substantially real time, the conversion function could be used to predict an estimated glucose value at a time corresponding to the time stamp of the reference analyte value (this may be required due to a time lag of the sensor data such as described elsewhere herein). Accordingly, a threshold may be set for the predicted estimated glucose value and the reference analyte value disparity, if any. 
     Referring again to  FIG. 8 , the results of the clinical acceptability evaluation are assessed. If clinical acceptability is determined with the received reference data, then processing continues to block  84  to optionally recalculate the conversion function using the received reference data in the calibration set. If, however, clinical acceptability is not determined, then the processing progresses to block  86  to control the user interface, such as will be described with reference to block  86  below. 
     At block  84 , the conversion function module optionally recreates the conversion function using the received reference data. In one embodiment, the conversion function module adds the newly received reference data (e.g., including the matched sensor data) into the calibration set, displaces the oldest, and/or least concordant matched data pair from the calibration set, and recalculates the conversion function accordingly. In another embodiment, the conversion function module evaluates the calibration set for best calibration based on inclusion criteria, such as described in more detail with reference to  FIG. 10 . 
     At  85 , the sensor data transformation module uses the conversion function to continually (or intermittently) convert sensor data into estimated analyte values, also referred to as calibrated data, such as described in more detail with reference to  FIG. 6 , block  68 . 
     At block  86 , the interface control module, also referred to as the fail-safe module, controls the user interface based upon the clinical acceptability of the reference data received. If the evaluation (block  82 ) deems clinical acceptability, then the user interface may function as normal; that is, providing output for the user such as described in more detail with reference to  FIG. 6 , block  69 . 
     If however the reference data is not considered clinically acceptable, then the fail-safe module begins the initial stages of fail-safe mode. In some embodiments, the initial stages of fail-safe mode include altering the user interface so that estimated sensor data is not displayed to the user. In some embodiments, the initial stages of fail-safe mode include prompting the user to repeat the reference analyte test and provide another reference analyte value. The repeated analyte value is then evaluated for clinical acceptability such as described with reference to blocks  81  to  83 , above. 
     If the results of the repeated analyte test are determined to be clinically unacceptable, then fail-safe module may alter the user interface to reflect full fail-safe mode. In one embodiment, full fail-safe mode includes discontinuing sensor analyte display output on the user interface. In other embodiments, color-coded information, trend information, directional information (e.g., arrows or angled lines), gauges, and/or fail-safe information may be displayed, for example. 
     If the results of the repeated analyte test are determined to be clinically acceptable, then the first analyte value is discarded, and the repeated analyte value is accepted. The process returns to block  84  to optionally recalculate the conversion function, such as described in more detail with reference to block  84 , above. 
     Reference is now made to  FIG. 10 , which is a flow chart that illustrates the process of evaluation of calibration data for best calibration based on inclusion criteria of matched data pairs in one embodiment. 
     It may be noted that calibration of analyte sensors may be variable over time; that is, the conversion function suitable for one point in time may not be suitable for another point in time (e.g., hours, days, weeks, or months later). For example, in an embodiment wherein the analyte sensor is subcutaneously implantable, the maturation of tissue ingrowth over time may cause variability in the calibration of the analyte sensor. As another example, physiological changes in the user (e.g., metabolism, interfering blood constituents, lifestyle changes) may cause variability in the calibration of the sensor. Accordingly, a continuously updating calibration algorithm is disclosed that includes reforming the calibration set, and thus recalculating the conversion function, over time according to a set of inclusion criteria. 
     At block  101 , the reference data receiving module, also referred to as the reference input module, receives a new reference analyte value (e.g., data point) from the reference analyte monitor. In some embodiments, the reference analyte value may be pre-screened according to criteria such as described in more detail with reference to  FIG. 6 , block  62 . In some embodiments, the reference analyte value may be evaluated for clinical acceptability such as described in more detail with reference to  FIG. 8 . 
     At block  102 , the data matching module, also referred to as the processor module, forms one or more updated matched data pairs by matching new reference data to substantially time corresponding sensor data, such as described in more detail with reference to  FIG. 6 , block  63 . 
     At block  103 , a calibration evaluation module evaluates the new matched pair(s) inclusion into the calibration set. In some embodiments, the receiver simply adds the updated matched data pair into the calibration set, displaces the oldest and/or least concordant matched pair from the calibration set, and proceeds to recalculate the conversion function accordingly (block  105 ). 
     In some embodiments, the calibration evaluation includes evaluating only the new matched data pair. In some embodiments, the calibration evaluation includes evaluating all of the matched data pairs in the existing calibration set and including the new matched data pair; in such embodiments not only is the new matched data pair evaluated for inclusion (or exclusion), but additionally each of the data pairs in the calibration set are individually evaluated for inclusion (or exclusion). In some alternative embodiments, the calibration evaluation includes evaluating all possible combinations of matched data pairs from the existing calibration set and including the new matched data pair to determine which combination best meets the inclusion criteria. In some additional alternative embodiments, the calibration evaluation includes a combination of at least two of the above-described embodiments. 
     Inclusion criteria comprise one or more criteria that define a set of matched data pairs that form a substantially optimal calibration set. One inclusion criterion comprises ensuring the time stamp of the matched data pairs (that make up the calibration set) span at least a set time period (e.g., three hours). Another inclusion criterion comprises ensuring that the time stamps of the matched data pairs are not more than a set age (e.g., one week old). Another inclusion criterion ensures that the matched pairs of the calibration set have a substantially distributed amount of high and low raw sensor data, estimated sensor analyte values, and/or reference analyte values. Another criterion comprises ensuring all raw sensor data, estimated sensor analyte values, and/or reference analyte values are within a predetermined range (e.g., 40 to 400 mg/dL for glucose values). Another criterion comprises evaluating the rate of change of the analyte concentration (e.g., from sensor data) during the time stamp of the matched pair(s). For example, sensor and reference data obtained during the time when the analyte concentration is undergoing a slow rate of change may be less susceptible inaccuracies caused by time lag and other physiological and non-physiological effects. Another criterion comprises evaluating the congruence of respective sensor and reference data in each matched data pair; the matched pairs with the most congruence may be chosen. Another criterion comprises evaluating physiological changes (e.g., low oxygen due to a user&#39;s posture that may effect the function of a subcutaneously implantable analyte sensor, or other effects such as described with reference to  FIG. 6 ) to ascertain a likelihood of error in the sensor value. It may be noted that evaluation of calibration set criteria may comprise evaluating one, some, or all of the above described inclusion criteria. It is contemplated that additional embodiments may comprise additional inclusion criteria not explicitly described herein. 
     At block  104 , the evaluation of the calibration set determines whether to maintain the previously established calibration set, or if the calibration set should be updated (e.g., modified) with the new matched data pair. In some embodiments, the oldest matched data pair is simply displaced when a new matched data pair is included. It may be noted however that a new calibration set may include not only the determination to include the new matched data pair, but in some embodiments, may also determine which of the previously matched data pairs should be displaced from the calibration set. 
     At block  105 , the conversion function module recreates the conversion function using the modified calibration set. The calculation of the conversion function is described in more detail with reference to  FIG. 6 . 
     At block  106 , the sensor data transformation module converts sensor data to calibrated data using the updated conversion function. Conversion of raw sensor data into estimated analyte values is described in more detail with reference to  FIG. 6 . 
     Reference is now made to  FIG. 11 , which is a flow chart that illustrates the process of evaluating the quality of the calibration in one embodiment. The calibration quality may be evaluated by determining the statistical association of data that forms the calibration set, which determines the confidence associated with the conversion function used in calibration and conversion of raw sensor data into estimated analyte values. 
     In one embodiment calibration quality may be evaluated after initial or updated calculation of the conversion function such as described elsewhere herein. However it may be noted that calibration quality may be performed at any time during the data processing. 
     At block  111 , a sensor data receiving module, also referred to as the sensor data module, receives the sensor data from the sensor such as described in more detail with reference to  FIG. 6 . 
     At block  112 , a reference data receiving module, also referred to as the reference input module, receives reference data from a reference analyte monitor, such as described in more detail with reference to  FIG. 6 . 
     At block  113 , the data matching module, also referred to as the processor module, matches received reference data with substantially time corresponding sensor data to provide one or more matched data pairs, such as described in more detail with reference to  FIG. 6 . 
     At block  114 , the calibration set module, also referred to as the processor module, forms a calibration set from one or more matched data pairs such as described in more detail with reference to  FIGS. 6 ,  8 , and  10 . 
     At block  115 , the conversion function module calculates a conversion function using the calibration set, such as described in more detail with reference to  FIGS. 6 ,  8 , and  10 . 
     At block  116 , the sensor data transformation module continuously (or intermittently) converts received sensor data into estimated analyte values, also referred to as calibrated data, such as described in more detail with reference to  FIGS. 6 ,  8 , and  10 . 
     At block  117 , a quality evaluation module evaluates the quality of the calibration. In one embodiment, the quality of the calibration is based on the association of the calibration set data using statistical analysis. Statistical analysis may comprise any known cost function such as linear regression, non-linear mapping/regression, rank (e.g., non-parametric) correlation, least mean square fit, mean absolute deviation (MAD), mean absolute relative difference, and the like. The result of the statistical analysis provides a measure of the association of data used in calibrating the system. A threshold of data association may be set to determine if sufficient quality is exhibited in a calibration set. 
     In another embodiment, the quality of the calibration is determined by evaluating the calibration set for clinical acceptability, such as described with reference to blocks  82  and  83  (e.g., Clarke Error Grid, Consensus Grid, or clinical acceptability test). As an example, the matched data pairs that form the calibration set may be plotted on a Clarke Error Grid, such that when all matched data pairs fall within the A and B regions of the Clarke Error Grid, then the calibration is determined to be clinically acceptable. 
     In yet another alternative embodiment, the quality of the calibration is determined based initially on the association of the calibration set data using statistical analysis, and then by evaluating the calibration set for clinical acceptability. If the calibration set fails the statistical and/or the clinical test, the processing returns to block  115  to recalculate the conversion function with a new (e.g., optimized) set of matched data pairs. In this embodiment, the processing loop (block  115  to block  117 ) iterates until the quality evaluation module 1) determines clinical acceptability, 2) determines sufficient statistical data association, 3) determines both clinical acceptability and sufficient statistical data association, or 4) surpasses a threshold of iterations; after which the processing continues to block  118 . 
       FIGS. 12A and 12B  illustrate one exemplary embodiment wherein the accuracy of the conversion function is determined by evaluating the correlation coefficient from linear regression of the calibration set that formed the conversion function. In this exemplary embodiment, a threshold (e.g., 0.79) is set for the R-value obtained from the correlation coefficient. 
       FIGS. 12A and 12B  are graphs that illustrate an evaluation of the quality of calibration based on data association in one exemplary embodiment using a correlation coefficient. Particularly,  FIGS. 12A and 12B  pictorially illustrate the results of the linear least squares regression performed on a first and a second calibration set ( FIGS. 12A and 12B , respectively). The x-axis represents reference analyte data; the y-axis represents sensor data. The graph pictorially illustrates regression that determines the conversion function. 
     It may be noted that the regression line (and thus the conversion function) formed by the regression of the first calibration set of  FIG. 12A  is the same as the regression line (and thus the conversion function) formed by the regression of the second calibration set of  FIG. 12B . However, the correlation of the data in the calibration set to the regression line in  FIG. 12A  is significantly different than the correlation of the data in the calibration set to the regression line in  FIG. 12A . In other words, there is a noticeably greater deviation of the data from the regression line in  FIG. 12B  than the deviation of the data from the regression line in  FIG. 12A . 
     In order to quantify this difference in correlation, an R-value may be used to summarize the residuals (e.g., root mean square deviations) of the data when fitted to a straight line via least squares method, in this exemplary embodiment. R-value may be calculated according to the following equation: 
     
       
         
           
             R 
             = 
             
               
                 
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     In the above equation: i is an index (1 to n), x is a reference analyte value, y is a sensor analyte value,  x  is an average of 1/n reference analyte values, and  y  is an average of 1/n sensor analyte values. 
     In the exemplary calibration set shown in  FIG. 12A , the calculated R-value is about 0.99, which may also be expressed as the correlation coefficient of regression. Accordingly, the calibration exhibits sufficient data association (and thus insufficient quality) because it falls above the 0.79 threshold set in this exemplary embodiment. 
     In the exemplary calibration set shown in  FIG. 12B , the calculated R-value is about 0.77, which may also be expressed as the correlation coefficient of regression. Accordingly, the calibration exhibits insufficient data association (and thus insufficient quality) because it falls below the 0.79 threshold set in this exemplary embodiment. 
     Reference is again made to  FIG. 11 , at block  118 , the interface control module, also referred to as the fail-safe module, controls the user interface based upon the quality of the calibration. If the calibration is exhibits sufficient quality, then the user interface may function as normal; that is providing output for the user such as described in more detail with reference to  FIG. 6 . 
     If however the calibration is not deemed sufficient in quality, then fail-safe module  118  begins the initial stages of fail-safe mode, which are described in more detail with reference to  FIG. 8 . In some embodiments, the initial stages of fail-safe mode include altering the user interface so that estimated sensor data is not displayed to the user. In some embodiments, the initial stages of fail-safe mode also include prompting the user to provide an updated reference analyte value. The updated analyte value is then processed as described above and the updated conversion function that results from the repeated reference analyte test, if any, is evaluated for statistical accuracy. 
     If the results of the updated evaluation again exhibit insufficient quality, then the fail-safe module alters user interface to reflect full fail-safe mode, which is described in more detail with reference to  FIG. 8 . If however the results of the updated evaluation exhibit sufficient quality, then the first reference analyte value is discarded, and the repeated reference analyte value is accepted and the process continues as described herein. 
     It may be noted that the initial stages of fail-safe mode and full fail safe mode may be similar to that described with reference to  FIG. 8 , including user interface control for example. Additionally, it is contemplated herein that a variety of difference modes between initial and full fail-safe mode may be provided depending on the relative quality of the calibration. In other words, the confidence level of the calibration quality may control a plurality of different user interface screens providing error bars, ±values, and the like. Similar screens may be implements in the clinical acceptability embodiments described with reference to  FIG. 8 . 
     The above description discloses several methods and materials of the disclosed invention. This invention is susceptible to modifications in the methods and materials, as well as alterations in the fabrication methods and equipment. Such modifications will become apparent to those skilled in the art from a consideration of this disclosure or practice of the invention disclosed herein. Consequently, it is not intended that this invention be limited to the specific embodiments disclosed herein, but that it cover all modifications and alternatives coming within the true scope and spirit of the invention as embodied in the attached claims. All patents, applications, and other references cited herein are hereby incorporated by reference in their entirety.