Improved method of detecting pre-spotting when dispensing sample

Abstract: It is known to detect pre-spots on slide elements using infrared radiation which is reflected therefrom by utilizing an expensive lamp and PbS photocell combination. This invention is based on the surprising discovery that the inexpensive versions of that system, namely, the infrared light-emitting diode (20) and the photosensitive transistor (24) will work effectively only if the light is detected in transmission rather than reflection. Most preferably, two parts of the system are disposed so that the light is directedin a direction which is counter to the flow of liquid through the element.

Description: The invention relates to a method of detecting the undesirable pre-spotting of slide test elements prior to sample dispensing, using improved apparatus. In analyzers of body liquids such as those available from the Eastman Kodak Company under the trademark "Ektachem 700" or "Ektachem 400", it has been conventional to use as part of the analyzer, apparatus for detecting liquid dispensing behavior on to a dried test element. The apparatus comprises an infrared emitting light source, a detector of infrared light, and a support to position a test element relative to the light source and detector. One such arrangement is described, by way of example, in US-A-4 420 566. In US-A-4 420 556, the light source is an incandescent lamp and the detector a PbS cell. The lamp is positioned to project radiation on to the surface of a test element and the detector is positioned to detect radiation received from the surface of a test element. The amount of radiation received from the surface of the test element changes if water or water-based serum is present, and the difference between wet and dry readings is compared with a stored value to determine if sufficient fluid has been metered on to the surface of the test element for analysis. Thus, the apparatus serves as a drop detector, and can be used to detect pre-spotting, an undesirable event which occasionally occurs. Pre-spotting occurs when fluid is dispensed on to the surface of a test element prior to the actual metering of the sample to be analysed. This may occur due to fluid being retained on the exterior of the dispensing pipette and dropping on to the test element prior to dispensing of a predetermined amount of the sample. This has the result that the test element is wetted prior to dispensing of the sample and may result in erroneous readings due to the presence of more fluid than is required for the analysis. One way of preventing pre-spotting is to modify the exterior surface of the dispensing pipette to prevent fluid being retained thereon, as disclosed for example in WO-A-9 014 162. Although the apparatus described in US-A-4 420 556 has been very useful, the lamp has proven to have a relatively short life, and the components are relatively expensive. Known substitutes for the lamp and PbS cell are, respectively, an infrared (IR) light-emitting diode (LED) and a photo-sensitive transistor or photodiode. Further, these are less expensive with a longer life. However, when these are substituted into the analyzer described above for reflection detection, they prove to be too insensitive to be useful. More precisely, they are not capable of detecting from noise, the existence of a pre-spot, a needed function of such a drop detector. This is considered to be due to the wavelengths of light emitted by the LED not being strongly absorbed by water. Thus, it has been a problem prior to this invention to identify a workable replacement for the drop detector already being used. It has been accidentally discovered that the sensitivity of such an IR light-emitting diode and a photo-sensitive transistor or photodiode are greatly increased, for water detection, if the two components are used to detect IR transmission, instead of reflection. More specifically, in accordance with one aspect of the present invention, there is provided a method for detecting undesirable pre-spots at a dispensing station in an analyzer using an IR light-emitting diode and a photo-sensitive transistor or photodiode, the method comprising the steps of:- a) positioning a test element which transmits infrared light and on to which liquid is to be dispensed in a predetermined amount at a prescribed time in between the light-emitting diode and transistor or photodiode; b) detecting only IR light which has been transmitted by the test element up to the prescribed time at which dispensing occurs; and c) comparing the amount of light transmitted by the test element during step b) in relation to a baseline level of light transmitted to determine whether a pre-spot of liquid has been inadvertently dispensed prior to the dispensing of the predetermined, desired amount at the prescribed time. Accordingly, it is an advantageous feature of this invention that the less expensive and more reliable IR light-emitting diode and photo-transistor or photodiode can be used for pre-spot detection, even though their sensitivity is unsatisfactory when used as taught by the prior art. For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:- Figure 1 is a fragmentary perspective view of apparatus constructed in accordance with the present invention shown at the dispensing station of an analyzer; Figure 2 is a schematic diagram of an electrical circuit used to process the output from the detector arrangement shown in Figure 1; Figures 3 and 4 are plots of the amount of signal, or the rate of change of that signal, produced by the receipt by a transistor of IR light transmitted through a test element, either with or without a pre-spot; and Figure 5 is a flow chart illustrating the method of the present invention. The invention is described as it is practiced with the preferred embodiments, featuring a particular preferred clinical analyzer and slide test elements. In addition, it is useful regardless of the construction of either the analyzer or the test elements, so long as a) the analyzer uses an infrared (IR) light-emitting diode and a photo-sensitive transistor or photodiode, and b) the test element will transmit IR light when wetted. As shown in Figure 1, apparatus according to the invention allows the wetting of a slide test element E to be detected at a dispensing station 10. Such a test element E is preferably either the dried, slide elements available from Eastman Kodak Company under the trademark "Ektachem" slides, or the dried, slide elements available from Fuji Photo Film Co. under the tradename "DryChem" slides. Further description of such elements is unnecessary, other than to point out that the element has an aperture 12 which allows a drop of liquid sample to be dispensed on to the element, and is transparent enough through the center portion to transmit IR light, particularly when wet. At station 10, a support surface 16 is provided to hold a test element E in place during liquid dispensing. Any suitable restraining means (not shown) can ensure that element E is restrained in all three orthogonal directions, above an aperture 18 in surface 16. Positioned below aperture 18 is any IR light-emitting diode 20, for example a GaAs or GaAlAs diode whose emission is centered at 935 and 890nm respectively, available from Optek Technology, Inc. Adjacent to the opposite side 22 of element E, namely the side with opening 12, are the light-detector 24 and the sample dispenser 26. Detector 24 is any photo-sensitive transistor or photodiode, for example, an NPN silicon transistor available from Optek Technology, Inc., or a PIN or PN junction photodiode available, for example, from Optek Technology, Inc. under the tradenames OP 913SL or OP 900SL respectively. Preferably, the center line of the diode light emission distribution is aligned with the center line of the detector distribution, and these center lines are inclined at an angle α to the vertical axis and hence to the axis of dispenser 26. Angle α preferably is between 40° and 60° so as to position these optical components clear of the sample dispenser yet close to element E. Regarding dispenser 26, any conventional dispenser can be used, for example, that available in the analyzers available from Eastman Kodak Co. under the trademark "Ektachem 700". Such a dispenser 26 includes a movable probe, not shown, on which is mounted a disposable tip 26. Pressure means (not shown) are effective to dispense an aliquot of sample in the direction of arrow 30, for example 10µl, on to element E at aperture 12. Diode 20 and transistor 24 generate an electrical signal which can be used to sense the wetting of element E, using circuitry 50 as shown in Figure 2. This circuitry 50 comprises a high-pass filter 52 to eliminate any DC component of the received square wave signal, a rectifying circuit 54 and a low-pass filter 56 which converts the square wave signal to a steady positive signal proportional to the amount of light detected by the photo-sensitive transistor or photodiode. An amplifier 58 is provided with an adjustable gain 60 to amplify the signal, a buffer 62 to convert the negativized signal out of amplifier 58 to a positive signal, and an A/D converter 64. All of these components are conventional, so that no details need be discussed. The resulting A/D counts from the converter 64 are supplied to computing means 66, which in turn controls the analyzer. In accordance with one aspect of the invention, the aforedescribed apparatus is used to detect whether or not a preliminary spot of liquid, herein called a "pre-spot", of liquid has fallen or otherwise been dispensed on to element E at aperture 12. Such pre-spots are objectionable since they alter the chemical reaction which occurs when the desired aliquot is dispensed. Broadly, the method features the steps of positioning element E as shown, and reading the amount of IR light transmitted through the element up until the time for the prescribed dispensing of the desired aliquot of liquid. No fixed time for the "dispense time" is necessary, although the examples which follow show a time of 1s following the onset of reading the IR transmission. It will be appreciated that enough read time is selected before the "dispense time" to ensure that all pre-spots, if any, will be detected. If during this read time, the amount of transmission deviates from a baseline value by more than a preselected threshold amount, then a pre-spot of liquid is present and the test is repeated. A preferred threshold amount to be exceeded before a "pre-spot" is announced is +8% of the baseline value. Only IR light which is transmitted, is detected, since the apparatus is not suited for reflected IR light. Figure 3 illustrates output curves obtained using this method in which the simple change in the amount of the signal is enough to identify a pre-spot, using a calcium "Ektachem" slide. (In this test, as well as those of Figures 4 and 5, angle α was about 40°, LED 20 was 12.3mm below the plane of the undersurface of element E, and a photosensitive transistor 24 was 6.4mm above surface 22 of element E. Additionally, the LED was driven with a 2/3 duty cycle square wave at a frequency of about 833Hz, the on-state collector current of the transistor was 35mA and the collector-emitter voltage was 14.5V. In each illustration of pre-spots, the pre-spot was deliberately created by pre-dispensing a 3µl pendant drop from the tip before the tip was seated above the element E. Then, when the tip was seated, the normal shock which occurred was sufficient to dislodge the 3µl as a pre-spot. Visual confirmation was not needed.) The analyzer was programmed to initiate the dispensing of 10µl at about T = 1.0s. Prior to this, at time T = 0, a reading was taken, indicated by curve 100, of about 100 A/D counts, and this was assigned as the "baseline" value. Because there was no plus deviation greater than the preselected threshold value of 8 counts before T = 1.0, the curve was interpreted as being normal, without any pre-spotting. However, when another identical test element (calcium) was subsequently positioned and curve 102 commenced, the results were different. (At time T = 0, a "baseline" of 110 counts was ascertained.) Shortly before T = 1.0s, the signal count rose to over 150, which was a difference of 40 which exceeded the threshold value of 8.8. Such increase is caused by the slide element being wetted with a pre-spot. Hence, this slide test element was labeled as having received a pre-spot, and was discarded. Other chemistries with which the raw A/D count measurement is adequate, include NH₃ and theophylline. The process was repeated and the test results are shown in Figure 4, except that the test elements were obtained from Eastman Kodak Co. under the trademark "Ektachem Total Protein", instead of calcium. Curve 200 was normal dispensing, whereas curve 202 was the curve which occurred with a pre-spot being dispensed at T = 0.7s approximately. In both cases, normal dispensing occurred at T = 1s. (The dip in curve 200 at T = 1.1s is believed to be due to the shadow created by the liquid beading up on the surface of element E before wetting occurs, due to the hydrophobic nature of the total protein chemistry.) For curve 200, the baseline value was about 143, creating a threshold of +11.44. For curve 202, the baseline value was 141, creating a threshold value of +11.28. This latter was exceeded before T = 1.0s, as the A/D count produced a deviation in excess of +15 at T = 0.95s. (The unusual decrease in the first derivative at T = 0.5s approximately, for curve 200, which also occurred for curve 202, is attributable to the shadow of the dispensing tip. This shadow is preferably ignored by commanding the apparatus to only detect positive, and not negative, deviations from the baseline value.) Similar results have been obtained when testing other "Ektachem" slide elements, for example, albumin, cholesterol and the like. The time for detecting whether the threshold has been exceeded can be the entire time between the time when the baseline is set and the time when normal dispensing occurs (T = 1.0s for the above examples.) However, since the A/D value does not decrease much once wetting occurs, the preferred reading time to ascertain whether the plus threshold has been exceeded is a one-time reading just prior to the normal dispense time, for example, 50 to 100ms before. It will be readily apparent that computing means 66 shown in Figure 2 is preferred to carry out the process, particularly regarding the processing of the signal to measure its changes against a standard level and a threshold level. Conventionally, one or more of the microprocessors of the analyzer are used for this purpose. Any convenient program can be used, following the logic of the flow chart shown in Figure 5. The first step, step 400, is to load a gain value of 1 into the auto gain register of gain 60 (Figure 2). This in turn provides a gain of 1 to the amplifier 58. The next several steps are used to ensure that the baseline signal produced by the A/D counts is not near the limits of detection, namely zero or 255. Specifically, step 402, an element E is positioned to receive patient liquid, and a baseline value of the IR transmitted is obtained, step 404, by taking and averaging 4 A/D readings. Such readings are preferably about 33ms apart, and can occur at T = 0 in the context of the above examples. Next, the A/D average reading is queried, step 406, to determine if it is less than 50. If yes, step 408, the gain value of the auto gain register is doubled, and steps 404 and 406 are repeated. If no, then the average count is queried as to whether it exceeds 100, step 410. If yes, the gain is divided by 1.5, step 412, and that is used in the auto gain register (and amplifier 58), and steps 404 and 406 are again repeated. If no, the new average is retained in a register "X", step 414. Then, just prior to time T = normal dispense time, for example, 50 to 100ms prior, four A/D readings are again taken and averaged, step 416, and the average is stored in register "Y". The difference between the values of register "Y" and "X" is calculated as a percent, step 418, and this percent is queried, step 420. If the percent change has exceeded +8% of the baseline value (the value in register "X"), then a pre-spot is "announced", step 422, the element E is discarded, and the procedure starts over at step 400. If the percent change is less than +8%, step 424, then there has been no pre-spot, and the analysis continues to completion, step 426. Other threshold values besides 8% can be used. The 8% has been found to be particularly useful, however, since it is just less than the smallest increase that is produced by a pre-spot on total protein elements E, the slide elements that produce the smallest increase when pre-spotted. On the other hand, 8% is larger than deviations that are "noise" in the signal. It is not necessary, although it is preferred, that the diode be opposite the side of the test element to which the sample dispenser is adjacent. That is, the positions of the diode and transistor shown in Figure 1 can be reversed, and the method will still work. However, when the light beam transmission is so reversed, so as to be concurrent with the direction of flow of sample into element E, then the proportion of emitted light which is detectable is decreased, producing a lower signal-to-noise ratio. Hence, the preferred of the two choices is that shown in Figure 1. By comparison, the use of the diode and the transistor or photodiode in measuring IR light reflected from such test elements, failed to reliably detect a pre-spot due to the change or rate of change of the signal.

Claims: A method for detecting undesirable pre-spots at a dispensing station in an analyzer using an IR light-emitting diode (20) and a photo-sensitive transistor or photodiode (24), the method comprising the steps of:- a) positioning a test element (E) which transmits infrared light and on to which liquid is to be dispensed in a predetermined amount at a prescribed time in between the light-emitting diode (20) and transistor or photodiode (24); b) detecting only IR light which has been transmitted by the test element (E) up to the prescribed time at which dispensing occurs; and c) comparing the amount of light transmitted by the test element (E) during step b) in relation to a baseline level of light transmitted to determine whether a pre-spot of liquid has been inadvertently dispensed prior to the dispensing of the predetermined, desired amount at the prescribed time. A method according to claim 1, wherein the comparing step c) comprises comparing the change in the amount of IR-light transmitted against the baseline level to determine if a preselected threshold has been exceeded. A method according to claim 2, wherein the threshold is +8% of the baseline level. A method according to any one of the preceding claims, wherein the IR light is transmitted by the test element (E) in a direction opposite to the direction of flow of liquid in the positioned element (E) during dispensing.

Inventors: BUTTURINI RANDAL STEVEN C O EA, BUTTURINI, RANDAL STEVEN, C/O EASTMAN KODAK CO.
Applicants: JOHNSON & JOHNSON CLIN DIAG, JOHNSON & JOHNSON CLINICAL DIAGNOSTICS, INC.
Application year: 1991
Main classification: G01N35
IPC codes: G01N1, G01N21, G01V9, G01V8, G01N35