Abstract:
Solutions, dialysates, and methods for measuring solutes in blood and/or for treating blood. In one aspect of the invention, a method of performing dialysis includes placing a solution in communication with blood of a subject, where a concentration of at least one electrically conductive solute in the solution, prior to being placed in communication with the blood of the subject, is substantially equal to a concentration of the at least one electrically conductive solute in the blood of the subject.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/940,531, filed on May 29, 2007, which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to solutions, dialysates, and related methods. 
     BACKGROUND 
     During hemodialysis, impurities and toxins are removed from the blood of a patient by drawing the blood out of the patient through a blood access site, typically via a catheter, and then passing the blood through an artificial kidney (often referred to as a “dialyzer”). The artificial kidney includes a semi-permeable membrane that separates a first conduit from a second conduit. Generally, a dialysis solution (often referred to as a “dialysate”) is flowed through the first conduit of the dialyzer while the patient&#39;s blood is flowed through the second conduit of the dialyzer, causing impurities and toxins to be transferred from the blood to the dialysate through the semi-permeable membrane. The impurities and toxins can, for example, be removed from the blood by a diffusion process. After passing through the dialyzer, the purified blood is then returned to the patient. 
     SUMMARY 
     In one aspect of the invention, a method of performing dialysis includes placing a solution in communication with blood of a subject, where a concentration of at least one electrically conductive solute in the solution, prior to being placed in communication with the blood of the subject, is substantially equal to a concentration of the at least one electrically conductive solute in the blood of the subject. 
     In another aspect of the invention, a method of performing dialysis includes measuring an electrical conductivity of a solution in communication with the blood of a subject. A concentration of at least one electrically conductive solute in the solution is substantially equal to a concentration of the at least one electrically conductive solute in the blood of the subject. The method further includes determining a sodium concentration of the blood of the subject as a function of the measured electrical conductivity of the solution and placing a dialysate in communication with the blood of the subject. The dialysate, prior to being placed in communication with the blood of the subject, has a sodium concentration that is substantially equal to the determined sodium concentration of the blood of the subject. 
     In an additional aspect of the invention, a method includes preparing a solution for determining a sodium concentration in blood of a subject. The solution has a level of one or more electrically conductive solutes, other than sodium, that is substantially equal to a determined average level of the one or more electrically conductive solutes in blood of a population of subjects. 
     In another aspect of the invention, a solution for determining a sodium concentration in blood of a subject includes a concentration of multiple electrically conductive solutes that is substantially equal to a predetermined concentration of the multiple electrically conductive solutes in the blood. 
     In an additional aspect of the invention, a method of preparing a dialysate includes measuring an electrical conductivity of a dialysate concentrate and combining an amount of the dialysate concentrate with an amount of a liquid. The amounts of the dialysate concentrate and the liquid are determined as a function of the measured electrical conductivity of the dialysate concentrate. 
     In another aspect of the invention, a method of performing dialysis includes placing a test solution in communication with a dialysis patient&#39;s blood across a semi-permeable membrane for sufficient time to cause sodium ions from the patient&#39;s blood to migrate across the membrane into the test solution. The test solution, prior to being placed in communication with the patient&#39;s blood, includes concentrations of conductive solutes approximately matching expected concentrations of corresponding conductive solutes in the patient&#39;s blood and has a sodium concentration that is lower than an expected sodium concentration of the patient&#39;s blood. The method further includes measuring conductivity of the test solution both before and after the test solution is placed in communication with the patient&#39;s blood and estimating a current concentration of sodium in the patient&#39;s blood as a function of a differential between the conductivity of the test solution before being placed in communication with the patient&#39;s blood and the conductivity of the test solution after being placed in communication with the patient&#39;s blood. The method also includes providing a dialysate having a sodium concentration substantially equal to the estimated current sodium concentration in the patient&#39;s blood and placing the dialysate in communication with the patient&#39;s blood across the semi-permeable membrane to perform a dialysis treatment on the patient&#39;s blood without substantially altering the sodium concentration of the patient&#39;s blood during the performance of the dialysis treatment. 
     Implementations can include one or more of the following features. 
     In some implementations, placing the solution in communication with the blood of the subject includes passing the solution through a dialyzer. 
     In certain implementations, the at least one electrically conductive solute includes phosphate, sulfate, bicarbonate, potassium, calcium, and/or magnesium. 
     In some implementations, the method further includes, after placing the solution in communication with the blood of the subject, measuring a conductivity of the solution and determining a concentration of sodium in the blood of the subject as a function of the measured conductivity of the solution. 
     In certain implementations, the method further includes placing a dialysate in communication with blood of the subject. The dialysate has a concentration of sodium that is substantially equal to the determined concentration of sodium in the blood. 
     In some implementations, placing the dialysate in communication with the blood of the subject includes passing the dialysate through a dialyzer. 
     In certain implementations, the method further includes determining a dialysance of the dialyzer. 
     In some implementations, the sodium concentration of the dialysate differs from the determined sodium concentration of the blood by no more than a predetermined amount and/or a predetermined percentage (e.g., no more than about five percent and/or about 7.0 mEq/L, no more than about one percent and/or about 1.5 mEq/L) of the determined sodium concentration. 
     In certain implementations, the method further includes preparing the dialysate by combining a dialysate concentrate with water. 
     In some implementations, placing the dialysate in communication with the blood of the subject includes passing the dialysate through a dialyzer. 
     In certain implementations, the method includes passing the solution through the dialyzer prior to passing the dialysate through the dialyzer. 
     In some implementations, the solution is passed through the dialyzer for substantially less time than the dialysate is passed through the dialyzer. 
     In certain implementations, the solution is passed through the dialyzer for about five minutes or less. 
     In some implementations, a sodium concentration in the blood of the subject, after passing the dialysate through the dialyzer for a predetermined period of time, differs from the determined sodium concentration of the blood of the subject by no more than a predetermined amount and/or a predetermined percentage (e.g., no more than about five percent and/or about 7.0 mEq/L, no more than about one percent and/or about 1.5 mEq/L) of the determined sodium concentration of the blood of the subject. 
     In certain implementations, the method further includes determining a dialysance of the dialyzer as a function of the measured electrical conductivity of the solution. 
     In some implementations, the method further includes measuring first electrical conductivities of the solution at an inlet and an outlet of the dialyzer, and then increasing a concentration of the solution and measuring second electrical conductivities of the solution at the inlet and the outlet of the dialyzer. 
     In certain implementations, the method further includes performing blood tests on each of the subjects to determine the average level of the one or more electrically conductive solutes in the blood of the population of subjects. 
     In some implementations, the population of subjects includes a population of hemodialysis patients. 
     In certain implementations, the plurality of electrically conductive solutes includes phosphate, sulfate, bicarbonate, potassium, calcium, and/or magnesium. 
     In some implementations, the solution further includes sodium. 
     In certain implementations, the predetermined concentration is an average determined concentration in blood of a population of subjects. 
     In some implementations, the method further includes determining a sodium concentration of the dialysate concentrate as a function of the measured electrical conductivity of the dialysate concentrate. 
     In certain implementations, a concentration of sodium in the dialysate is substantially equal to a predetermined concentration of sodium in blood of a subject. 
     In some implementations, the method further includes measuring conductivities of the dialysate before and after the dialysate is placed in communication with the patient&#39;s blood and adjusting the sodium concentration of the dialysate during the dialysis treatment as a function of the before and after conductivities of the dialysate in order to substantially maintain the same sodium level in the patient&#39;s blood throughout the dialysis treatment. 
     In certain implementations, the method further includes preparing the test solution. 
     In some implementations, preparing the test solution includes determining average levels of conductive solutes in blood of a population of subjects and preparing the test solution to have levels of conductive solutes that substantially match the average levels of conductive solutes in the blood of the population of subjects. 
     In certain implementations, the method further includes preparing the dialysate. 
     In some implementations, preparing the dialysate includes determining an actual concentration of a dialysate concentrate, and combining the dialysate concentrate with an amount of water that is determined as a function of the actual concentration of the dialysate concentrate. 
     In certain implementations, the actual concentration of the dialysate concentrate is determined as a function of a measured conductivity of the dialysate concentrate. 
     Implementations can include one or more of the following advantages. 
     In some implementations, the concentration of the one or more electrically conductive solutes in the solution (e.g., the conductivity-testing solution) is selected to be substantially equal to the concentration of the one or more electrically conductive solutes in the subject&#39;s blood. This can help to improve the accuracy with which the sodium concentration of the blood is determined. The sodium concentration of the blood can be determined as a function of the conductivity of the solution, and the presence of differing concentrations of electrically conductive solutes in the solution and in the patient&#39;s blood can degrade the accuracy of this measurement technique. Maintaining an approximate equilibrium between the blood and the solution with respect to the one or more electrically conductive solutes can help to negate the effect of those electrically conductive solutes on the conductivity measurement, which can help to ensure that the conductivity measurement is more closely correlated with the actual concentration of sodium in the blood. 
     In certain implementations, the sodium concentration of the dialysate is selected to be substantially equal to the sodium concentration of the subject&#39;s blood. This can help to ensure that the sodium concentration of the blood remains substantially constant throughout the dialysis (e.g., the hemodialysis). Maintaining the sodium level of the blood at a substantially constant concentration throughout the treatment can help to reduce or prevent discomfort experienced by the subject as a result of the treatment. 
     In some implementations, the actual concentration (e.g., the actual sodium concentration) of the dialysate concentrate is determined and compared to the labeled concentration of the dialysate concentrate, and then the dialysate is prepared (e.g., by mixing the dialysate concentrate with water) based on the actual concentration of the dialysate concentrate, rather than the labeled concentration of the dialysate concentrate. Determining the actual concentration of the dialysate concentrate can, for example, help the preparer of the dialysate to more accurately determine desired amounts of the concentrate and another liquid (e.g., water) to combine to form the dialysate. The use of dialysates prepared in such a manner can help to maintain the sodium concentration in the subject&#39;s blood at a substantially constant level during treatment, which, as discussed above, can help to reduce or prevent discomfort experienced by the subject as a result of the treatment. 
     Other aspects, features, and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a hemodialysis apparatus. 
         FIG. 2  is a more detailed diagram of a dialysate supply device of the dialysis apparatus of  FIG. 1 . 
         FIG. 3  is a schematic diagram illustrating a method of determining a concentration of sodium in blood of a subject using the apparatus of  FIG. 1  in measurement mode. 
         FIG. 4  is a schematic diagram illustrating a method of treating blood of a subject using the apparatus of  FIG. 1  in treatment mode. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure relates to solutions (e.g., conductivity-testing solutions) having a concentration of one or more electrically conductive solutes (e.g., phosphate, sulfate, bicarbonate, potassium, calcium, and/or magnesium) that is substantially equal to a concentration of those one or more electrically conductive solutes in the blood of a subject (e.g., a dialysis patient). Methods include passing the solution through a dialysate/solution conduit of a dialyzer while passing blood through a blood conduit of the dialyzer and measuring the conductivity of the solution (e.g., measuring a conductivity differential of the solution across the dialyzer). The concentration of sodium in the blood of the subject is then determined as a function of the measured conductivity (e.g., as a function of the measured conductivity differential across the dialyzer). The determined concentration of sodium in the subject&#39;s blood can be used as a model or can be taken into account to prepare a dialysate to be used to perform hemodialysis on the subject. The dialysate can, for example, be prepared to have a sodium concentration that is substantially equal to the determined sodium concentration of the subject&#39;s blood. As a result, the level of sodium in the subject&#39;s blood can be maintained at a substantially constant level throughout the blood treatment. 
     Referring to  FIG. 1 , a dialysis apparatus  100  includes a dialyzer  105  that can be used to filter or purify blood. Dialyzer  105  includes a dialysate/solution conduit  110  separated from a blood conduit  115  by a semi-permeable membrane  120 . Semi-permeable membrane  120  is permeable to certain impurities and toxins commonly found in uremic blood, such as phosphate, sulfate, bicarbonate, potassium, urea, creatinine, low molecular weight proteins, and other byproducts of metabolism. Semi-permeable membrane  120  is substantially impermeable to certain other blood components, such as high molecular weight proteins. Semi-permeable membrane can, for example, include (e.g., be formed of) one or more porous materials, such as a porous polysulfone. During hemodialysis, as described below, a patient&#39;s blood flows through blood conduit  115  and a dialysate  130  flows through dialysate/solution conduit  110 , causing solutes to pass across membrane  120  from the blood to the dialysate and vice versa. 
     A dialysate supply device  125  containing dialysate  130  is in fluid communication with dialyzer  105 . A dialysate supply line  135  is connected at a first end to dialysate supply device  125  and at a second end to a valve  140 . Valve  140  can be arranged in a first position (shown in  FIG. 1 ) in which dialysate  130  is permitted to flow therethrough, and in a second position in which dialysate  130  is prevented from flowing therethrough. Valve  140  can be any of various different types of valves, such as electronically controlled solenoid valves, hydraulically controlled solenoid valves, pinch valves, etc. A dialysate/solution inlet line  145  is fluidly connected to valve  105  at one end and to a dialyzer inlet opening  150  of dialyzer  105  at an opposite end. During use of apparatus  100 , dialysate  130  can be transported from dialysate supply device  125  to dialyzer  105  via lines  135 ,  145 . 
     As shown in  FIG. 2 , dialysate supply device  125  includes a mixer  155  that is fluidly connected to a concentrate supply tank  160  via a concentrate supply line  165 . A metering pump  175  is disposed within concentrate supply line  165  to enable a controllable amount of dialysate concentrate  180  to be pumped from concentrate supply tank  160  into mixer  155 . Mixer  155  is also fluidly connected to a water supply source (not shown) via a water supply line  170 . Water supply line  170  is equipped with a valve (e.g., a solenoid valve)  190  in order to allow the flow of water into mixer  155  to be regulated. Concentrate  180  and water can be delivered to mixer  155  and mixed together therein to form dialysate  130 . Pump  175  and valve  190  can be used to control the proportions of concentrate and water used to make the dialysate. 
     Referring again to  FIG. 1 , a solution supply device  195 , which contains a conductivity-testing solution  200 , is in fluid communication with dialyzer  105 . Solution supply device  195  is a container (e.g., a tank) that is at least partially filled with conductivity-testing solution  200 . A solution supply line  205  is connected at a first end to solution supply device  195  and at a second end to valve  140 . When valve  140  is arranged in the first position (shown in  FIG. 1 ), conductivity-testing solution  200  is prevented from flowing therethrough, and when valve  140  is arranged in the second position, conductivity-testing solution  200  is permitted to flow therethrough. During use of apparatus  100 , conductivity-testing solution  200  can be transported from solution supply device  195  to dialyzer  105  via lines  205  and  145 . 
     A dialysate/solution outlet line  210  is connected to dialyzer  105  at a dialyzer outlet opening  145  located at the opposite end of dialysate/solution conduit  110 . Dialysate/solution outlet line  210  is also connected to a pump  215 . Pump  215  can be any of various different types of pumping devices that are capable of moving the dialysate and/or solution through dialyzer  105 . Pump  215 , when activated, can draw dialysate  130  from dialysate supply device  125  or conductivity-testing solution  200  from solution supply device  195  (depending on the position of valve  140 ) through dialysate/solution conduit  110  of dialyzer  105 . 
     A first electrical conductivity detector  220  is disposed in dialysis/solution inlet line  145  between valve  140  and dialyzer  105 , and a second electrical conductivity detector  225  is disposed in dialysate/solution outlet line  210  between dialyzer  105  and pump  215 . Conductivity detectors  220 ,  225  are arranged to detect the conductivity of dialysate  130  and/or conductivity-testing solution  200  passing through dialysate/solution inlet and outlet lines  145 ,  210 , respectively. Conductivity detectors  220 ,  225  can be any of various types of devices capable of measuring the conductivity of the dialysate and/or solution. 
     Conductivity detectors  220 ,  225  provide signals to a control unit (e.g., a microprocessor)  230  by lines (e.g., electrical wires)  235  and  240 , respectively. Control unit  230  provides signals to valve  190  of water supply line  170  ( FIG. 2 ) and pump  175  of concentrate supply line  165  ( FIG. 2 ) via lines (e.g., electrical wires)  245  and  250 . Thus, control unit  230  can, based on input received from conductivity detectors  220 ,  225 , control the amount of concentrate and water transported to mixer  155 , thereby controlling the proportions of concentrate and water used to prepare dialysate  130 . Other arrangements are possible using conventional metering systems and/or proportioning valves. For example, a single signal could be output from controller  230  representing the desired volumetric and/or mass ratio of concentrate to water. 
     On the right-hand side of dialyzer  105 , a blood inlet line  255  is fluidly connected to a blood inlet opening  260  of dialyzer  105 , and a blood outlet line  265  is fluidly connected to a blood outlet opening  270  of dialyzer  105 . Needle catheters  275  and  280  are attached to the free ends of blood inlet and outlet lines  255  and  265 , respectively. During use, needle catheters  275  and  280  can be inserted into a blood access (e.g., a fistula) in a patient in order to draw blood from the patient and return the blood to the patient. Blood inlet line  255  is fluidly connected to a blood pump (e.g., a peristaltic roller pump)  285 . When blood lines  255 ,  265  are connected to a patient and pump  285  is activated, blood is drawn from the patient and delivered to the blood side of dialyzer  105  through blood inlet line  255 . After passing through blood conduit  115  of dialyzer  105 , the blood is returned to the blood access of the patient via blood outlet line  265 . 
     Blood lines  255 ,  265  and dialysate/solution lines  135 ,  145 ,  205 ,  210  can be any of various types of tubing capable of transporting blood or dialysate/solution therethrough. In some implementations, the blood lines and/or dialysate/solution lines are formed of one or more relatively compliant materials. Examples of materials from which the blood lines and/or dialysate/solution lines can be formed include polyvinylchloride (PVC), Di(2-ethylhexyl) phthalate (DEHP), and polyolefins. 
     Without wishing to be bound by theory, it is believed that subjects, including hemodialysis patients, have a natural or set level of sodium in their bodies, often referred to as their “set point.” It is further believed that the set point of a subject tends to remain relatively constant, and that sodium levels deviating too far from the set point can cause discomfort to the subject. Maintaining hemodialysis patients&#39; levels of sodium at or near their set point during hemodialysis would tend to alleviate or eliminate this cause of discomfort. Using a dialysate with a sodium concentration that is substantially equal to the sodium concentration in the blood of the hemodialysis patient can help to maintain the patient&#39;s blood sodium level at or near the patient&#39;s set point throughout the hemodialysis procedure. Methods of determining the sodium concentration in the blood of a hemodialysis patient and methods of preparing and using a dialysate having a sodium concentration that is substantially equal to the sodium concentration of the blood of the hemodialysis patient are described below. 
       FIG. 3  illustrates a method of determining a sodium concentration in a patient&#39;s blood using dialysis apparatus  100 . Referring to  FIG. 3 , blood lines  255 ,  265  are connected to a blood access (e.g., a fistula)  290  of a patient by inserting needle catheters  275 ,  280  into blood access  290 . After connecting blood lines  255 ,  265  to blood access  290 , blood pump  285  is activated, causing blood to flow through blood conduit  115  of dialyzer  105  in the direction indicated by arrow  295 . While flowing the blood through blood conduit  115  of dialyzer  105 , conductivity-testing solution  200  is flowed in the opposite direction (i.e., in the direction of arrow  300 ) through dialysate/solution conduit  110  of dialyzer  105 . Valve  140  can, for example, be arranged in its second position in which substantially only conductivity-testing solution  200  (and not dialysate  130 ) is permitted to flow into dialyzer  105 . Conductivity-testing solution  200  is generally flowed through dialyzer  105  for about five minutes or less (e.g., about three minutes to about five minutes). While flowing conductivity-testing solution  200  through dialyzer  105 , detectors  220 ,  225  measure the conductivity of conductivity-testing solution  200  as the solution enters and exits dialyzer  105 . 
     Conductivity-testing solution  200  is formulated such that electrically conductive solutes other than sodium in the patient&#39;s blood have little or no effect on the conductivity measurements of conductivity-testing solution  200  (e.g., on the differential of conductivity of conductivity-testing solution  200  across dialyzer  105 ). Conductivity-testing solution  200  can, for example, be formulated such that the concentrations of phosphate, sulfate, bicarbonate, potassium, calcium, and magnesium in conductivity-testing solution  200  closely match the concentrations of those respective solutes in the patient&#39;s blood. Thus, conductivity-testing solution  200  can estimate serum sodium concentration of the patient (based on a measured conductivity of the solution) with an increased level of accuracy. 
     Conductivity-testing solution  200  can include about 135 mEq/L to about 145 mEq/L of sodium, about 100 mEq/L to about 110 mEq/L of chloride, about 20 mEq/L to about 45 mEq/L of bicarbonate, and about 3.0 mEq/L to about 5.0 mEq/L of potassium, about 6.0 mg/dl to about 8.0 mg/dl of phosphate, and about 6.0 mg/dl to about 8.0 mg/dl of sulfate. In some implementations, solution  200  includes about 140 mEq/L of sodium, about 100 mEq/L of chloride, about 20 mEq/L of bicarbonate, about 4.0 mEq/L of potassium, about 7.0 mg/dl of phosphates, and about 7.0 mg/dl of sulfates. 
     Various techniques can be used to determine and prepare a desirable composition of conductivity-testing solution  200 . For example, to prepare conductivity-testing solution  200 , a large sample of dialysis patients can be tested to determine an average level of the above-noted electrically conductive solutes in their blood. Conductivity-testing solution  200  can then be formulated to include similar (e.g., identical) levels of those solutes. By using the average level of blood solute concentrations in a targeted population of subjects, relatively accurate predictions of other hemodialysis patients&#39; blood levels can be predicted without having to use more invasive and/or time-consuming procedures, such as drawing and testing the blood of each patient undergoing treatment. In addition, because conductivity-testing solution  200  need not be specifically tailored for each individual patient, the solution can be manufactured and distributed in large batches, and can thus be manufactured relatively inexpensively. 
     Due to the closely matched concentrations of electrically conductive solutes, such as phosphate, sulfate, bicarbonate, potassium, calcium, and magnesium, in conductivity-testing solution  200  and in the patient&#39;s blood, little if any diffusion of those electrically conductive solutes occurs across membrane  120 . Consequently, the conductivity measurements of conductivity-testing solution  200 , as measured by detectors  220 ,  225 , can be more closely correlated with the level of sodium in the patient&#39;s blood. Therefore, as compared to traditional dialysate solutions, conductivity-testing solution  200  can be used to more accurately determine the level of sodium in the patient&#39;s blood as a function of the conductivity (e.g., change in conductivity across dialyzer  105 ) of the conductivity-testing solution. 
     After determining the conductivity of conductivity-testing solution  200  at detectors  220  and  225 , the conductivity values are transferred from detectors  220 ,  225  to control unit  230 . Control unit  230  then determines (e.g., calculates) the level of sodium in the patient&#39;s blood as a function of the measured conductivity values. The level of sodium in the patient&#39;s blood can, for example, be determined using the following equation:
 
 Cb   i   =Cd   i [(( D (1 −Q   f   /Q   e )− Qd (1 −Cnd   o   /Cnd   i )+ Qf ( Cnd   o   /Cnd   i ))/( D (1 −Q   f   /Q   e )+ Q   f )]  (1)
 
     In equation (1) above, Cd i  is the base sodium concentration of the conductivity-testing solution; Q f  is the ultrafiltration rate; Q e  is the blood water flow rate which can be approximated as 0.85 Qb; D is the conductivity dialysance; and Cnd o , Cnd i  are the conductivies of the conductivity testing solution in the outlet and inlet streams, respectively. 
     As an alternative to or in addition to calculating the level of sodium in the patient&#39;s blood, control unit  230  can use a look-up table that includes multiple conductivity values (e.g., multiple differential of conductivity values) and corresponding blood sodium concentrations. In some implementations, the look-up table is used as a quality control mechanism to compare a calculated blood sodium level to a predetermined theoretical blood sodium level. In such implementations, if the calculated blood sodium level differs from the theoretical blood sodium level by greater than a predetermined acceptable amount, an indicator (e.g., an audio and/or visual indicator) can be activated to notify the user of the disparity. 
     After determining the concentration of sodium in the patient&#39;s blood, dialysate  130  can be prepared to include a concentration of sodium that is substantially equal to the concentration of sodium determined to exist in the patient&#39;s blood. Dialysate  130  can, for example, be prepared to have a sodium concentration that differs from the sodium concentration in the subject&#39;s blood by no more than about five percent (e.g., no more than about one percent) and/or no more than about 7.0 mEq/L (e.g. no more than about 1.5 mEq/L). Referring to  FIGS. 2 and 3 , to formulate dialysate  130  such that it has a sodium concentration substantially equal to that of the patient&#39;s blood, the proportion of dialysate concentrate to water can be altered by control unit  230 . Control unit  230 , which is in electrical communication with pump  175  of concentrate supply line  165  and valve  190  of water supply line  170  can operate pump  175  and valve  190  to deliver desired proportions of dialysate concentrate and water to mixer  155  of dialysate supply device  125 . The sodium level of the dialysate can, for example, be increased or decreased, by altering the proportions of dialysate concentrate and water. The levels of other solutes within the dialysate concentrate will be increased or decreased along with the sodium when the proportions of dialysate concentrate and water are changed. However, it is believed that, due to the relatively small amounts of those other solutes in the dialysate, the increased or decreased levels of those other solutes will have a negligible effect on the treatment of the patient&#39;s blood. 
     After preparing dialysate  130 , hemodialysis can be performed using dialysate  130  in order to remove impurities and toxins as well as excess water and sodium from the blood of the patient. Referring to  FIG. 4 , to perform hemodialysis, blood lines  255 ,  265  are connected (e.g., remain connected) to blood access  290  of the patient, and pump  285  is activated (e.g., remains activated) such that the patient&#39;s blood is caused to flow through blood conduit  115  of dialyzer  105  in the direction indicated by arrow  295 . While flowing the patient&#39;s blood through blood conduit  115  of dialyzer  105 , dialysate  130  is pumped through dialysate/solution conduit  110  of dialyzer  105  in a direction opposite to that of the blood (i.e., the direction indicated by arrow  305 ). 
     Due to the differing compositions of dialysate  130  and the patient&#39;s blood, certain impurities and toxins, such as phosphate, sulfate, bicarbonate, potassium, calcium, and magnesium, are diffused through membrane  120  of dialyzer  105 , from the blood into the dialysate or vice versa. Other impurities, such as urea and creatinine, can similarly be diffused across membrane  120 . Dialysate  130 , for example, generally has concentrations of phosphate, sulfate, potassium, calcium, and magnesium that are lower than the concentrations of phosphate, sulfate, potassium, calcium, and magnesium, respectively, found in blood  220 , and dialysate  130  generally has a concentration of bicarbonate that is higher than the concentration of bicarbonate in blood  220 . As a result, phosphate, sulfate, potassium, calcium, and magnesium will generally diffuse from blood  220  into dialysate  130  during treatment while bicarbonate will generally diffuse from dialysate  130  into blood  220  during treatment. Typical concentrations within dialysate  130  are about 140 mEq/L of sodium, about 100 mEq/L of chloride, about 35-40 mEq/L of bicarbonate, about 2.0 mEq/L of potassium, 2.5 mEq/L of calcium, 2.0 mEq/L of magnesium and no phosphate or sulfate. However, as discussed above, the sodium concentration in dialysate  130  will vary depending on the sodium level in the blood of the particular patient being treated. The other components of dialysate  130  can also vary depending on, among other things, the type of treatment to be performed. 
     In addition to the above-described diffusion technique, an ultrafiltration process is used to remove certain components, such as water, from the blood by convection. To remove components from the blood using ultrafiltration, a pressure gradient is produced between dialysate/solution conduit  110  and blood conduit  115 . Due to the pressure gradient across membrane  120 , water and certain solutes contained therein, such as sodium, pass across membrane  120  from the blood to dialysate  130  during treatment. 
     Due to the substantially equal levels of sodium in the blood and the dialysate, after passing the dialysate and blood through dialyzer  105  for a desired period of time (e.g., about 150 minutes to about 210 minutes), the sodium concentration in the blood will differ from the predicted sodium concentration of the blood by no more than about five percent (e.g., by no more than about one percent) of the predicted sodium concentration of the blood and/or by no more than about 7.0 mEq/L (e.g., by no more than about 1.5 mEq/L). 
     After purifying the blood and removing the excess liquid and sodium from the blood, the treated blood is returned to the patient via blood outlet line  265  and the spent dialysate is transferred to a waste drain. 
     While certain implementations have been described above, other implementations are possible. 
     While implementations above describe predicting a patient&#39;s blood sodium concentration based on an average level of sodium found in a previously tested group of hemodialysis patients, the levels of electrically conductive solutes in a patient&#39;s blood can alternatively or additionally be determined on an individual basis by, for example, drawing blood from the patient prior to his/her treatment and testing the levels of the electrically conductive solutes in the blood. A conductivity-testing solution having levels of electrically conductive solutes substantially equal to those found in the patient&#39;s blood can then be produced and used in a process similar to those described above to determine the level of sodium in the patient&#39;s blood. For example, the concentrations of phosphate, sulfate, bicarbonate, potassium, calcium, and magnesium in the conductivity-testing solution can be approximately matched to the patient&#39;s plasma solute concentrations at the beginning of dialysis. 
     In some implementations, using pre-packaged dialysate concentrate  180 , the sodium concentration of dialysate concentrate  180  is tested prior to formulating dialysate  130  even though the label indicates that the concentrate has a particular concentration. In such implementations, the proportions of water and concentrate  180  used to form dialysate  130  can be adjusted prior to hemodialysis if it is determined that the actual sodium concentration of the dialysate concentrate differs from the labeled concentration (e.g., the concentration provided by the manufacturer of the concentrate). To test the sodium concentration of dialysate concentrate  180 , the dialysate concentrate is flowed through dialysate/solution conduit  110  of dialyzer  105 , and the conductivity of the dialysate concentrate is measured by one or both of detectors  220 ,  225 . A technique similar to that described above with respect to the delivery of dialysate  130  to dialyzer  105  can be used to deliver dialysate concentrate  180  to dialyzer  105 . However, rather than supplying both water and concentrate to mixer  155  ( FIG. 2 ), the concentrate alone is delivered to mixer  155 . While flowing dialysate concentrate  180  through dialysate/solution conduit  110  of dialyzer  105 , the blood of the patient is prevented from passing through blood conduit  115  of dialyzer  105 . Blood lines  255 ,  265  can, for example, be disconnected from the patient prior to flowing dialysate concentrate  180  through dialyzer  105 . Blood lines  255 ,  265  can alternatively or additionally include shut-off valve or a bypass valve to prevent the blood from passing through the dialyzer. By preventing the blood from flowing through dialyzer  105 , diffusion and/or convection of blood components or solutes into dialysate concentrate  180  through membrane  120  of dialyzer  105  can be prevented. Consequently, the true composition of the dialysate can be tested. 
     After measuring the conductivity of the concentrate, the conductivity values are communicated (e.g., electronically transferred) to control unit  230 . Control unit  230  can then determine the sodium concentration of dialysate concentrate  180 . While dialysate concentrate  180  includes certain electrically conductive solutes (e.g., bicarbonate, potassium, calcium, and/or magnesium) in addition to sodium, the relative amount of those electrically conductive solutes, as compared to the sodium, remains very small throughout the testing process. This is due, at least in part, to the fact that electrically conductive solutes are not transferred from the blood to the dialysate concentrate during the testing process. Due to the relatively small amount of those electrically conductive solutes in dialysate concentrate  180 , it is believed that the effect of those solutes on the measured conductivity and the determination of sodium concentration of the dialysate concentrate will be negligible. 
     After determining the actual concentration of sodium in dialysate concentrate  180 , control unit  230  determines whether the difference (if any) between the determined sodium concentration and the labeled sodium concentration warrants an adjustment to the proportions of water and concentrate used to prepare dialysate  130 . If, for example, the sodium concentration determined by control unit  230  differs from the labeled concentration by more than about one percent (e.g., by more than about 0.5 percent) of the labeled concentration and/or by more than about 1.5 mEq/L (e.g., by more than about 1.0 mEq/L), then the proportions of the concentrate and water used to form dialysate  130  can be altered to compensate for the erroneous labeled concentration. Adjusting the proportions of the concentrate and water used to form dialysate  130  can help to ensure that dialysate  130  has sodium concentration that is substantially equal to the sodium concentration of the blood to be treated. 
     While the implementations above describe the use of a dialysate concentrate (e.g., the use of a dialysate concentrate mixed with varying amounts of water) to form dialysate  130 , other techniques can alternatively or additionally be used. In some implementations, for example, one or more of the individual components found in the dialysate concentrate are separately combined with water to form the dialysate. As a result, the concentration of one component, such as sodium, can be altered without altering the concentrations of other components of the dialysate, such as bicarbonate, potassium, calcium, and magnesium. In such implementations, as an alternative to or in addition to concentrate supply tank  160  ( FIG. 2 ), multiple different concentrate or concentrate component supply tanks can be fluidly connected to mixer  155  of dialysate supply device  125  to enable the concentrate or concentrate components to be added to mixer  155  when forming the dialysate. 
     While implementations above describe preparing the dialysate by mixing dialysate concentrate or concentrate components with water, the user can alternatively or additionally select the dialysate from a variety of premixed dialysates of different concentrations. To treat a patient, the treatment provider can select and use the premixed dialysate having a sodium concentration that most closely matches the sodium concentration of the patient&#39;s blood. In such implementations, the dialysate supply device could merely be a container (e.g., a tank) that contains the premixed dialysate. 
     In some implementations, dialysis apparatus  100  also includes temperature detectors (not shown) positioned proximate to detectors  220 ,  225 . The temperature detectors can be configured to determine the temperature of dialysate  130  and/or conductivity-testing solution  200  flowing through dialysate/solution lines  145 ,  210 . The temperature detectors can be in communication with control unit  230  such that the detected temperature data can be transferred to control unit  230 . Control unit  230  can, for example, use the temperature readings to adjust conductivity readings and/or sodium concentration determinations that may be affected by a temperature differential in the dialysate or solution across dialyzer  105 . 
     While detectors  220 ,  225  have been described as conductivity detectors, other types of detectors and/or techniques can alternatively or additionally be used to determine the sodium concentration of the blood. Examples of other types of detectors include ion selective electrodes and flame photometers. 
     While blood access  290  has been described as a fistula, other types of blood accesses, such as grafts, shunts, and catheters, can alternatively or additionally be used. 
     While solution supply device  195  has been described as a container containing a premixed solution, other types of solution supply devices can alternatively or additionally be used. In some implementations, for example, solution supply device  195  includes a solution concentrate supply and a water supply. In such implementations, the solution concentrate and water can be mixed together (e.g., in a mixing tank) to form conductivity-testing solution  200 . 
     While the methods, solutions, and dialysates described in implementations above relate to hemodialysis, the methods, solutions, and/or dialysates can alternatively or additionally be used for other types of dialysis, such as peritoneal dialysis. 
     Other implementations are within the scope of the following claims.