Method and device for the assay of ions

Abstract: A test device and method of determining the presence or concentration of a predetermined ion, such as a cation of medical interest, like sodium, potassium, calcium, magnesium or lithium, in an aqueous test sample are disclosed. The test device includes a test pad comprising a hydrophilic indicator reagent composition capable of interacting with a predetermined ion, or electrolyte, to produce a detectable or measurable response. The hydrophilic indicator reagent composition comprises an ion specific ionophore, a reporter substance, and a stabilizer compound, such a gelatin. The hydrophilic indicator reagent composition, either as a film or layer, or after incorporation into a suitable carrier matrix, provides a test pad of sufficient sensitivity to the predetermined ion to achieve an accurate and trustworthy electrolyte assay of an aqueous test sample, such as the plasma or serum of a whole blood sample, by a dry phase test strip assay.

Description: The present invention relates to a composition, a test device and a method of assaying an aqueous test sample for the presence or concentration of an electrolyte, or ion, especially an ion of medical interest, such as sodium, potassium, calcium, magnesium and lithium. More particularly, the present invention relates to a hydrophilic indicator reagent composition, a method and a device for assaying an aqueous test sample, for a cation by utilizing a test device having a test pad including a hydrophilic indicator reagent composition. The hydrophilic indicator reagent composition comprises a hydrophilic ion-specific ionophore, a hydrophobic reporter substance and a stabilizer compound (a hydrophilic polymer compound), that can be used as a film or a layer, or incorporated into a carrier matrix, to provide a test pad that generates a detectable or measureable response upon contact with an aqueous cation-containing test sample. The hydrophilic indicator reagent composition of the present invention, either as a film or a layer, or after incorporation into a hydrophilic carrier matrix, provides a test pad of sufficient sensitivity to a predetermined cation, or cations, to afford an accurate and trustworthy electrolyte assay of an aqueous test sample, such as the plasma or serum of a whole blood sample, by a dry phase test strip assay. BACKGROUND OF THE INVENTION AND PRIOR ART The detection or concentration measurement of an ion in an aqueous test sample has applications in numerous technologies. For example, in the field of water purification, calcium ion concentration is monitored to determine the degree of calcium saturation of an ion exchange resin deionizer. Furthermore, the measurement of sodium ion concentration and other ion concentrations in seawater is important in the preparation of drinking water aboard a ship at sea. However, among the most important ion concentration determinations are measurements related to the electrolyte levels of an individual. For example, physicians routinely assay for the amount of potassium ion in the blood as an aid in diagnosing conditions leading to muscle irritability and excitatory changes in myocardial function and conditions such as oliguria, anuria, urinary obstruction and renal failure due to shock. Since the clinical range of serum potassium is only from about 2 to about 10 millimolar (mM), with a normal range of from about 3.5 to about 5.5 mM, potassium ion measurements require particular sensitivity and precision. Similarly, assays for sodium ion require high sensitivity and precision because the total clinical range for sodium ion is from about 120 to about 170 mM, with a normal range of from about 135 to about 155 mM. Furthermore, an accurate and reliable measurement of lithium levels in serum also is important because the toxic dose level of lithium ion is only slightly higher than the therapeutic level used in psychiatric treatment. Other cations, such as calcium and magnesium, also are considered medically important and must be assayed accurately in order to provide the physician with sufficient information to diagnose an abnormal condition and to institute, maintain and monitor a proper medical treatment. Although the present invention is particularly useful in the measurement of metal cations, or electrolytes, found in body fluids, other ions, especially other cations, also can be accurately and reliably detected and measured. The measurement of potassium ion concentration in aqueous test samples, such as the serum or plasma of a whole blood sample, is a widely used and important diagnostic test. A reliable and fast method of determining the concentration of a clinically important ion, like potassium ion, would advance the art of medical assays where rapid, accurate ion determinations are required. Thus, for example, if a medical laboratory technician could accurately measure the sodium, lithium, magnesium, potassium or calcium level of a whole blood sample in matter of seconds or minutes, such rapid assay results would increase laboratory efficiency and would aid the physician in diagnosis. Furthermore, an easy and accurate method of determining electrolyte concentrations in plasma or serum of a whole blood sample would allow an individual to perform the test at home in order to better maintain and monitor a physician-prescribed treatment. Therefore, it would be useful to have a fast, convenient method of detecting or measuring the ion concentration of an aqueous test sample, whereby accurate, reliable assay results are available within minutes of contacting a test device with the aqueous test sample. Therefore, in accordance with an important feature of the present invention, the cumbersome, expensive electronic equipment that is presently used, such as ion-specific electrodes, flame photometers, atomic absorption photometers or the like, can be avoided. The present invention enables the medical practitioner, or the individual at home, simply to contact the test sample with a test device, then correlate a detectable response to the concentration of the particular ion of interest. As stated above, the present-day methods of determining ion concentration in solution include flame photometry, atomic absorption photometry and ion-specific electrodes. However, more recently, and as will be described more fully hereinafter, dry phase test strip formats have been used to assay for ions, such as potassium. In addition, with respect to the dry phase test strips and to ion-specific electrodes, the use of certain compounds and compositions that selectively isolate the particular ion of interest from the aqueous test sample has become popular. These compounds, known as ionophores, have the capability of transporting the particular ion of interest into an electrode membrane causing a difference in potential that can be measured. The ion assay methods that utilize the selective ion/ionophore phenomenon include membrane electrodes, liquid/liquid partitioning, fluorescence and dry phase test strips. The ion-specific electrode method of determining the ion concentration of an aqueous test sample is based on the electrical potential (EMF) generated when two solutions having different concentrations of the ion are separated by an electrically-conductive membrane. In membrane separation cells, the membrane can be a simple fritted glass barrier, allowing a small but measurable degree of ion diffusion from one solution to the other. Alternatively, a nonporous, electrically-nonconductive film, such as polyvinyl chloride, impregnated with an ionophore can be employed. In the absence of the ionophore, the nonconductive film is an insulator and no EMF can be measured. However, when blended with an ionophore, the charged ions are bound to the film and a small, measurable current can be induced to flow. Such cells are ion selective because the ionophore is selective in its affinity to a particular ion, and thus binds almost exclusively to a certain specific ion. Therefore, any measurable EMF is due solely to the presence of the particular ion of interest bound to the ionophore. However, ion-specific electrodes do have disadvantages. The current flowing across the membrane is sufficiently small such that the actual quantity of the ion, or its counterion, transported across the membrane is insignificant. Therefore, such small changes in ion concentration produce very small changes in EMF that require sophisticated voltmeter equipment for detection. Further, the electrical neutrality of the membrane is maintained either by a reverse flow of hydrogen ions, or by a parallel flow of hydroxyl ions. However, this flow of hydroxyl ions can reduce the specificity of the electrode towards the specific ion to be determined, and therefore is an interference that has to be minimized. In addition, a major difficulty in the use of ion-selective electrodes has been a significant reduction of assay accuracy and of speed of response over time. The disadvantages of ion-specific electrodes are somewhat overcome by using certain antibiotics, such as valinomycin, that effect the electrical properties of biological phospholipid bilayer membranes. These antibiotics solubilize cations within the membrane in the form of mobile charged complexes, thereby providing a "carrier" mechanism wherein cations can cross the insulating hydrocarbon interior of the membrane. The complexes carry electrical charges through the membrane such that a voltage differential can be determined between the solutions on either side of the membrane. Prior art disclosures of ion-specific electrodes include Simon U.S. Patent No. 3,562,129, teaching the use of porous membranes, like glass frits, impregnated with macrocyclic derivatives of amino and oxy-acids, in ion-specific electrodes. These electrodes are said to be effective in measuring ion activities. U.S. Patent No. 4,053,381, issued to Hemblen et al., disclosed a similar technology, and utilized an ion-specific membrane with ion mobility. Similarly, Simon et al., in U.S. Patent No. 3,957,607, disclosed a process for the electrochemical determination of cations by utilizing an electrode having a membrane containing neutral ionophores capable of forming lipid soluble complexes with cations. Another method of using ionophores in ion determinations is liquid/liquid partitioning. Eisenman et al., Membrane Biol. , 1:294-345 (1969) taught the selective extraction of cations from aqueous solutions into organic solvents by macrotetralide actin antibiotics. Therefore, in the Eisenman et al. procedure, a hydrophobic ionophore is dissolved in a water-immiscible organic solvent. Then, the organic solvent phase containing the antibiotics is shaken with an aqueous solution containing cationic salts of lipid-soluble, colored anions, such as picrates and dinitrophenolates. The intensity of color developed in the organic phase then is measured spectrophotometrically to indicate the amount of extracted salt. Phase transfer has also been studied by Dix et al., Angew. Chem. Int. Ed. Engl. , 17S:857 (1978) and discussed in reviews including Burgermeister et al., Top Curr. Chem. , 69:91 (1977); Yu et al., "Membrane Active Complexones", Elsevier, Amsterdan (1974); and Duncan, "Calcium in Biological Systems", Cambridge University Press (1976). The partitioning of a compound between liquids is rapid and effective because the mobility of the ionophore carrier and ions allows the transported species to diffuse rapidly away from the interface. Such a mechanism is normally impossible in the solid phase because of the rigidity, immobility and essentially zero diffusion of material in a solid phase. Another method used to detect and measure ion concentrations with an ionophore is disclosed in U.S. Patent No. 4,367,072. The method disclosed the use of a chromogenic ionophore, i.e., an ionophore covalently linked to a chromogen. A charged chromogen-ionophore complex having the same charge as the ion to be determined also is taught. In use, a chromogenic ionophore, or a charged chromogen-ionophore complex, is added to a liquid sample and the color of the solution is monitored spectrophotometrically. It was disclosed that the ionophore could be incorporated into a carrier such as paper, synthetic resin, film, silicon oxide, natural or synthetic fibers or metal. As illustrated by the above-cited prior art, at present, diagnostic assays for the determination of electrolytes, such as sodium, potassium and lithium, routinely are measured by ion selective electrodes, or by flame photometry. However, two other assay methods for electrolytes recently have become available. Dry phase reagent strips are available for the colorimetric determination of potassium ion in serum or plasma, but not whole blood, on a SERALYZER® reflectance photometer, with both the reagent strips and the instrument marketed by Miles Inc., Elkhart, Indiana. In this commercial procedure, it is necessary to separate the serum or plasma from the coagulated or packed cells of the whole blood sample. The dry phase reagent strip technology is disclosed in U.S. Patent Nos. 4,540,520; 4,552,697; 4,649,123; 4,645,744; and 4,670,218. In addition, Abbott Laboratories has marketed a specialized instrument and reagent package that measures potassium ion concentration by partitioning a chromogenic counterion between liquid organic and aqueous phases. The liquid organic phase contains an ionophore specific for potassium ion and the amount of chromogenic counterion partitioning into the organic phase is a function of potassium ion concentration. The instrument performs the required centrifugation to effect phase separation and to measure the chromogenic counterion concentration in the liquid organic phase by absorbence. This technology is disclosed in European Patent Applications 85105106.0 (EP-A-0 160 282), 85105108.6 (EP-A-0 160 901) and U.S. Patent No. 4,639,424. Similarly, Sumiyoshi et al., Talanta , 24, 763-765 (1977) describes a method of determining potassium ion concentration in serum wherein the serum first is deproteinated by trichloracetic acid. Then after an indicator dye is added to the serum, the serum is shaken with a solvent, such as chloroform, containing valinomycin. Therefore, most methods of measuring cation concentration with ionophores require the transport of the ion by the ionophore either into a liquid organic phase (phase transfer measurements), or through an ion selective membrane containing an ionophore (electrodes or optodes). In the dry reagent strip method, the ionophore and an indicator compound are isolated within a hydrophobic matrix, commonly provided by a polymeric material or an organic liquid, and the transport of the ion of interest into the hydrophobic matrix produces a detectable colorimetric response. For example, Charlton et al. in U.S. Patent No. 4,649,123 teaches incorporating finely divided globules of a hydrophobic mixture including a hydrophobic vehicle, such as a polymer, like polyvinyl chloride, or a water-insoluble liquid, like dioctylphosphate or a nitrophenyl ether, into a hydrophilic carrier matrix. The globules also contain an ionophore and an indicator compound, i.e., a reporter substance, and the globules are formed by preparing an emulsion between the hydrophobic mixture and a mixture of water and a hydrophilic polymer, such as gelatin. The emulsion then is coated onto a support member and the water evaporated leaving the hydrophilic polymer and the finely divided hydrophobic globules. Alternatively, the emulsion can be coated onto filter paper, then the water is evaporated and the coated filter paper is affixed to a support. In either case, the test pad of the test device comprises finely-divided globules of a hydrophobic mixture, containing an ionophore and a reporter substance in a hydrophobic vehicle, in a hydrophilic matrix. Similarly, Gantzer et al. in U.S. Patent No. 4,670,218 disclose incorporating the ionophore and the reporter substance directly into a porous carrier matrix from a homogeneous, non-aqueous, hydrophobic composition, as opposed to an emulsion. The method of Gantzer then requires a second manipulative step in order to incorporate a water-soluble buffer compound into the carrier matrix. The exception to the requirement of isolating the ionophore in a hydrophobic, or organic, phase is described by Feinstein et al., in Proc. Nat. Acad. Sci. USA , 68 (9):2037-2041 (Sept. 1971). Feinstein et al. disclose the formation of a cation-ionophore complex in water in the presence of the anionic form of two fluorescent dyes, 1-anilino-8-naphthalene sulfonate (ANS) and 2-p-toluidinyl-6-naphthalene sulfonate (TNS). The method of Feinstein included antibiotics, such as valinomycin and the macrotetralide actins, as the ionophores. However, the method used by the authors to produce the aqueous solutions is not clearly described. In general, the authors theorized that the observed increase in fluorescence depolarization is due to the binding sites of the cation-ionophore complex becoming saturated, but also theorized that the depolarization is caused by energy transfer between fluorophore molecules within a micelle or caused by dye interaction with aggregates of cation-ionophore complexes. Feinstein therefore proposed that a cation-ionophore complex could be detected in an aqueous environment. This proposal was contrary to the prevailing theory that the stability of such cation-ionophore complexes was drastically reduced by the addition of water to an organic solution containing the complexes. Another exception to the requirement of isolating an ionophore in a hydrophobic, or organic, phase is disclosed Gibbons U.S. Patent No. 4,820,647. Gibbons disclosed the wet phase diagnostic detection of metal ions with an aqueous solution including an ionophore, a reporter substance and a micelle-forming material. The micelle-forming material acts to solubilize the hydrophobic ionophore and reporter substance. Other pertinent prior art includes Vogtle et al., U.S. Patent No. 4,367,072, disclosing a color change upon the addition of an ion-containing test sample to a solution of hydrazinium hydrochloride dissolved in methanol/water and mixed with molar amounts of a crown ether. A change of the absorption (brightening) is observed because the dye is displaced from its inclusion complex by a cation, such as potassium or sodium. In addition, surface active properties of ionophores have been studied by Ping-Lin Kau et al., Tenside Detergents , 19 (4):204-206 (1982) and by J. Le Moigne et al., J. Phys. Chem. , 84 :170-177 (1980). EP 0 319 863 relates to a homogeneous aqueous test solution of an ionophore, a reporter substance, a stabilizer and a buffer, said solution being capable of measuring the concentration of an ion. However, EP 0 319 863 fails to teach or suggest using a film or a dry test strip method of assaying for an ion. Therefore, many methods are known for assaying ions in solution. Instrumental methods include such sophisticated techniques as ion-specific potentiometry, flame photometry and atomic absorption photometry. In addition, the use of ionophores that selectively complex with a specific ion has led to five additional approaches: ion selective electrodes, liquid/liquid partitioning, fluorescence enhancement, chromophorelabeled ionophores and test strips. However, prior to the present invention, very few of the assay methods provide the user with rapid and accurate assay results by simply contacting a test sample with a test device. The present invention, on the other hand, permits the assayer simply to contact the aqueous test sample with an improved test device of the present invention and to observe a detectable or measurable change in color or other detectable response. As will be described more fully hereinafter, the method of the present invention allows the fast, accurate and trustworthy ion, or electrolyte, assay of aqueous test samples by utilizing a test strip that includes a test pad comprising a new hydrophilic indicator reagent composition. The hydrophilic indicator reagent composition comprises an ionophore, a reporter substance and a stabilizer compound, such as gelatin. Surprisingly and unexpectedly, and in contrast to the prior art, it is not necessary to include the ionophore and the reporter substance in a hydrophobic phase in order to mediate the color transition or other detectable change that occurs in response to the ionophore complexing with the electrolyte. The hydrophilic indicator reagent composition of the present invention, either as a layer or film, or after incorporation into a carrier matrix, provides a test pad of sufficient stability and of sufficient sensitivity to a predetermined electrolyte to afford an accurate and trustworthy electrolyte assay of aqueous test samples, such as the plasma or serum of a whole blood sample, by a dry phase test strip. All known prior art test strip assay techniques for ions require a hydrophobic phase to isolate the ionophore. Surprisingly and unexpectedly, the method and device of the present invention provide an accurate ion assay without isolating the ionophore in a hydrophobic phase. Until the method and device of the present invention, dry phase test strips to assay for the presence or concentration of a predetermined ion, or electrolyte, in an aqueous test sample required a hydrophobic organic phase. However, the new and improved hydrophilic indicator reagent composition of the present invention, either as a film or layer, or after incorporation into a carrier matrix, provides an improved assay pad for the detection or measurement of the electrolyte concentration, like sodium, potassium, lithium, magnesium or calcium ion concentration, of an aqueous test sample, such as the plasma and serum of a whole blood sample. The device of the present invention exhibits excellent storage stability and provides sufficient differentiation and resolution of the color transition resulting from contacting the ion-containing test sample with the hydrophilic indicator reagent composition to provide sufficient assay sensitivity and allow a reliable and accurate detection or measurement of electrolyte content in an aqueous test sample. Any method of assaying for electrolytes in whole blood or other aqueous test samples must yield accurate, trustworthy and reproducible results by providing a detectable or measurable response, like a color transition, as a result of an interaction between the hydrophilic indicator reagent composition and the electrolyte, and not as a result of a competing chemical or physical interaction, such as a preferential interaction with a test sample component other than the electrolyte of interest. Moreover, it would be advantageous if the electrolyte assay method is a dry phase test strip assay for the rapid, economical and accurate determination of an electrolyte in the plasma or serum of a whole blood sample, or other aqueous test samples. Additionally, the test pad, comprising the hydrophilic indicator reagent composition, utilized in the assay for an electrolyte should not adversely affect or interfere with the other reagent test pads that are present on multiple test pad strips. Prior to the present invention, no known method of assaying aqueous test samples for an electrolyte utilized a test device including a test pad comprising a hydrophilic indicator reagent composition that provides sufficient test strip stability; sufficient color differentiation and resolution; and sufficient assay sensitivity to allow the assay of low electrolyte levels in the test sample. Therefore, surprisingly and unexpectedly, the method and device of the present invention permits the dry phase test strip assay of plasma, serum and other aqueous test samples for low levels of an electrolyte without including a hydrophobic phase in the test pad. In contrast to the prior art, and in contrast to the presently available commercial test strips, the method of the present invention provides sufficient sensitivity to a predetermined electrolyte to allow the detection and measurement of the electrolyte in an aqueous sample. Therefore, the method and test device of the present invention allow the simple, accurate and essentially immediate detection or measurement of an electrolyte concentration of an aqueous test sample. The method and test device are suitable for use either in the physician's office or at home. Hence, in accordance with the method of the present invention, new and unexpected results are achieved in the dry phase reagent strip assay of plasma, serum and other aqueous test samples for a predetermined electrolyte, like potassium, sodium, lithium, magnesium or calcium, by utilizing a test pad comprising a hydrophilic indicator reagent composition of the present invention. SUMMARY OF THE INVENTION In brief, the present invention is directed to a new hydrophilic indicator reagent composition, a new and improved test device, a method manufacturing the test device, and a method of determining the presence or concentration of a predetermined component in a test sample. The device includes a test pad comprising a hydrophilic indicator reagent composition that interacts with a predetermined test sample component, like an electrolyte, to produce a detectable response. For laboratory and home use, the hydrophilic indicator reagent composition produces a response that is detectable visually or by instrument. The new and improved test pad of the test device of the present invention comprises a hydrophilic indicator reagent composition either as a film or a layer, or incorporated into a suitable carrier matrix, such as filter paper. The carrier matrix then holds the hydrophilic indicator reagent composition homogeneously throughout the carrier matrix while maintaining carrier matrix penetrability by the test sample and by the test sample component to be assayed. The hydrophilic indicator reagent composition comprises an ionophore, a reporter substance and a stabilizer compound (a hydrophilic polymer compound), such as gelatin. More particularly, the present invention is directed to a method of assaying an aqueous test sample, like the plasma or serum of a whole blood sample, for a predetermined electrolyte, like potassium ion, by utilizing a test device including a test pad comprising a hydrophilic indicator reagent composition. Surprisingly and unexpectedly, it has been demonstrated that the hydrophilic indicator reagent composition, either as a film or layer, or after incorporation into a suitable carrier matrix, provides a sufficiently stable dry phase test strip and provides a sufficiently differentiable and resolvable color transition, to permit the sensitive detection of a low electrolyte concentration, like a potassium ion concentration of from 0 to about 10 mMol/L. Therefore, and in accordance with an important feature of the present invention, the qualitative or quantitative determination of an electrolyte concentration between 0 mMol/L and about 200 mMol/L in the plasma or serum of a whole blood sample or in other aqueous test samples is accomplished. By utilizing the test device of the present invention in clinical test methods, the qualitative or quantitative concentration of an ion, such as sodium, calcium, lithium, magnesium or potassium, in plasma, serum or other aqueous test samples can be determined accurately because the color resolution and differentiation exhibited by a test pad comprising the hydrophilic indicator reagent composition demonstrates sufficient sensitivity for a dry phase assay for low concentrations of ions. Furthermore, surprisingly and unexpectedly, the new and improved test device allows the detection and measurement of a predetermined ion, such as potassium, sodium, calcium, magnesium or lithium, in other nonbiological aqueous test samples. Thus, the present invention provides a hydrophilic indicator reagent composition for determining the presence or concentration of an electrolyte, which composition consists essentially of a film or layer of: a) a hydrophobic ion-specific ionophore capable of forming a complex with a specific ion to be determined, b) a hydrophobic reporter substance capable of interacting with said complex of said ionophore and a specific ion to be determined to produce a detectable response, and c) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound. The present invention also provides a method of determining the presence or concentration of a predetermined ion in a test sample comprising: a) contacting the test sample with an analyte detection device, wherein said analyte detection device includes a hydrophilic test pad impregnated with a homogenous hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion; ii) a hydrophobic reporter substance capable of interacting with said complex formed by the ionophore and the predetermined ion to provide a detectable response; and iii) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound; b) examining the analyte detection device for a detectable response; and c) correlating the detectable response to the concentration of the predetermined ion in the test sample. The present invention also provides an analyte detection device to determine the presence or concentration of a predetermined ion in a liquid test sample comprising: a) a support strip; and b) a hydrophilic test pad operatively connected to the support strip said hydrophilic test pad comprising; c) a hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion; ii) a hydrophobic reporter substance capable of interacting with a complex of the hydrophobic ionophore and the predetermined ion to provide detectable response; and iii) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound. The present invention also provides a test pad for determining the presence of or the concentration of specific ion in a test sample, said test pad comprising a layer of hydrophilic indicator reagent composition, said hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with a specific ion to be determined; ii) a hydrophobic reporter substance capable of interacting with the complex of the hydrophobic ionophore and the specific ion to produce a detectable response; and iii) about 2 % to about 20 % by weight of the compositon of a hydrophilic polymer compound. The present invention also provides a method of manufacturing a film or layer of a hydrophilic indicator reagent composition for determining the presence or concentration of a predetermined ion in a liquid test sample, said method comprising the steps of: a) preparing a high pH aqueous solution of a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion and a hydrophobic reporter substance capable of interacting with the complex of the hydrophobic ionophore and a predetermined ion to provide a detectable response and then forming a second aqueous solution from the first aqueous solution and a hydrophilic polymer, and b) removing water from the second aqueous solution to provide a film or layer of composition, said composition containing about 2 % to about 20 % by weight of the hydrophilic polymer. A further object of the present invention is the use ot the present composition or of the present test pad for determining the presence or concentration of an electrolyte. Moreover there is described: a new and improved test device and method of determining the relative concentration of a predetermined chemical compound in a liquid. a simple, trustworthy, accurate and reproducible method of assaying plasma, serum or other aqueous test samples for a predetermined ion, or electrolyte. a new and improved electrolyte interactive test device for interaction with a predetermined electrolyte in a test fluid to produce a visible response, such as a change in color, indicative of the electrolyte concentration in the test fluid. a method of assaying plasma or serum of a whole blood sample, or other aqueous test samples, for a predetermined ion, or electrolyte, such as sodium or potassium. a method of assaying plasma, serum or other aqueous test samples that provides sufficient visual color differentiation and resolution, and therefore sufficient sensitivity, to low electrolyte concentrations. a method of assaying plasma, serum or other aqueous test samples that is sensitive to, and that quantitatively discriminates between, an electrolyte level of from 0 mMol/L to about 200 mMol/L. a method of assaying the serum, or plasma of a whole blood sample, or other aqueous test samples, that utilizes a test device including a test pad comprising a hydrophilic indicator reagent composition. method of assaying plasma, serum or other aqueous test samples by utilizing a hydrophilic indicator reagent composition that, when used as a film or a layer, or after incorporation into a suitable carrier matrix, can interact with a predetermined electrolyte and undergo a detectable or measurable color transition to establish the presence or to measure the concentration of the electrolyte in a test sample. a test device including a test pad, comprising a hydrophilic indicator reagent composition, either as a film or a layer, or incorporated into a carrier matrix, that can interact with a predetermined electrolyte and undergo a visually or instrumentally differentiable and resolvable color transition to allow the quantitative determination of the predetermined electrolyte concentration in the serum, plasma or other aqueous sample at levels between 0 mMol/L and about 200 mMol/L. a test device including a test pad, comprising a hydrophilic indicator reagent composition comprising an ionophore capable of forming a complex with the predetermined electrolyte of interest, a reporter substance capable of interacting with the ionophore-electrolyte complex to produce a detectable or measurable response, and a stabilizer compound, such as gelatin. a new and improved test device, and a method of manufacturing the test device, including a test pad comprising a hydrophilic indicator reagent composition capable of interacting with a predetermined chemical compound in an aqueous test sample. a method of assaying for a predetermined electrolyte by incorporating a hydrophilic indicator reagent composition into a carrier matrix to provide a dry phase detection device of sufficient sensitivity to allow the detection and measurement of a low concentration of the electrolyte. a new and improved test device and method of manufacturing the test device for sensing the presence of a predetermined electrolyte in an aqueous sample, wherein the electrolyte is capable of interacting with a test pad comprising a film or layer of a hydrophilic indicator reagent composition, said hydrophilic indicator reagent composition comprising an ionophore capable of forming a complex with the predetermined electrolyte, a reporter substance capable of interacting with the ionophore-electrolyte complex to produce a detectable or pleasurable response, and a stabilizer compound, such as gelatin. a new and improved test device for the accurate quantitative analysis of an electrolyte, like sodium, lithium, potassium, calcium or magnesium, in a test sample within about six minutes. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages and novel features of the present invention will become apparent from the following detailed description of the invention illustrated in the accompanying figure demonstrating the accurate and sensitive quantitative electrolyte assays achieved by the device and method of the present invention wherein: FIG. 1 includes a graph of potassium ion concentration in mMol/L (mM or millimoles per liter) vs. the optical density measured at 593nm (nanometers), and a graph of potassium ion concentration vs. the ratio of the optical density measured at 593nm to the optical density measured at 491.5nm, the isospestic point. DETAILED DESCRIPTION OF THE INVENTION In accordance with the method of the present invention, the qualitative or quantitative assay for a predetermined electrolyte, including sodium, potassium, lithium, magnesium or calcium, in serum, plasma and other aqueous test samples is accomplished by utilizing a test device including a test pad comprising a hydrophilic indicator reagent composition. By employing a test device of the present invention including a new and improved test pad, the color differentiation and color resolution of the color transitions provided by different concentrations of the electrolyte is sufficient to provide a sensitive and accurate assay for the electrolyte concentration in a test sample. In addition, the sensitivity of the assay is sufficient to allow the accurate determination of a low concentration of a predetermined electrolyte. The sensitivity to low electrolyte levels afforded by the method of the present invention is especially useful in plasma and serum assays. Prior to the present invention, a sensitive and accurate, ionophore-based dry phase test strip for a predetermined ion and absent a hydrophobic organic phase was unavailable. Although, as described above, Feinstein et al. purported to disclose the formation of the ionophore/ion complex in aqueous solution, the solutions disclosed therein included a high concentration of a crystalline ionophorous antibiotic that easily is separated from the solution by gentle centrifugation. Furthermore, even though the concentration of the electrolyte in solution was very high, the Feinstein et al. disclosure exhibited such a slight change in fluorescence in the clinical range for potassium, the primary ion of interest, that a person of ordinary skill in the art would consider the technique impractical and therefore not a viable avenue of investigation or research. In addition, Feinstein et al. state that no fluorescence changes were observed with synthetic "crown" ether ionophores. The use of a homogeneous aqueous test solution to assay for a predetermined electrolyte in a test sample is disclosed by Gibbons in U.S. Patent No. 4,820,647. The method disclosed by Gibbons is directed to the wet phase determination of a predetermined ion in solution by utilizing a solution of an ionophore, a reporter substance and a micelle-forming material. Furthermore, the method disclosed by Gibbons does not teach or suggest using a dry phase test strip including a test pad comprising a hydrophilic indicator reagent composition. Gibbons teaches that the micelle-forming material solubilizes the hydrophobic ionophore and reporter substances. The composition disclosed by Gibbons has not proved successful in dry phase test strips, wherein an organic hydrophobic phase is needed to mediate the ionophore-ion complexation and the resulting color transition. Therefore, most investigators continued to utilize the ability of an ionophore to transport a specific ion either into an organic liquid phase, or through an ion selective membrane, or into a solid hydrophobic environment. Researchers conducting investigations into the solid phase ion test strips also attempted, relatively unsuccessfully, to increase the reactivity of nonporous film formats. Therefore, present-day commercial assays for a predetermined electrolyte, such as potassium ion, are based on a colorimetric, ionophore-mediated, cationproton exchange reaction between an aqueous phase and an organic phase. The assay is based upon the quantitative measurement of a color transition produced when potassium ions in a diluted aqueous test sample are exchanged simultaneously for hydrogen ions from a reporter substance in the carrier matrix. The test pad of the present day electrolyte test device includes a carrier matrix containing the hydrophobic organic phase and the components of an aqueous phase, like a buffer. When an aqueous test sample is applied to the test pad, the components of the aqueous phase are solubilized and the aqueous phase is reconstituted. An ionophore selective for potassium ions, 2,3-(naphtho)-15-crown-5, present in the organic phase, mediates a transfer of potassium ions from the aqueous phase to the organic phase. Charge neutrality in the hydrophobic organic phase is maintained by the simultaneous loss of a proton from a reporter substance, the indicator dye, 7-decyl-MEDPIN, in the organic phase. Deprotonation of the dye results in the appearance of an absorption peak at 640nm. The overall reaction scheme is shown below: The degree and intensity of the color transition due to the above reaction is proportional to the amount of potassium ion in the test sample and is monitored by reflectance spectroscopy at 640nm with reflectance measurements taken at five second intervals. Results then are displayed digitally by the instrument in units of mMol/L (millimoles per liter). The present-day potassium ion test strip assay combines the convenience of dry phase chemistry with a simple, quick and highly-selective method of quantitatively determining the concentration of serum or plasma potassium. However, in accordance with the present invention, it has been found that conventional teachings concerning the requirement of an organic, or hydrophobic, phase in the test device for a successful ion/ionophore-based assay were erroneous. As a result, the present test device and method were found to provide a hydrophilic indicator reagent composition, used either as a film or a layer, or incorporated into a suitable hydrophilic carrier matrix, as the test pad of the test device. In addition, the dry phase test strips of the present invention are compatible with conventional analyzers, including diagnostic analyzers based on absorbence measurements of a dry phase test strip that contacts a body fluid sample. Therefore, in accordance with an important feature of the present invention, an accurate and reliable assay for a predetermined electrolyte in an aqueous test sample is provided by contacting the aqueous test sample, like a body fluid, with a test device including a test pad comprising a hydrophilic indicator reagent composition comprising an ionophore, a reporter substance and a stabilizer compound. Surprisingly and unexpectedly, the presence of a hydrophobic organic phase, such as an organic solvent or a polymer, is not required to achieve an accurate and reliable electrolyte assay. It should be noted that the term "hydrophilic" is defined as that physical property of a substance to have a strong or pronounced tendency to bind or absorb water. Therefore, the term hydrophilic includes those materials that swell or form reversible gels upon contact with water; that are wettable or permeable by water; or that form aqueous solutions. Accordingly, the hydrophilic indicator reagent composition of the present invention includes an ionophore. Ionophores are well known in the art, and, as organic molecules, generally exhibit minimal water solubility. The term "ionophore" includes organic molecules capable of selectively forming a complex with a particular ion to the substantial exclusion of forming complexes with other ions. For example the cyclic polyether 2,3-naphtho-1,4,7,10,13-pentaoxacyclopentadeca -2-ene, also known as 2,3-naphtho-15-crown-5, selectively interacts with potassium ions to form a cationic complex. Therefore, the term "ionophore" includes neutral, electron-rich compounds that are capable of complexing with cations. Accordingly, the term includes multidentate cyclic compounds that contain electron-rich donor atoms, like oxygen or nitrogen, in their cyclic chains. Such multidentate cyclic compounds can be monocyclic or polycyclic. Alternatively, the ionophore can be an open chain compound containing donor atoms. Thus, included in the term ionophore are monocyclic ion-specific compounds known as coronands; open chain ion-specific compounds known as podands; and antibiotic type ionophores such as valinomycin and macrotetralide actins. The coronand class, referred to as crown ethers, are particularly useful ionophores. However, all members of the class of compounds generally known as ionophores can be used on the method and device of the present invention. Numerous examples of ionophore/ion pairs can be found in the literature. For example, U.S. Patent Nos. 4,649,123 and 4,670,218, include a full discussion and several nonlimiting examples of the various types of ionophores that are useful in the present invention. In general, ionophores are capable of complexing with ions, usually a specific ion, and of transporting the ion through lipid membranes or into an organic phase as an ionophore/ion complex. This property, i.e. the ability to transport an ion into a hydrophobic phase, is used to distinguish ionophores as a class from the broader class of compounds known as "chelates". Chelates, while technically a broader classification, are commonly considered as charged, water-soluble molecules capable of complexing ions and solubilizing the ions in aqueous media. Examples of ionophores found especially useful in the present invention are listed in TABLE I. It should be understood that TABLE I is merely illustrative of nonlimiting examples of ionophores that are useful in the present invention, and that several other, unlisted ionophores known to workers skilled in the art also can be included in the hydrophilic indicator reagent composition. TABLE I also lists the cation most selectively complexed by each ionophore. It should be noted that an individual ionophore, although considered to specifically complex with a particular cation, also complexes with other cations to varying degrees. Structural formulas for some of the ionophores listed in TABLE I can be found in U.S. Patent No. 4,639,123. Although these specific ionophores can be used advantageously in the test device of the present invention, they are presented only as examples, and other ionophores, or mixtures thereof, also can be used as long as the ionophore has sufficient electrolyte specificity for the particular ion of interest. The hydrophilic indicator reagent composition of the present invention also includes a reporter substance. The reporter substance provides a detectable response upon formation of the ionophore/ion complex. A reporter substance usually is capable of interacting with an ionophore/ion complex to produce a color transition or other detectable response in the test device. In general, "interacting" means any mutual action between a reporter substance and an ionophore/ion complex that leads to a detectable response. For example, the interaction between an ionophore/ion complex and a reporter substance having a dissociable proton causes the reporter substance to lose a proton thus producing a detectable response. The expression "detectable response" is defined as a change in a test device that is capable of being perceived, either visually or instrumentally, and that can be correlated to the concentration of a specific ion in an aqueous test sample. Therefore, detectable responses include, but are not limited to, a change in color, fluorescence, reflectance, pH, chemiluminescence or infrared spectra. A colorimetric response is preferred, although a fluorometric response is useful with suitable measuring equipment. An example of a reporter substance interacting with an ionophore/ion complex and generating a detectable response is the change of p-nitrophenol from a colorless to a colored form by the ion/ionophore complex. If the ion of interest is present in the aqueous test sample, the reporter substance provides a detectable or measurable response by interacting with the ionophore/ion complex. The reporter substance can be a single compound that ionizes in response to the formation of the ionophore/ion complex, or the reporter substance can be a mixture of compounds that produce a detectable product when a reaction chain is triggered by the presence of the ion/ionophore complex. Thus, if the electrolyte of interest is not present in the test sample, no ionophore/ion complex results, and therefore the reporter substance remains unchanged and no detectable response is observed. Alternatively, when the particular ion of interest is present, the resulting ionophore/ion complex in turn interacts with the reporter substance and the reporter substance undergoes a detectable change. To achieve the full advantage of the present invention, the reporter substance is a neutral compound having a dissociable proton. Furthermore, the reporter substance is capable of dissociating upon interaction with the ionophore/ion complex to produce a detectable response. The preferred reporter substances can be referred to as having an acidic proton. Therefore, under suitable pH conditions, the proton of the reporter substance dissociates upon formation of the ionophore/ion complex to produce the detectable response. With the loss of the dissociable proton, the reporter substance has a negative electrical charge, thereby causing a change in the electron distribution in the reporter substance that produces the detectable response. The expression "reporter substance" includes both organic compounds having a dissociable proton, for example, phenolic compounds, like p-nitrophenol, that are relatively colorless in the nonionized state, but are colored upon loss of the dissociable proton; and organic fluorescent compounds that produce either more or less fluorescence upon a change in electron distribution. The reporter substance also can be a compound that can trigger a detectable response when combined with other compounds. For example, the change in electron distribution in the reporter substance, caused by interaction with the ionophore/ion complex, can in turn facilitate the interaction of the reporter substance with another component that would then produce a detectable response. It has been found that reporter substances that contain an aromatic hydroxyl moiety, i.e., a phenolic-type compound having a proton that can dissociate from the molecular to produce a detectable response, are preferred in the method of the present invention. Two classes of indophenol reporter substances of this type have been described in U.S. Patent Nos. 4,540,520 and 4,552,697. A more comprehensive discussion of suitable reporter substances that can be used in the method of the present invention can be found in U.S. Patent Nos. 4,649,123 and 4,670,218. However, other classes of reporter substances also are suitable for use in the present invention, including the triphenylmethanes, such as tetrabromophenolphthalein decyl ester (TBDE), and the fluoresceins, such as methyl eosin, fluorescein esters, 7-hydroxy coumarins, resorufins, pyren-3-ols, and flavones. Other suitable dyes include phenolphthalein, bromocresol purple, cresophthalein, chlorophenol red, tetrabromophenol blue, thymophthalein, 5-amino-eosin, eosin-5-maleamic acid, 5-(N-dodecanoyl)amino-eosin and acid blue #45. In general, reporter substances that have a dissociable proton are preferred for the detection or measurement of the presence or concentration of a cation in a test sample. Particularly preferred is the indophenol, 7-(n-decyl)-2-methyl-4-(3',5'-dichlorophen-4'-one)indophenol, referred to herein as 7-decyl MEDPIN. Other suitable reporter substances include 2-methyl-4-(3',5'-dichlorophen-4'-one)indonaphth-1-ol (MEDPIN), tetrabromophenolphthalein decyl ester (TBDE), and methyl(tetrabromo fluorescein) (methyl eosin). It should be noted that although these specific reporter substances have been used advantageously in the test device and method of the present invention, other reporter substances, or mixtures thereof, also can be used as long as the reporter substance changes color, or undergoes some other detectable change, in response to an interaction between the ion/ionophore complex and the reporter substance. The hydrophilic indicator reagent composition also includes a stabilizer compound, such as gelatin, in an amount of from about 2% to about 20% by weight of the composition. It is theorized that the stabilizer compound either helps maintain the homogeneous dispersion of the reporter substance and the ionophore in aqueous solution or helps solubilize these components in water. The hydrophobic organic phase utilized in the prior art dry phase test strips served to isolate the ionophore and the reporter substance from the aqueous phase produced by contact of the test strip with the test sample. The primary function of the hydrophobic organic phase is to increase the detectable response of the test device. It has been speculated that the hydrophobic phase interacts with the ionophore/ion complex to overcome the strong attraction of ions into the aqueous phase, and therefore to stabilize the ionophore/ion complex when the complex is formed in the hydrophobic phase. Surprisingly and unexpectedly, the hydrophilic stabilizer compound of the present invention may perform a function similar to the hydrophobic organic phase of the prior art test strips because the test strip of the present invention demonstrates a sufficiently detectable or measurable response to the predetermined ion of interest to allow the accurate and reliable assay of low concentrations of clinically significant ions. In selecting a hydrophilic stabilizer compound it is important that the substance or combination of substances selected to act as a stabilizer does not interfere with the interaction between the ionophore and the ion to form the ionophore/ion complex, and with the interaction between the compound ion complex and the reporter substance. However, as will be demonstrated more fully hereinafter, a person of ordinary skill in the art is able to choose from several substances or combinations of substances to provide a suitable hydrophilic stabilizer compound. In general, stabilizer compounds useful in the hydrophilic indicator reagent composition of the present invention are themselves hydrophilic. Useful stabilizer compounds include water-soluble or water-swellable polymers, either synthetic or naturally-occurring. In particular, a water-swellable polymer is a slightly crosslinked polymer capable of absorbing several times its weight in water. Suitable synthetic polymers include noncrosslinked and slightly crosslinked polymers, such as noncrosslinked and lightly crosslinked acrylates, poly(vinyl alcohol), copolymers of acrylic acid, carboxymethylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxymethylcellulose and silicone gums. The hydrophilic stabilizer compound also can be a naturally-occurring polymer, and preferably a naturally-occurring polymer that forms a gel when dissolved in water. Such naturally-occurring polymers include, but are not limited to, agarose, gelatin, carageenan, alginates, alginic acid, xanthan gums, guar gums, and similar polymers. All of these materials, synthetic and naturally-occurring, are water-soluble or water-swellable polymers that, in their dry state, exhibit a marked wettability by aqueous media. Surprisingly and unexpectedly, an aqueous composition including the ionophore, the reporter substance and the hydrophilic stabilizer compound can be formed into a film or a layer, or can be incorporated into a suitable carrier matrix, then dried to form a test pad of a test device to assay for the presence or concentration of a predetermined electrolyte in an aqueous test sample. However, the aqueous composition including the ionophore, the reporter substance and the stabilizer compound also can include optional ingredients such as buffers, surfactants, chelating agents, preservatives, photochemical stabilizers, light scattering centers and background dyes as long as the optional ingredients do not interfere with the production of a detectable change in response to the presence of an electrolyte. Therefore, in accordance with an important feature of the present invention, it is well within the experimental techniques of those skilled in the art of preparing test devices to design a device for the dry phase assay of an electrolyte in an aqueous test sample by varying the relative amounts and relative identity of the ionophore, reporter substance, stabilizer compound and any optional ingredients to provide a test pad that yields a detectable and differentiable color transition, such that a comparison, either visually or by instrument, to color standards derived from solutions having a standard electrolyte concentration is possible. For example, the reporter substance can provide a detectable response by loss of a proton-upon formation of the ionophore/ion complex. Therefore, a buffering compound can be included to maintain a pH that allows a proton loss from the reporter substance upon an interaction between the ionophore/ion complex and the reporter substance, and that inhibits proton loss from purely pH or other environmental effects. Generally the desired pH range for proton loss to occur because of an interaction between the ionophore/ion complex and the reporter substance is between about 5 to about 10, but the particular pH choice depends on the hydrophilic indicator reagent composition components, the test sample and the desired sensitivity of the assay. For example, at a higher pH, proton loss occurs more readily, and therefore a higher pH provides an apparent increase in reactivity. At a lower pH, the opposite is true. The desired pH range therefore provides a neutral, essentially completely protonated, reporter substance prior to contact of the hydrophilic indicator reagent composition with the ion of interest. With respect to the preferred colorimetric reporter substances that lose a proton upon interaction with an ionophore/ion complex, the chemical form of the reporter substance, either protonated or unprotonated, usually is evident because the color of the reporter substance changes radically upon conversion from the protonated to the nonprotonated form. The determination of a suitable pH to assay for the predetermined electrolyte of interest is a routine laboratory experiment well within the skill of those trained in the art. Therefore, to achieve a suitable pH for the assay of the electrolyte of interest a buffering compound can be included, if necessary, in the hydrophilic indicator reagent composition. The buffering compound can be any buffer, or combination of buffers, capable of providing a pH in the range of from about 5 to about 10. The buffering compound is chosen, and acts, to maintain the pH of the aqueous phase, produced by contact of the aqueous test sample with the dry phase test device, within a pH range that facilitates the interaction between the ionophore/ion complex and a reporter substance. The interaction of the ionophore/ion complex with the reporter substance causes the reporter substance to lose a dissociable proton and a detectable response results. The presence of a buffering compound in the aqueous phase allows the proton loss to occur in response to the predetermined electrolyte concentration of the test sample. Therefore, an accurate and sensitive electrolyte determination is provided. Suitable buffers include citrate, lactate, malonate, trichloracetate, sulfosalicylate, tartarate, phosphate, acetate, borate, bis[2-hydroxyethyl]-imino]tris[hydroxymethyl]methane, 1,3-bis(trishydroxymethyl)methylaminopropane, piperazine-N,N'-bis[2-ethanesulfonic acid], N-2-hydroethylpiperazine-N'-2-ethanesulfonic acid (HEPES), 4-morpholinoethanesulfonic acid, N,N-bis(2-hydroxyethyl)glycine, piperazine-N,N'-bis(2-hydroxypropane)sulfonic acid (POPSO), 2-([tris-(hydroxymethyl)methyl]amino)ethanesulfonic acid (TES), tris(hydroxymethyl)aminomethane, N-[2-acetamido]-2-iminodiacetic acid, N-2-hydroxyethylpiperazine-N'-3-propanesulfonic acid, 3-N-(tris-hydroxymethyl)methylamino-2-hydroxypropanesulfonic acid (TAPSO), tetramethylammonium borate, and tetramethylammonium phosphate. The preferred pH range for the assay of a particular electrolyte is related to the reporter substance. Therefore, the choice of buffer is determined by the particular reporter substance used in the electrolyte assay, and to some extent by the desired detectable response. For example, when 7-decyl MEDPIN is used as the reporter substance, the preferred pH range is from about 6 to about 8.5. However, when using a reporter substance that has a higher pKa than 7-decyl MEDPIN, a higher pH range is preferred. Similarly, when a reporter substance having a lower pKa than 7-decyl MEDPIN is used, a lower pH range is preferred. Furthermore, when the detectable response is a color change, the buffer can influence the degree of the detectable response, and a particular buffer can be chosen to provide an optimum color intensity. For example, the preferred pH range for the reporter substance, 7-decyl MEDPIN, is from about pH 6 to about 8.5, wherein the color change is detectable and differentiable color change from orange to blue. At a higher pH, like pH 8.5 to 10, the color transition is between shades of dark blue that are difficult to differentiate visually. At a lower pH, like pH 5 to 6, the color transition is between shades of pale yellow, that also are difficult to differentiate visually. It should be understood that both pH extremes (i.e. pH of from 5 to 10) can be used when an instrumental method of detection is used, however the optimum instrumental precision also occurs within the pH range of from about 6 to about 8.5. The hydrophilic indicator reagent composition also can include a chelating agent to remove interfering substances from the test sample. Although an ionophore usually is chosen because it selectively complexes with the predetermined electrolyte of interest, in some cases the presence of other cations interferes with the interaction of the ionophore with the predetermined electrolyte of interest. For example, body fluids normally contain many different cations, such as sodium ion (Na + ), potassium ion (K + ), calcium ion (Ca ++ ) and magnesium ion (Mg ++ ). Therefore, the presence of different cations could interfere with the assay for a particular ion, such as sodium, because the ionophore does not complex with sodium to the exclusion of complexing with any other cation. For example, a particular ionophore preferentially can bind to a sodium ion as opposed to a calcium ion in a ratio of approximately 4 to 1. Therefore, in a test sample having a ratio of sodium ion to calcium ion of less than 4 to 1, it may be necessary to prevent the interaction of calcium ions with the ionophore in order to ensure the proper correlation between sodium ion concentration in the test sample and the detectable response. Accordingly, a chelating agent can be included in the hydrophilic indicator reagent composition as an interferant removal compound to eliminate calcium ion interference. In particular, ethylenediaminetetraacetic acid (EDTA) and ethylene glycol bis(aminoethyl)tetraacetic acid are water-soluble compounds that form complexes with divalent cations, such as calcium ions. If EDTA is included in a hydrophilic indicator reagent composition designed to assay for sodium ion, upon contact with an aqueous test sample containing sodium and calcium ions, the EDTA preferentially complexes with the calcium ions. As a result, the complexed calcium ion is not available to substantially interfere with the formation of the ionophore/sodium ion compiex. Therefore, the resulting color change in the reporter substance is due to an interaction essentially only with the ionophore/sodium ion complex to give an accurate and reliable sodium ion assay. In addition, ionophores can be used to remove interfering cations if they are specific for the interfering cation and if they are water-soluble, or if they are chemically-modified to increase their water solubility without decreasing their ability to interact with the interfering cation. For example, an ionophore having aryl moieties can be modified by the addition of water solubilizing functional groups, such as sulfonate (-SO 3 H) groups, to the aryl moieties to increase the water solubility of the ionophore without decreasing the ability of the ionophore to interact with the interfering ion. In addition, a hydrophilic surfactant can be included in the hydrophilic indicator reagent composition. In some instances, the addition of a surfactant enhances the operation of the test device. For example, when an aqueous test sample is pipetted onto the test pad of the test device, the test sample can form a liquid bead on the surface of the test device. The addition of a hydrophilic surfactant to the hydrophilic indicator reagent composition can reduce the surface tension of the aqueous test sample thereby allowing the test sample to level out and efficiently and evenly contact the test pad of the test device. Of course, the surfactant must be chosen such that it does not interfere with the formation of the ionophore/ion complex or with the interaction between the ionophore/ion complex and the reporter substance to produce a detectable or measurable response. The addition of a surfactant to the hydrophilic indicator reagent composition is not always necessary. However, when a surfactant is added, suitable surfactants include, but are not limited to, nonionic surfactants, such as the nonoxyols, the octoxynols and polyoxyethylene ethers of fatty alcohols. Other surfactants, as are well known to those skilled in the art, such as anionic or cationic surfactants, also can be used provided that the surfactant does not interfere with the production of a detectable response to the predetermined electrolyte of interest. Similarly, other optional ingredients like preservatives or background dyes can be added to the hydrophilic indicator reagent composition of the present invention. As previously described, and as will be evident from the following examples and methods of preparing a test device of the present invention, the hydrophilic indicator reagent composition comprises an ionophore, a reporter substance and a stabilizer compound. These three components of the hydrophilic indicator reagent composition, plus any optional ingredients, are dissolved in water, or in water containing a small amount of a water-miscible solvent, such as lower alcohols, like methanol or ethanol; tetrahydrofuran; acetone; and like organic solvents. The concentration of the water-miscible organic solvent in the aqueous solution of the indicator reagent composition is generally less than about 5% by volume, and preferably less than about 4% by volume. Further, the organic solvent is evaporated after incorporating the aqueous solution of the indicator reagent composition into a carrier matrix, or after applying the aqueous solution of the indicator reagent composition, as a film or as a layer, onto a suitable substrate. In addition, the concentration of each of the components of the aqueous solution of the indicator reagent composition is not critical to the invention provided that the concentration of the ionophore, reporter substance and stabilizer compound each is sufficient to produce a detectable response to the predetermined electrolyte content of the test sample after evaporation of the water and any organic solvent present in the aqueous solution. For qualitative electrolyte assays, neither the concentration of the ionophore nor the concentration of the reporter substance is directly related to the concentration range of the ion to be determined. Determination of the optimum concentration of the components of the aqueous solution of the indicator reagent composition is within the ability of one skilled in the art of designing dry phase test strips. However, the following concentration ranges are provided to demonstrate the range wherein each component can be included in the aqueous solution of the hydrophilic indicator reagent composition. The concentration of the water-miscible organic solvent usually is kept to a minimum, such as less than about 5% by volume. The concentration of stabilizer compound can vary widely. Different stabilizer compounds have drastically different solubility, gelling and film-forming properties, therefore, the amount of stabilizer compound included in the aqueous solution depends upon the particular physical properties of the stabilizer compound and the particular physical properties desired in the test pad. The concentration of the optional surfactant is less than the critical micelle concentration for the surfactant because surfactant concentrations above this level tend to produce anomalous results. However, it has been found that, preferably, the ionophore is present in a molar excess over the reporter substance (i.e., a greater than 1:1 molar ratio of ionophore to reporter substance). It also has been found that the concentration of the ionophore can range from about 2 g /L to saturation. The stabilizer compound usually is present in concentrations ranging from about 10 g /L to about 100 g /L. The interferent-removing chelating agent, if included at all, is included at levels ranging from 0 g /L to about 30 g /L. The concentration of the chelating agent included in the hydrophilic indicator reagent composition usually is related to the concentration of the interfering ion in the test sample of interest. For example, in an assay for lithium ion, the concentration of chelating agent for calcium ion removal is much lower than the concentration of chelating agent required for sodium ion removal. Therefore, the concentration of chelating agent can vary widely, but it is expected that approximately equimolar quantities of the chelating agent to the interfering ion would be used. The useful and the preferred concentration ranges of the components of an aqueous solution of the hydrophilic indicator reagent composition used in a test pad of electrolyte test devices, using either instrumental or visual detection techniques, are given below. The concentration ranges usually are given in g /L, or alternatively, in millimolar quantities, to eliminate the differences in molecular weight of various ionophores. Concentrations are given in grams per liter of the aqueous solution of the hydrophilic indicator reagent composition. Useful Preferred Instrumental Detection Ionophore 2-200 g /L 5-100 g /L (6-600 mM) (15-30 mM) Reporter Substance 1-50 g /L 5-30 g /L (2-110 mM) (10-70 mM) Surfactant 0-3 g /L 0-2 g /L Buffer 0-1 M 0-0.8 M Chelating Agent 0-30 g /L 10-20 g/L Visual Detection Ionophore 2-50 g /L 5-15 g /L (3-80 mM) (7-25 mM) Reporter Substance .5-5 g /L 1.5-4 g /L (1-11 mM) (3-9 mM) Buffer 0-1 M 0-0.7 M Chelating Agent 0-30 g /L 0-20 g /L Surfactant 0-3 g /L 0-2 g /L To demonstrate the new and unexpected results achieved by the method of the present invention, an aqueous solution of the hydrophilic indicator reagent composition, including the ionophore 2,3-naphtho-15-crcwn-5, the reporter substance 7-decyl MEDPIN and the stabilizer compound gelatin, was prepared. This aqueous solution can be incorporated into a carrier matrix that, after drying, is used as a test pad in a dry phase test strip assay for the presence or concentration of a predetermined electrolyte in an aqueous test sample. Alternatively, a layer of the aqueous solution can be applied directly to a suitable substrate, then, after drying, the resulting film of the hydrophilic indicator reagent composition is used as a test pad in a dry phase test strip assay for the presence or concentration of a predetermined electrolyte in an aqueous test sample. In addition to preparing a test device including a hydrophilic, ion-interactive indicator reagent composition, it also was found that several methods of preparing an aqueous solution of the hydrophilic indicator reagent composition are available. However, it was found that the most straightforward methods, such as those used by Feinstein et al., did not produce a homogeneous aqueous solution of the hydrophilic indicator reagent composition. Neither dissolving the components of the hydrophilic indicator reagent composition directly in a buffer solution having a suitable pH; nor dissolving the components of the hydrophilic indicator reagent composition in a water-miscible organic solvent and then adjusting the pH with a buffer having the desired pH, thereby also diluting the organic solvent concentration to produce an aqueous solution containing less than about 5% by volume organic solvent, produced a homogeneous, aqueous solution capable either of incorporation into a suitable carrier matrix or of being applied as a film or layer to form a test pad for measuring the concentration of a predetermined ion at clinically significant levels. However, methods of producing a homogeneous aqueous solution of the hydrophilic indicator reagent composition were found and are fully-described below. For example, in one suitable method, the ionophore and the reporter substance are dissolved in a water-miscible organic solvent as the first step in preparing a homogeneous aqueous solution. After the ionophore and reporter substance are dissolved in a water-miscible organic solvent, the resulting organic solvent-based solution then is added, with stirring, to an aqueous, alkaline solution having a pH sufficiently high to cause dissociation of the dissociable proton from the reporter substance. For a colorimetric reporter substance, this proton dissociation usually is easily observed because the color of the dissociated form of the reporter substance is different from the color of the neutral form of the reporter substance. The pH of the high pH aqueous alkaline solution then is adjusted by adding an aliquot of the solution containing the dissociated reporter substance to an aqueous solution having a pH in the range wherein the reporter substance is in the neutral form. Consequently, the reporter substance now is present in the aqueous solution in the neutral form. The dissociation of the reporter substance can reoccur upon formation of a ionophore/ion complex resulting from contact between the test sample and the hydrophilic indicator reagent composition. The presence of a sufficient amount of the buffering compound prohibits proton dissociation due to the pH of the test sample. It should be noted that the water-miscible organic solvent can be chosen from any of the solvents that dissolve the ionophore and the reporter substance in the desired concentrations. For example, tetrahydrofuran (THF), acetone, cyclohexanone, methanol and ethanol have been used successfully. However, THF is the preferred organic solvent. In preparing the aqueous solution of the indicator reagent composition, the amount and speed of stirring can be important because the ionophore and the reporter substance have limited solubility in water. Occasionally, in each of the methods disclosed herein, some particulate matter remains in the aqueous solution of the hydrophilic indicator reagent composition. Often the amount of particular matter is sufficiently small that it can be ignored. However, any incompletely dissolved particulate matter found to be the ionophoce or the reporter substance can be removed from the aqueous solution by mild centrifugation, i.e. 0.1X10 6 g force (g-mins) applied for one minute. The resulting homogeneous aqueous solution then is thoroughly admixed with an aqueous solution of the stabilizer compound, such as an aqueous solution of gelatin. The homogeneous aqueous solution of the indicator reagent composition is available for incorporation into a suitable carrier matrix, or for application to a suitable substrate as a film or layer, such that, after drying a test pad of the test device is provided. In accordance with an important feature of the present invention, the majority of the ionophore and the reporter substance remain in aqueous solution even after prolonged centrifugation (1 × 10 6 g force [g-mins]). Similar experiments carried out with an aqueous solution made by the method of Feinstein et al. produced nonreactive aqueous solutions after only very mild centrifugation (i. e., 0.01 × 10 6 g-force [g-mins]), showing that the ionophore probably is present in particulate form, and therefore is easily centrifuged from the aqueous solution. Another method of preparing an aqueous solution of the hydrophilic indicator reagent composition eliminates solubilizing the ionophore and the reporter substance in a water-miscible organic solvent. The ionophore and the reporter substance are dissolved directly, with stirring, in an alkaline acueous solution of sufficiently high pH to dissociate the proton of the reporter substance. Then the pH of the resulting alkaline solution including the ionophore and the reporter substance is adjusted to the desired pH range with an aqueous buffer solution. The pH adjusted solution contains a decreased amount of ionophore and reporter substance because of dilution. Therefore, the concentration of the ionophore and the reporter substance initially added to the alkaline aqueous solution should be sufficiently high such that the desired concentration of each ingredient is attained upon dilution with the buffer solution to attain the final pH, and upon further dilution with the aqueous stabilizer compound solution. After forming the homogeneous aqueous solution containing the ionophore and the reporter substance, the aqueous solution is admixed with an aqueous solution of the stabilizer compound, such as an aqueous solution of a water-soluble polymer, like gelatin, to form an aqueous solution of the hydrophilic indicator reagent composition of the present invention. It has been theorized, although not relied upon herein, that the stabilizer ingredient further promotes the solubilization of the ionophore and the reporter substance. It has been found that each of the above methods produced a homogeneous aqueous solution of a hydrophilic indicator reagent composition that, after evaporation of the aqueous, and, if present, organic, solvent to form a test pad, is capable of interacting with a predetermined ion that specifically complexes to the ionophore present in the aqueous solution. This interaction leads to a detectable or measurable response, such as a color transition, that is correlated to the amount of the predetermined ion in a test sample. As previously stated, the above methods inherently decrease the concentration of the ionophore and the reporter substance initially added to the aqueous or organic solvent. Therefore, this dilution effect should be considered when the concentration of the ionophore and of the reporter substance initially added to the aqueous or organic solvent is calculated. Preparing an aqueous solution having sufficient ionophore and reporter substance concentrations for forming a suitable test pad for the assay of a predetermined cation of clinical interest can be found in the following general method and specific Example. In general, a homogeneous aqueous solution of a hydrophilic indicator reagent composition used to form a test pad of a dry phase test strip to determine the presence and concentration of an electrolyte in an aqueous solution can be achieved by the following method: (a) Dissolve an ionophore and a neutral, protonated reporter substance in a suitable water-miscible organic solvent to produce a homogeneous, organic solvent-based solution; (b) Add the organic solvent-based solution, dropwise, to a vortexed aqueous alkaline solution to produce a high pH aqueous solution wherein the reporter substance is present in the deprotonated form; and (c) Add the high pH aqueous solution including the reporter substance essentially in the deprotonated form to an aqueous solution of the stabilizer compound; (d) Optionally, add the resulting aqueous, high pH solution including the ionophore, the reporter substance and the stabilizer compound to a buffer solution to produce a homogeneous aqueous solution having a pH wherein the reporter substance is essentially in the neutral, protonated form. The aqueous solution resulting from steps (a) through (c), or from steps (a) through (d), of the above-described general method then can be applied as a film to a suitable substrate, or can be incorporated into a suitable carrier matrix, then dried to produce a test pad capable of detecting and measuring a particular electrolyte. It also should be understood that step (a) of the above-described general method can be omitted. Then, the ionophore and the neutral, protonated reporter then are dissolved directly in the aqueous alkaline solution (step (b)). Example 1 specifically demonstrates the formation of an aqueous solution of a hydrophilic indicator reagent composition that can be incorporated into a suitable carrier matrix, or applied as a film to a suitable substrate, then dried and used as a test pad to detect and measure potassium ion concentrations in solution. Example 1 Aqueous Solution of A Hydrophilic Indicator Reagent Composition For Detecting And Measuring Potassium Ion Concentrations A homogeneous aqueous solution suitable for forming a test pad of a test device used to assay for potassium ions was prepared according to the procedure described in the general method. In this particular aqueous solution, 2,3-naphtho-15-crown-5 was used as the ionophore; 7-decyl-MEDPIN was used as the reporter substance; and gelatin was used as the stabilizer compound. In particular, a homogeneous organic solvent-based solution first was prepared by dissolving a sufficient amount of the 2,3-naphtho-15-crown-5 and of the 7-decyl-MEDPIN in tetrahydrofuran (THF) such that each component was present at a concentration of 200 mM. Then, to 5 ml of a 50 mM aqueous solution of lithium hydroxide-was added 500 µL (microliters) of the homogeneous organic solvent-based solution of the ionophore and the reporter substance, with vortexing. The resulting aqueous solution was blue in color, and the ionophore and the reporter substance present at concentrations 1/10 of their original concentration. The resulting blue, aqueous solution then was mixed with 1.67 ml of a 20% by weight aqueous gelatin solution to produce another clear blue solution than can be incorporated into a suitable carrier matrix or applied onto a suitable substrate as a film. Optionally, the clear blue solution, including the gelatin, can be adjusted to a buffered pH of about 6.5 by adding an aqueous buffer solution containing 250 mM of the buffer HEPES. The resulting buffered solution, is reddishorange in color and contains 14 mM ionophore, 14 mM reporter substance, 200 mM buffer and 4.7% by weight stabilizer compound. This buffered solution of a hydrophilic indicator reagent composition can be incorporated into a suitable carrier matrix or applied to a suitable substrate as a film, then dried, to form a test pad for the determination of potassium ion concentration in a test sample. The composition of EX. 1 also is prepared by adding a sufficient amount of reporter substance and a sufficient amount of ionophore directly to a 50 mM solution of lithium hydroxide to provide an aqueous solution including 20 mM each of reporter substance and ionophore. As stated above, the aqueous solution of the hydrophilic indicator reagent composition can be used to form the test pad of the test device. The test pad of the present invention can be produced by one of two methods. In the first method, the aqueous solution of the hydrophilic indicator reagent composition is applied as a film, such as with a Meyer rod, directly to a suitable substrate. The substrate should be sufficiently hydrophilic, or, if hydrophobic, possess a sufficiently rough or sufficiently treated surface, such that the hydrophilic indicator reagent composition can sufficiently adhere to the substrate surface. The wet film then is dried to form a test pad of the hydrophilic indicator reagent composition comprising the ionophore, the reporter substance and the stabilizer compound. Alternatively, the aqueous solution of the hydrophilic indicator reagent composition first is incorporated into a suitable bibulous or nonbibulous carrier matrix, like filter paper, glass fibers, or a layer or film of a polymeric material. Then, the carrier matrix incorporating the hydrophilic indicator reagent composition is dried to yield a test pad for assaying an aqueous test sample for a predetermined electrolyte concentration. More particularly, in one embodiment of the present invention, the aqueous solution of the hydrophilic indicator reagent composition of Example 1 was coated, such as stripe-coated, directly onto a plastic substrate, such as polystyrene, with a #75 Meyer rod to provide a film having a thickness of 6.75x10 -3 inches. After removing the aqueous solvent, and any organic solvent, by drying in an air oven at about 50°C for about 20 to 30 minutes, the dried film of the hydrophilic indicator reagent composition served as the test pad of a test device for a predetermined electrolyte, i.e., potassium, in an aqueous test sample. Alternatively, the aqueous composition can be applied to a film support, like a polyester film, that can accept gelatin. Such a polyester film is available commercially from 3M Co., St. Paul, Minn. under the brandname to 40 GAB 25. In either case, the test device is a test strip, designed either as a single pad test strip (to assay only for a single analyte) or as a multiple pad test strip (to assay for several analytes simultaneously), including a support strip, or handle, normally constructed from a hydrophobic plastic, and a test pad comprising the hydrophilic indicator reagent composition. The handle usually is formed from hydrophobic materials such as cellulose acetate, polyethylene terephthalate, polycarbonate or polystyrene. The aqueous solution of the hydrophilic indicator reagent composition can be coated onto a suitable substrate, then dried to form a test pad on the suitable substrate. This test pad and substrate can be secured to the handle by a double-sided adhesive. Preferably, the aqueous solution of the hydrophilic indicator reagent composition initially is applied to a handle of suitable construction as a stripe-coated wet film. Drying the aqueous solution of the hydrophilic indicator reagent composition forms the test pad of the present invention. In this embodiment, the test pad is formed directly on the handle of the test device. The handle of the test device can be opaque, or it can transmit light or other energy. When the detectable response is fluorescence, or if an optional layer is placed over the upper surface of the test pad of the test device to allow any excess test sample to be wiped off, the test device can be read through a transparent support material. A transparent support material is especially useful in the assay of whole blood. The optional layer over the upper surface of the test pad can be used to filter, or otherwise separate, the cellular components of the whole blood sample from the serum or plasma. The serum or plasma then can migrate to the test pad to interact with the hydrophilic indicator reagent composition, and bottom surface of the transparent handle can be examined for a detectable response. This particular test device allows a small whole blood sample to be assayed without the need to separate the cellular components from the serum or plasma prior to the assay. Therefore, preferred handles include transparent materials capable of transmitting electromagnetic radiation of a wavelength in the range of about 200 nanometers (nm) to about 900nm. The support need not transmit over the entire 200-900nm region, although for fluorometric detection it is desirable that the support be transparent over a band wider than, or at least equal to, the absorption and emission spectra of the fluorescent materials used for detection. It also may be desirable to have a support that transmits one or more narrow wavelength bands and is opaque to adjacent wavelength bands. This could be accomplished, for example, by impregnating or coating the support with one or more colorants having suitable absorption characteristics. In another embodiment of the present invention, the aqueous solution of the hydrophilic indicator reagent composition of Example 1 first is incorporated into a suitable-hydrophilic carrier matrix. The carrier matrix incorporating the hydrophilic indicator reagent composition then is dried to form a test pad of the present invention. The test pad then is secured to a plastic handle of the same construction as described above to produce a test device of the present invention. Similarly, an optional coating can be applied above the test pad to separate the cellular material from a whole blood sample. In this embodiment of the present invention, the hydrophilic carrier matrix is an absorbent material that allows the hydrophilic indicator reagent composition to move, in response to capillary forces, through the matrix to homogeneously incorporate the hydrophilic indicator reagent composition into the matrix. Subsequent contact with a test sample produces a detectable or measurable color transition in response to the predetermined electrolyte concentration of the test sample. The carrier matrix can be any substance capable of incorporating the chemical reagents required to perform the assay of interest, as long as the carrier matrix is substantially inert with respect to the chemical reagents, and is porous or absorbent relative to the liquid test sample. The expression "carrier matrix" refers to either bibulous or nonbibulous matrices that are insoluble in water and other physiological fluids and maintain their structural integrity when exposed to water and other physiological fluids. Suitable bibulous matrices include filter paper, sponge materials, cellulose, wood, woven and nonwoven fabrices and the like. Nonbibulous matrices include glass fiber, sintered ceramic frits, polymeric films, and preformed or microporous membranes. Other suitable carrier matrices include hydrophilic inorganic powders, such as silica gel, alumina, diatomaceous earth and the like; argillaceous substances; cloth; hydrophilic natural polymeric materials, particularly cellulosic material, like cellulosic beads, and especially fiber-containing papers such as filter paper or chromatographic paper; synthetic or modified naturally-occuring polymers, such as cellulose acetate, polyacrylamide, polyacrylates, polyurethanes, crosslinked dextran, agarose and other such crosslinked and noncrosslinked water-insoluble hydrophilic polymers. Hydrophobic and nonabsorptive substances are not suitable for use as the carrier matrix of the present invention. The carrier matrix can be of different chemical compositions or a mixture of chemical compositions. The matrix also can vary in regards to smoothness and roughness combined with hardness and softness. However, in every instance, the carrier matrix should include a hydrophilic or absorptive material. In accordance with this embodiment of the present invention, to perform a dry phase, test strip assay for an electrolyte, an aqueous solution of the hydrophilic indicator reagent composition of Example 1 first is prepared. A suitable carrier matrix, such as filter paper, then is saturated with the aqueous solution of the hydrophilic indicator reagent composition either by spreading, by immersing or by spraying the aqueous solution onto sheets or precut strips of the filter paper. After removing the aqueous solvent by oven drying in an air oven at about 50°C for about 20 to 30 minutes, the filter paper incorporating the hydrophilic indicator reagent composition is cut to an appropriate size, such as a pad having dimensions from about 0.25 cm by about 0.5 cm to about 0.5 cm by about 1.0 cm. The filter paper incorporating the hydrophilic indicator reagent composition then is secured to an opaque or transparent hydrophobic plastic handle with double-sided adhesive tape, available from 3M Company, St. Paul, MN. The devices of the present invention can be adapted to carry out a wide variety of chemical analyses, not only in the field of clinical chemistry, but in chemical research and chemical process control laboratories. The devices are especially well-suited for use in clinical testing of body fluids, such as blood, plasma, serum, cerebrospinal fluid, and urine, because a large number of repetitive assays are frequently conducted and assay results often are needed within a very short time. In regard to the analysis of blood, the present invention can be adapted for use in performing a quantitative analysis for blood electrolytes of clinical interest, such as potassium, sodium, magnesium or calcium. The present invention provides a particularly useful test for the determination of serum potassium wherein the assay for low concentrations of potassium requires measurements of high sensitivity and high precision. Similarly, the present invention provides a very useful test for the determination of serum lithium in individuals undergoing a lithium treatment for depression. Such a lithium test is extremely important because the clinically useful dose of lithium is close to a toxic dose of lithium, therefore lithium levels in the blood must be carefully monitored. The test device is used by contacting the test device with a test sample for a sufficient period of time. In the case of whole blood or urine testing, merely dipping the test device into the sample is sufficient. Although it is usually unnecessary to remove excess sample, in some cases, such as with whole blood samples, any excess test sample, or the cellular components of the whole blood sample separated from the plasma or serum by an optional coating layer over the test pad, is removed by wiping or blotting. If the predetermined ion of interest is present in the test sample, the resulting ionophore/ion complex interacts with the reporter substance, and the response, like a color transition, is detected and measured. When the reporter substance is a proton-dissociable compound, wherein a dissociated compound having a different color from the undissociated compound is produced, an observable color change forms in the test device that can be visually or instrumentally monitored. If a transparent handle is used, the observable color change can be monitored from either side of the device. In the case of a whole blood sample, the observable color change is monitored from the bottom surface of the transparent handle. If the reporter substance is a fluorophore, such as fluorescein or a fluorescein derivative, a fluorescence spectrophotometer can be utilized to measure the detectable response formed in the test device (i.e., the appearance of or the change in fluorescence). Other techniques useful in observing a detectable response include reflectance spectrophotometry, absorption spectrophotometry and light transmission measurements. Various calibration techniques are applicable as a control for the assay of the predetermined assay of interest. For example, a sample of a standard solution of a particular electrolyte can be applied to a separate test device as a comparison or to permit the use of differential measurements in the analysis. Test devices can be designed that are suitable for quantitative visual determinations when an appropriate color chart is supplied. In accordance with either embodiment of the present invention, the test strip was dipped into an aqueous test sample for a sufficient time to insure sufficient contact of the test pad with the test sample. After waiting a predetermined time, such as from about 30 seconds to 360 seconds, the test strip is examined, either visually or by instrument, for a detectable response. The color transition, if any, of the test pad reveals the presence or concentration of the predetermined electrolyte of interest in the aqueous test sample. Analogous to designing the aqueous solution of the hydrophilic indicator reagent composition as described above, it is well within the experimental techniques of those skilled in the art of preparing test devices to determine the proper balance between size of the test pad, the strength of aqueous solution of the hydrophilic indicator reagent composition, the amount of test sample, and the method of introducing the test sample to the test strip, such as by pipetting rather than dipping, in order to design a quantitative assay for a predetermined electrolyte utilizing the method and test device of the present invention. In many cases, such as in qualitative assays, simple visual observation of the test strip provides the desired information. If more accurate information is required, a spectrophotometer or colorimeter can be used to more precisely determine the intensity and degree of the color transition. In addition, the dry phase test strip assay can be made quantitative by employing spectrophotometric or colorimetric techniques, as opposed to visual techniques, in order to more reliably and more accurately measure the intensity and degree of the color transition, and therefore more accurately measure the concentration of a predetermined electrolyte in the aqueous test sample, especially at the lower, clinically-significant electrolyte concentrations, such as below 10 mM, and especially below about 2 mM. To demonstrate the utility of the present invention in the assay of a predetermined electrolyte, the reddish-orange, buffered aqueous solution of the hydrophilic indicator reagent composition of Example 1 was applied directly, as a stripe-coated film, to a hydrophobic plastic handle, constructed of GEL BOND, available from Marine Colloids, with a #75 Meyer rod. The wet film was allowed to dry in an air oven, as described above, to form a test pad directly on the plastic handle. The resulting test strip was used to assay standardized aqueous solutions containing known amounts of potassium ion. As demonstrated in FIG. 1, standardized aqueous solutions containing potassium ions ranging in concentration of from 0 mM to 2.0 mM were assayed for potassium ion concentration with test devices of the present invention. After allowing the test device to contact the standardized solution for about 30 sec., the color transition response was detected and measured by normal spectroscopic transmission techniques. FIG. 1 shows that the plot of absorbence (optical density or OD) at 593nm (the blue form) vs. potassium ion concentration is essentially linear, especially over the concentration range of 0 mM to about 1.75 mM. Therefore, the unknown potassium ion concentration of a test sample can be determined from this graph of absorption (OD) vs. potassium ion concentration for known standard solutions. The second plot of FIG. 1 is a plot of potassium ion concentration of standardized solutions vs. the ratio of the absorbence of the test strip at 593nm to the absorbence of the test strip at 491.5nm, the isospestic point. The isospestic point is defined as the wavelength wherein the extinction coefficient for both the protonated form and the deprotonated form of the indicator are identical. This plot corrects variations in test pad film thickness. This plot also is essentially linear, especially over the potassium concentration range of 0 mM to about 1.75 mM, therefore allowing a more accurate determination of potassium ion concentration in a test sample. In regard to the assay of a whole blood sample for a predetermined electrolyte, five cations having particular clinical significance can be assayed by the method and device of the present invention. However, it should be understood that other cations present in a whole blood sample, and in general other cations present in other test samples, also can be assayed by the method and device of the present invention by the selection of a suitable ionophore and a suitable reporter substance. This selection is easily performed by a person of ordinary skill in the art of designed diagnostic test strips. The five cations, potassium, sodium, lithium, magnesium and calcium, are present in the blood in different concentrations. For example, sodium generally is present in the blood in amounts ranging from 120 mM to 170 mM, and normally in the range of from about 135 mM to about 155 mM. Likewise, potassium generally is present in amounts ranging from about 2 mM to about 10 mM, and normally from about 3.5 mM to about 5.5. The normal ranges for magnesium and calcium in the blood are about 0.5 mM to about 1.5 mM, and about 2 mM to about 3 mM respectively. Lithium normally is not found in the blood, however an individual undergoing lithium therapy for depression should maintain a serum lithium level of from about 0.6 mM to about 1.2 mM. The device and method of the present invention can assay for each of these ions in the clinically-significant concentration ranges without diluting the blood sample. An undiluted blood sample especially can be used to assay for magnesium and lithium because these ions are present in such low amounts. Furthermore, the linearity of the graphs presented in FIG. 1 over the range of from 0 mM to about 1.75 mM shows that accurate and sensitive assays for these ions can be achieved. Assays for calcium, potassium and sodium also can be performed on undiluted blood samples, but because of the greater concentration of these ions in the blood and because of the linearity of the test method demonstrated in the range of from 0 mM to 1.75 mM of ion, the assays for these ions also can be performed on diluted whole blood, diluted serum or diluted plasma samples. For example, in the assay for calcium, the neat, undiluted whole blood sample can be assayed, or the sample can be diluted with up to two parts of water per part of whole blood sample. This dilution rate provides a diluted blood sample having a calcium level of about 1 mM calcium. Similarly, in an assay of whole blood for potassium ions, the whole blood sample can be diluted with 5 parts of water per part of blood sample to provide a sample including about 1 mM of potassium ions. Finally, in the assay for sodium, diluting the whole blood sample with up to 50 times with water provides a test sample including about 1 mM in sodium ions. The above dilution ratios for the different cations provide a test sample having a sufficient amount of ion for detection and an amount of ion in the concentration region of FIG. 1 showing the most accurate response of the hydrophilic indicator reagent composition to the predetermined ion of interest, i.e. from 0 mM to about 1.75 mM of ion in the test sample. The composition method and test device of the present invention therefore afford an economical, accurate and reliable assay for the concentration of a predetermined electrolyte in an aqueous test sample. The assay can be performed at home or in the laboratory. In addition, the method of the present invention allows detection, differentiation and measurement of low electrolyte concentrations in the test sample, such as from 0 mM to about 10 mM, therefore making the assay more useful clinically. Moreover, the test device of the present invention, by avoiding the use of an organic hydrophobic phase, eliminates a costly and time-consuming step in the manufacture of the dry phase test strip. More importantly, the composition method and test device of the present invention allow the detection and measurement of the concentration of a predetermined electrolyte by simple and accurate absorption/transmission measurements on a single layer test device. As a result, the costly and time-consuming manipulative steps that are required to manufacture multilayer test devices can be avoided. The single layer test device thereby allows, a simple dip-and-read assay procedure, as opposed to the prior art electrolyte assays using a hydrophobic phase that utilized less accurate reflectance techniques to assay for electrolyte concentration in aqueous test samples. However, the method and test device of the present invention also can be used to assay for whole blood when a layer of suitable material is positioned over the test strip to filter or otherwise separate the cellular components of the whole blood sample from the plasma or serum. The composition method and test device of the present invention provide rapid assay results with a detectable response exhibited within a few minutes. Cumbersome and expensive testing equipment is not required. In addition, the detectable response is of sufficient magnitude to enable direct visual detection. Moreover, it has been found that the color or other detectable response produced in the test device is usually stable, in some instances for a period of days, such that a number of used test devices can be set aside for reading at some future time. Therefore, in accordance with an important feature of the present invention, more accurate and reliable assays for the concentration of a predetermined electrolyte in whole blood, plasma, serum and other aqueous test samples can be performed by utilizing a hydrophilic indicator reagent composition. The hydrophilic indicator reagent composition provides sufficient color resolution to perform an accurate assay and improves assay sensitivity by allowing the electrolyte concentration to be determined by absorbence/transmission detection techniques as opposed to reflectance techniques. The use of absorbence/transmission techniques also allows the use of a single layer test device that is more economical to manufacture than the multilayer test strips required for reflectance measurements.

Claims: A hydrophilic indicator reagent composition for determining the presence or concentration of an electrolyte, which composition consists essentially of a film or layer of: a) a hydrophobic ion-specific ionophore capable of forming a complex with a specific ion to be determined, b) a hydrophobic reporter substance capable of interacting with said complex of said ionophore and a specific ion to be determined to produce a detectable response, and c) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound. A method of determining the presence or concentration of a predetermined ion in a test sample comprising: a) contacting the test sample with an analyte detection device, wherein said analyte detection device includes a hydrophilic test pad impregnated with a homogenous hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion; ii) a hydrophobic reporter substance capable of interacting with said complex formed by the ionophore and the predetermined ion to provide a detectable response; and iii) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound; b) examining the analyte detection device for a detectable response; and c) correlating the detectable response to the concentration of the predetermined ion in the test sample. The method of claim 2 wherein the hydrophobic reporter substance is selected from the group consisting of a phenol, an indophenol compound, a triphenylmethane, a fluorescein, a fluorescein ester, a 7-hydroxy coumarin, a resorufin, a pyren-3-ol, a flavone and combinations thereof. An analyte detection device to determine the presence or concentration of a predetermined ion in a liquid test sample comprising: a) a support strip; and b) a hydrophilic test pad operatively connected to the support strip said hydrophilic test pad comprising; c) a hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion; ii) a hydrophobic reporter substance capable of interacting with a complex of the hydrophobic ionophore and the predetermined ion to provide a detectable response; and iii) about 2 % to about 20 % by weight of the composition of a hydrophilic polymer compound. The analyte detection device of claim 4 wherein the hydrophilic test pad indicator reagent composition further comprises a buffer capable of providing a pH in the range of from about 5 to about 10, a chelating agent, a hydrophilic surfactant or a combination thereof. A test pad for determining the presence of or the concentration of a specific ion in a test sample, said test pad comprising a layer of hydrophilic indicator reagent composition, said hydrophilic indicator reagent composition consisting essentially of: i) a hydrophobic ion-specific ionophore capable of forming a complex with a specific ion to be determined; ii) a hydrophobic reporter substance capable of interacting with the complex of the hydrophobic ionophore and the specific ion to produce a detectable response; and iii) about 2 % to about 20 % by weight of the compositon of a hydrophilic polymer compound. The test pad of claim 6 wherein the reporter substance includes a dissociable proton. The test pad of claim 6 wherein the hydrophilic polymer is selected from the group consisting of agarose, gelatin, carageenan, an alginate, alginic acid, a xanthan gum, a guar gum, an acrylate, a poly(vinylalcohol), a copolymer of acrylic acid, carboxymethylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxymethylcellulose and combinations thereof. A method of manufacturing a film or layer of a hydrophilic indicator reagent composition for determining the presence or concentration of a predetermined ion in a liquid test sample, said method comprising the steps of: a) preparing a high pH aqueous alkaline solution of a hydrophobic ion-specific ionophore capable of forming a complex with the predetermined ion and a hydrophobic reporter substance capable of interacting with the complex of the hydrophobic ionophore and a predetermined ion to provide a detectable response and then forming a second aqueous solution from the first aqueous solution and a hydrophilic polymer, and b) removing water from the second aqueous solution to provide a film or layer of composition, said composition containing about 2 % to about 20 % by weight of the hydrophilic polymer. Use of the composition of claim 1, of the detection device of claims 4 or 5, or of the test pad of any of claims 6 to 8 for determining the presence or concentration of an electrolyte.

Inventors: CHARLTON STEVEN C, CHARLTON, STEVEN C.
Applicants: BAYER AG, BAYER CORPORATION
Application year: 1991
Main classification: G01N33
IPC codes: G01N33, G01N31