Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/327—Biochemical electrodes, e.g. electrical or mechanical details for in vitro measurements
  • G01N27/3271—Amperometric enzyme electrodes for analytes in body fluids, e.g. glucose in blood
  • G01N27/3274—Corrective measures, e.g. error detection, compensation for temperature or hematocrit, calibration
• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
  • C12Q—MEASURING OR TESTING PROCESSES INVOLVING ENZYMES, NUCLEIC ACIDS OR MICROORGANISMS; COMPOSITIONS OR TEST PAPERS THEREFOR; PROCESSES OF PREPARING SUCH COMPOSITIONS; CONDITION-RESPONSIVE CONTROL IN MICROBIOLOGICAL OR ENZYMOLOGICAL PROCESSES
  • C12Q1/00—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions
  • C12Q1/001—Enzyme electrodes
  • C12Q1/004—Enzyme electrodes mediator-assisted
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/327—Biochemical electrodes, e.g. electrical or mechanical details for in vitro measurements
  • G01N27/3271—Amperometric enzyme electrodes for analytes in body fluids, e.g. glucose in blood
  • G01N27/3272—Test elements therefor, i.e. disposable laminated substrates with electrodes, reagent and channels
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/50—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing
  • G01N33/62—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving urea
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/49002—Electrical device making

Definitions

• the present invention relates generally to biosensor tests systems, and more particularly to systems, compositions, and methods for protecting the dry-film reagent of test element from degradation by atmospheric humidity or water vapor.
• Biosensors are available to permit an individual to test the glucose level in a small sample of blood.
• Common meter designs use a disposable test element which, in combination with the meter, measures the amount of glucose in the blood sample electrochemically or optically.
• the information is typically displayed as a blood glucose value and perhaps the time and date the measurement was performed. This information is in most cases sufficient to allow diabetics to adjust their dietary intake and/or insulin dosage, and in the case of low glucose values may indicate the need for intake of sugar to avoid hypoglycemia.
• the electrochemical measurement of an analyte such as glucose may be achieved by dosing a sensor with a sample containing the analyte (for example, glucose in an aqueous blood sample) to initiate a chain of reactions such as the chain shown below for glucose.
• a sample containing the analyte for example, glucose in an aqueous blood sample
• the reduced form of the mediator is converted to the oxidized form.
• the current associated with this reaction is proportional with the mass of reduced mediator, and consequently with the glucose
• biosensor test elements such as glucose test strips
• Exposure of biosensor test elements (such as glucose test strips) to high humidity conditions may cause the mediator to degrade to the reduced form or to other products that are electrochemically active in the same potential range. Additionally, high humidity may cause the enzyme to degrade to a reduced form, and the reduced enzyme may react with the mediator, producing mediator in its reduced form.
• test strip reagent matrix The components of a test strip reagent matrix are often selected to dissolve quickly in an aqueous blood sample, thus providing a fast reaction time and a quick display of the result for the user.
• this provides the undesired consequence of making the matrix susceptible to degradation by environmental humidity, thus compromising the shelf stability of disposable test strips.
• test elements can be packaged and distributed in containers configured to prevent undesired ingress of moisture from the atmosphere or other sources, and while test elements can be packaged in containers with desiccant materials incorporated within the container (such as in the lid or cap of the container) or in an insert that is left in the container with the test elements, known methods have not been effective for eliminating the risks of sensor degradation.
• the present invention addresses that need.
• test element for determining the amount of an analyte in a sample fluid.
• the test element may comprise a working electrode and a counter electrode, and a reagent matrix extending between the working electrode and the counter electrode and configured for an electrode reaction.
• the reagent matrix is provided with a deliquescent material, and the deliquescent material is selected and provided in an amount effective to protect one or more of the active ingredients from humidity in the atmosphere.
• a reagent matrix for a blood glucose biosensor comprising a mediator, an enzyme, and a deliquescent material selected and provided in an amount effective to preferentially and/or preemptively absorb water from the air when compared to the water absorption properties and rates of the other components of the reagent matrix, thus protecting the other components from degradation by water when the relative humidity exceeds the deliquescence point of the deliquescent material.
• the deliquescent material may be selected to preferentially absorb water when the environmental relative humidity exceeds any predetermined level, such as 40%, or 50%>, or 60%>, or 70%>, or 75%, or 80%>, etc., as desired.
• a method for improving the stability of a biosensor test element with respect to degradation of reagent components by humidity in the air wherein the test element comprises a base/supporting substrate, working and counter electrodes, and a reagent matrix extending between the working electrode and the counter electrode and configured for an electrode reaction.
• the inventive method may comprise providing in the reagent matrix a deliquescent material selected and in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• the method may comprise providing in the reagent matrix a deliquescent material selected and in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• the deliquescent material may be selected to be a material having a deliquescence point between about 50% relative humidity and about 80% relative humidity. In some embodiments the deliquescence point is preferably between about 70%> relative humidity and about 80%> relative humidity, while in other embodiments the deliquescent material has a deliquescence point at about 75% relative humidity.
• the test element may be a glucose meter test strip having a glucose reagent matrix thereon, with the test strip reagent matrix including an enzyme system and a mediator.
• the enzyme system may comprise glucose dehydrogenase (GDH) enzyme and/or flavin adenine dinucleotide (FAD) cofactor.
• the mediator may comprise a nitrosoaniline mediator.
• a film former comprising polyvinylpyrrolidone (PVP) may also be included.
• the deliquescent material may comprise one or more members selected from the group consisting of sodium chloride, calcium chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium phosphate, carnallite, ferric ammonium citrate, potassium hydroxide and sodium hydroxide.
• the deliquescent material may be provided in an amount effective to absorb water from the atmosphere at a rate that is faster than the rate at which the other components of the reagent matrix absorb water from the atmosphere.
• the deliquescent material may be provided in an amount effective to absorb water from the atmosphere at a rate that is faster than the rate at which the other components of the reagent matrix absorb water from the atmosphere when the relative humidity in the atmosphere exceeds about 75%.
• the deliquescent material may alternatively or additionally be selected and provided in an amount effective to preferentially absorb water from the atmosphere when compared to the water absorption properties of the other reagent components, thus protecting the other components of the reagent matrix from absorbing water from the atmosphere when the relative humidity exceeds the deliquescence point of the deliquescent material.
• FIG. 1 shows a graph of the blank current (nA) over time for strips having varying amounts of NaCl added to the reagent mixture.
• the strips were dosed with buffer solution and exposed to high heat and humidity conditions (30°C/80%RH).
• FIG. 2 shows a graph of the blank current (nA) over time for strips having varying amounts of NaCl added to the reagent mixture.
• the strips were dosed with unspiked blood and exposed to high heat and humidity conditions (30°C/80%RH).
• FIG. 3 shows a graph of the fraction of mediator remaining on the strip over time for strips having varying amounts of NaCl added to the reagent mixture. The strips were exposed to high heat and humidity conditions (30°C/80%RH).
• FIG. 4 is a graph of sorption data for PVP according to test measurements (red diamonds) and reported literature values (other symbols).
• FIG. 5 is a graph of sorption data for NaCl according to test measurements (red diamonds) and reported literature values (other symbols), as well as the deliquescence point (green line) and the crystallization point (blue line).
• FIG. 6 is a graph of sorption isotherms for PVP, NaCl, and mixtures of the two components.
• FIG. 7 is a graph of the blank currents observed when dry film reagents having various additives are protected by deliquescent materials, as provided by certain aspects of the present invention.
• Glucose biosensors typically use a reagent matrix comprising an enzyme/co factor system and a mediator to test for glucose concentration in a blood sample.
• the strips may lose their functionality or become unstable.
• the present invention addresses that problem by formulating the test strip reagent matrix to include a deliquescent material that absorbs water from the atmosphere at a selected humidity level, thereby preventing the water from degrading the reagent matrix components.
• the deliquescent material is selected to protect the reagent components when exposed to a defined relative humidity, such as when the relative humidity in the atmosphere exceeds about 75%.
• the deliquescent material is preferably a salt, and is most preferably sodium chloride.
• Other known deliquescent materials include calcium chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium phosphate, carnallite, ferric ammonium citrate, potassium hydroxide and sodium hydroxide.
• the reagent matrix stabilization materials and methods of the present invention have particular utility when used with test elements (such as a glucose test strip) having dry- film reagent systems that may degrade if exposed to excessive humidity, such as a relative humidity of 75%, or even 70% or less.
• test elements such as a glucose test strip
• dry- film reagent systems that may degrade if exposed to excessive humidity, such as a relative humidity of 75%, or even 70% or less.
• test elements such as a glucose test strip
• dry- film reagent systems that may degrade if exposed to excessive humidity, such as a relative humidity of 75%, or even 70% or less.
• test elements such as a glucose test strip
• dry- film reagent systems that may degrade if exposed to excessive humidity, such as a relative humidity of 75%, or even 70% or less.
• commonly-used glucose reaction mediators may degrade when exposed to humidity (water) in the air, producing a reduced form or other products which are electrochemically active and can result in a blank current in the
• the issue is exacerbated by the fact that the reagent matrix needs a degree of hydrophilicity in order to provide quick wetting of the reagent (and thus, a quick test readout).
• This issue is specifically presented, for example, when PVP-containing materials are used as a film former for a glucose reagent system. PVP adsorbs moisture from the air - bringing it into contact with the reagent system.
• the problem noted above is addressed by adding a component to the reagent matrix that causes the matrix as a whole to adsorb less water. While the invention is illustrated in this disclosure by adding a deliquescent material such as NaCl to a glucose reagent system including FADGDH and nitrosoaniline, the invention when construed broadly is intended to include other reagent systems and other deliquescent materials.
• the biosensor is a glucose test strip.
• test strips typically have working and counter electrodes, and are provided with a reagent matrix to electrochemically analyze a test strip reagent matrix.
• test elements are known and available in different forms, to which the present invention as a whole is applicable.
• the test element typically comprises at least one test field having a test field surface.
• a test field is understood to mean a two- or three-dimensional region of the test element, which region is usable in principle for the detection of the analyte, which can be carried out qualitatively and/or quantitatively.
• the test field may be a dry test field, and may comprises at least one detection reagent which is selected to carry out a detectable reaction in the presence of the analyte.
• a test strip reagent matrix comprises an enzyme system and a mediator.
• the enzyme system may comprises glucose dehydrogenase enzyme (GDH) and/or flavin adenine dinucleotide cofactor (FAD), and the mediator may comprises a nitrosoaniline mediator.
• the reagent matrix may also include a film former, which may comprise polyvinylpyrrolidone (PVP). Examples of other glucose-specific enzymatic detection reagents include, but are not limited to, deoxy reductases (e.g.,
• GlucDOR/PQQ dehydrogenases
• oxidases or similar enzymes, for example glucose oxidase (GOD) or glucose dehydrogenase.
• a mediator may also be included in the reagent mixture.
• the mediator may be any chemical species (generally electroactive), which can participate in a reaction scheme involving an enzyme, an analyte, and optionally a cofactor (and reaction products thereof), to produce a detectable electroactive reaction product.
• participation of the mediator in this reaction involves a change in its oxidation state (e.g., a reduction), upon interaction with any one of the analyte, the enzyme, or a cofactor, or a species that is a reaction product of one of these (e.g., a cofactor reacted to a different oxidation state).
• a variety of mediators exhibit suitable electrochemical behavior.
• a mediator can preferably also be stable in its oxidized form; may optionally exhibit reversible redox electrochemistry; can preferably exhibit good solubility in aqueous solutions; and preferably reacts rapidly to produce an electroactive reaction product.
• suitable mediators include benzoquinone, meldola blue, other transition metal complexes, potassium ferricyanide, and nitrosoanilines, see U.S. Patent No. 5,286,362, the contents of which are incorporated herein by reference.
• the test field may further include carrier substances, auxiliary substances, pigments, fillers, buffer substances, etc.
• carrier substances auxiliary substances, pigments, fillers, buffer substances, etc.
• the chemistry of the reaction scheme of any chosen electrochemical detection method can be chosen in light of various chemical factors relating to the system, including the identity of the analyte and of the sample substance. Even then, for a given analyte or substance, various different reactive components may be useful in terms of a catalyst (often, a variety of enzymes will be useful), co-reactants (e.g., a variety of mediators may be useful), and cofactors (if needed, a variety may be useful). Many such reaction schemes and their reactive components and reaction products are known, and examples of a few different enzymes include those listed in Table 1.
• a sample containing glucose can react with an enzyme (e.g., Glucose-Dye-Oxidoreductase (Glue-Dor)) and optionally a cofactor, (e.g., pyrrolo- quinoline-quinone), in the presence a redox mediator (e.g., benzoquinone, ferricyanide, or nitrosoaniline derivatives), to produce the oxidized form of the analyte, gluconolactone, and the reduced form of the redox mediator.
• an enzyme e.g., Glucose-Dye-Oxidoreductase (Glue-Dor)
• a cofactor e.g., pyrrolo- quinoline-quinone
• a redox mediator e.g., benzoquinone, ferricyanide, or nitrosoaniline derivatives
• a deliquescent material is added to the reagent matrix to protect components of the reagent from degradation by water.
• Preferred deliquescent materials include sodium chloride, calcium chloride, magnesium chloride, magnesium sulfate, zinc chloride, potassium carbonate, potassium phosphate, carnallite, ferric ammonium citrate, potassium hydroxide and sodium hydroxide, with salts such as sodium chloride being most preferred.
• the term "deliquescence” generally refers to a phase transition from solid to solution that occurs when the vapor pressure of a saturated aqueous solution of a substance is less than the vapor pressure of water in ambient air.
• a mixture of the solid and its saturated solution or an aqueous solution of the compound forms until the substance is dissolved and is in equilibrium with its environment.
• the relative humidity at which deliquescence occurs is a property of the specific substance. At this point the vapor pressure of water over the aqueous solution will equal the partial pressure of water in the atmosphere in contact with it. Accordingly, a salt particle will deliquesce in the atmosphere when the relative humidity surpasses the deliquescence point of the material.
• delivery material generally refers to materials having deliquescent properties, and may refer to materials, such as salts, that have a strong affinity for moisture and that will absorb relatively large amounts of water from the atmosphere if exposed to it, thus protecting other materials from absorbing water.
• the deliquescent material may be selected to be a material having a deliquescence point that protects the other reagent components when exposed to environmental humidity at a pre-determined level.
• a user identifies a particular environmental stress to be avoided, such as the environmental stress of a 75% relative humidity, and includes in the reagent mixture a deliquescent material selected to have a deliquescence point at slightly below that level.
• the deliquescence point of NaCl is near a relative humidity (RH) of 75%, so the addition of NaCl is particularly effective for use with test elements that may be exposed to a relative humidity that may exceed 75%.
• RH relative humidity
• the sodium chloride crystals absorb water faster than the other reagent materials, thus preempting absorption by the other components and protecting the other reagent materials from water degradation.
• the amount of deliquescent material added to the reagent matrix will depend on the material being used, the other components in the reagent matrix, and the environmental conditions likely to be faced.
• the deliquescent material will be provided in the reagent matrix in an amount effective to allow the deliquescent material to preemptively take on the water that would otherwise be taken on by other reagent components, and thus to prevent the other reagent components from deteriorating.
• the deliquescent material is preferably provided in an amount effective to absorb water from the atmosphere at a rate that is faster than the rate at which the other components of the reagent matrix will absorb water from the atmosphere, thus preventing the other components from absorbing water and degrading.
• the deliquescent material may be a combination of two or more deliquescent materials.
• a mixture of salts such as NaCl , MgSC ⁇ and CaCl 2 may be used.
• the deliquescent material(s) may be added to the reagent matrix in amounts effective to provide from about 1% to about 10% by weight percent deliquescent material in the dry film reagent, with amounts between about 2% and about 8% being more preferred.
• the deliquescent material is provided in the dry film reagent matrix in an amount between about 3% and about 7% by weight, with amounts between about 3% and about 6%> being more preferred.
• the deliquescent material is included in the reagent material in an amount effective to provide at least about 2% deliquescent material in the dry film reagent matrix, while in another embodiment the deliquescent material is included in the reagent material in an amount effective to provide at least about 3% deliquescent material in the dry film reagent matrix.
• the deliquescent material may be added to the reagent material in amounts effective to provide no more than about 10% deliquescent material in the dry film reagent matrix.
• the deliquescent material was provided in an amount effective to provide about 3.0% NaCl in the dry film reagent. In another preferred embodiment the deliquescent material was provided in an amount effective to provide about 5.8% NaCl in the dry film reagent. In a third preferred embodiment the deliquescent material was provided in an amount that provides about 6.5% MgSC ⁇ in the dry film reagent.
• the principles of the invention described herein find particular utility in stabilizing reagents used in a test element for detecting at least one analyte in a sample, such as the detection of at least one metabolite in a body fluid, and particularly the detection of glucose in a blood sample. Accordingly, certain aspects and principles of the present invention are illustrated herein by describing one particularly preferred embodiment, namely, an embodiment in which NaCl is used as a deliquescent material in a reagent mixture containing PVP or a PVP-containing material.
• NaCl sodium chloride
• ⁇ is the volume fraction of water (w) and polymer (P), and is the interaction parameter between the water and polymer.
• the amount of water adsorbed below the deliquescence point at relative humidity of 75% is, at most, a few molecular layers.
• the sodium chloride crystals dissolve and the concentration of the solution is a function of the water activity in the gas phase and is calculated as:
• Deliquescent materials other than NaCl may also be used, as indicated above.
• dry film reagents that are protected with a MgSC ⁇ deliquescent material have been shown to be effective using the procedures described generally above.
• FIG. 7 shows that including MgSC ⁇ in the dry film reagent matrix provides protection against water degradation in humid environments.
• roll 2 shows data from a test of a dry film reagent comprising a FADGDH enzyme/co factor system, a nitrosoaline mediator, PVP, and no deliquescent material.
• roll 11 shows data from a test of a dry film reagent having a reagent mixture that is comparable to the reagent mixture of roll 2, but with about 6% MgSC ⁇ added as a deliquescent material. In these tests the temperature was about 30°C, and the relative humidity was about 82%.
• FIG. 7 Inspection of the data provided in FIG. 7 shows a significantly higher blank current from roll 2, where the dry film reagent matrix does not include a deliquescent material, than for roll 11, where the dry film reagent matrix does include a MgSC ⁇ deliquescent material.
• phase equilibrium data is reported in the following papers, the contents of which are hereby incorporated herein by reference.
• a test element for determining the amount of an analyte in a sample fluid comprising:
• a dry- film reagent matrix extending between the working electrode and the counter electrode and configured for an electrode reaction; wherein said reagent matrix comprises a deliquescent material and one or more active ingredients effective for facilitating a desired electrochemical reaction; and wherein the deliquescent material is selected and provided in an amount effective to preemptively absorb water from the atmosphere when the relative humidity exceeds a predetermined level.
• a dry-film reagent matrix for a blood glucose biosensor comprising:
• a stabilizing agent comprising a deliquescent material selected and provided in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• test element comprises a working electrode and a counter electrode, and a dry- film reagent matrix extending between the working electrode and the counter electrode and configured for an electrode reaction; the method comprising providing in the reagent matrix a deliquescent material selected and in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• a method for improving the stability of a dry- film reagent matrix of an electrochemical biosensor comprising providing in the reagent matrix a deliquescent material selected and in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• a dry-film reagent matrix effective for facilitating an electrochemical reaction on a biosensor test strip, wherein the reagent matrix comprises an enzyme and a mediator
• the improvement comprising providing in the reagent matrix a deliquescent material selected and in an amount effective to decrease the sorption of water from the air by other components of the reagent matrix.
• test element of embodiment 1 wherein said deliquescent material has a deliquescence point between about 50% relative humidity and about 80% relative humidity.
• test element of embodiment 1 wherein said test element is a glucose meter test strip having a glucose reagent matrix thereon, and wherein said test strip reagent matrix includes an enzyme system and a mediator.
• test element of embodiment 8 wherein said test element is a glucose meter test strip having a glucose reagent matrix thereon, and wherein said test strip reagent matrix includes an enzyme system and a mediator.
• test element of embodiment 11 wherein said enzyme system comprises glucose dehydrogenase (GDH) enzyme and/or flavin adenine dinucleotide (FAD) cofactor.
• GDH glucose dehydrogenase
• FAD flavin adenine dinucleotide
• test element of embodiment 16 wherein said film former comprises polyvinylpyrrolidone (PVP) and/or a PVP-containing material.
• PVP polyvinylpyrrolidone
• said reagent matrix comprises a glucose dehydrogenase (GDH) enzyme, a flavin adenine dinucleotide (FAD) cofactor, a nitrosoaniline mediator, and a film former comprising polyvinylpyrrolidone (PVP).
• GDH glucose dehydrogenase
• FAD flavin adenine dinucleotide
• PVP polyvinylpyrrolidone
• deliquescent material comprises one or more members selected from the group consisting of sodium chloride, calcium chloride, magnesium chloride, magnesium sulfate, zinc chloride, potassium carbonate, potassium phosphate, carnallite, ferric ammonium citrate, potassium hydroxide and sodium hydroxide. 20. The test element of embodiment 19 wherein said deliquescent material comprises
• test element of embodiment 19 wherein said deliquescent material comprises MgS0 4 .
• test element of embodiment 11 wherein said deliquescent material is provided in an amount effective to absorb water from the atmosphere at a rate that is faster than the rate at which the other components of the reagent matrix absorb water from the atmosphere.
• test element of embodiment 11 wherein said deliquescent material is provided in an amount effective to absorb water from the atmosphere at a rate that is faster than the rate at which the other components of the reagent matrix absorb water from the atmosphere when the relative humidity in the atmosphere exceeds about 75%.
• test element of embodiment 1 wherein said deliquescent material is selected and provided in an amount effective to preferentially absorb water from the atmosphere when compared to the water absorption properties of the other reagent components, thus protecting the other components of the reagent matrix from absorbing water from the atmosphere when the relative humidity exceeds the deliquescence point of the deliquescent material.
• test element of embodiment 1 wherein said deliquescent material is provided in the reagent matrix in an amount effective to provide from about 1% to about 10% by weight percent deliquescent material in the dry film reagent.
• test element of embodiment 1 wherein said deliquescent material is provided in the reagent matrix in an amount effective to provide between about 2% and about 8% by weight percent deliquescent material in the dry film reagent.
• test element of embodiment 1 wherein said deliquescent material is provided in the reagent matrix in an amount effective to provide between about 3% and about 7% by weight percent deliquescent material in the dry film reagent.
• test element of embodiment 1 wherein said deliquescent material is NaCl, and is provided in the reagent matrix in an amount effective to provide between about 3.0% and about 6.0% NaCl in the dry film reagent.
• test element of embodiment 1 wherein said deliquescent material is MgS04, and is provided in the reagent matrix in an amount effective to provide between about 3.0% and about 7.0% MgS04 in the dry film reagent.

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