Abstract:
Unitary test means, such as a composition or device, purified uricase for use therein, method of making the test device and process for determination of uric acid therewith are disclosed. More particularly, there is provided in test means for the detection of uric acid comprising a uricase-active substance, at least one chromogen, and a peroxidatively active substance, the improvement wherein the uricase active substance is animal-originated uricase which is free of pH sensitive contaminants having a molecular weight of less than about 6000. The test means can take the form of a composition which can further include at least one coupling agent and at least one stabilizing agent. The compositions can optionally be incorporated with a carrier, such as a tablet or matrix, to provide a test device. The uricase-active substance is purified by fractionation of standard animal uricase preparations to remove low molecular weight contaminants and is stable over a heretofore unattainable pH range, permitting a unitized test.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of diagnostic tests and, more particularly, to a unitary reagent test for determining uric acid in a fluid sample. 
     In primate metabolism, there is a constant endogenous conversion of ingested nucleoproteins to purines and pyrimidines. The purines, by a catabolic process, then undergo further deamination and partial oxidation to uric acid, which in man is normally excreted in the urine. Thus, a nominal concentration of uric acid is present in human blood and urine at all times. 
     Ingestion of purine-containing food normally has no affect on the uric acid blood serum content, except in the case of renal insufficiency, in which event the concentration is elevated. In certain other pathological conditions not associated with dietary ingestion of purine-containing food, for example uremia and gout, there is an abnormal increase in the amount of uric acid found in the blood serum. Also, uric acid concentration in the serum is elevated in conditions associated with excessive destruction of leucocyte nuclei, such as leukemia and pneumonia. 
     A serum uric acid test has been recognized as useful as an aid in diagnosing the foregoing conditions and, in some instances, distinguishing between closely related abnormal conditions, for example, gout and arthritis. Gout is characterized by an abnormal increase in blood serum uric acid, whereas arthritis does not exhibit such increase. It is therefore desirable to provide a simple and economical test which affords a precise and specific determination of the concentration of uric acid in blood serum. 
     Uric acid is normally found in blood serum in quantities from about 0.7 to about 6.0 milligrams per 100 ml of blood serum, generally reported as milligrams percent (mg%). In the abnormal conditions enumerated above, the uric acid content in the blood serum often attains values of 10 mg% or higher. 
     The prior art has disclosed a number of methods for determining uric acid in blood serum. Among the more widely used conventional methods are colorimetric procedures utilizing blood filtrates. Some of these procedures depend upon the precipitation of uric acid from the blood filtrate, for example, as a silver salt, and the formation therewith of a chromogenic adduct by reaction with either a phosphotungstate or arsenotungstate. Other methods utilizing the blood filtrate depend upon the direct treatment of the filtrate with a tungstic acid in the presence of a cyanide-urea solution to develop a color which is then measured using conventional techniques for quantitative estimation of uric acid concentration. 
     More recently, methods have been proposed which involve the catalyzed oxidation of uric acid to allantoin and hydrogen peroxide. This oxidation is usually accomplished in the presence of atmospheric oxygen and utilizes a material having uricase activity, the reaction occurring at or near pH 9. In such methods a spectrophotometer can be employed to measure the disappearance of the characteristic uric acid spectrum during its conversion to allantoin and hydrogen peroxide. Another method utilizes a colorimetric means for measuring the hydrogen peroxide produced in stoichiometric amounts during such degradation of uric acid. See, for example, Albaum U.S. Pat. No. 3,349,006 and Wachter U.S. Pat. No. 3,335,069 (both assigned to the assignee of the present invention). 
     In the enzymatic conversion test, where the amount of hydrogen peroxide produced is directly proportional to the amount of uric acid present in the blood serum, the hydrogen peroxide is detected by means of a color change produced upon oxidation of a color forming substance in the presence of a substance having peroxidative activity. This reaction occurs at acid pH. A catalase inhibitor, such as sodium azide or sodium cyanide, has usually been required to prevent catalase destruction of the peroxide. The color obtained is then compared visually to standards, or measured electronically, to give a quantitative estimation of uric acid present in the fluid being tested. 
     French Pat. No. 72/31557 discloses a fundamentally different enzyme catalyzed reaction wherein catalase and aldehyde-free methanol are added along with the uricase to the sample (buffered to pH 8), and 3-methyl-2-benzothiazolinone hydrazone (buffered to pH 3) and FeCl 3  in HCl are then added to give a blue coloration. 
     Kano U.S. Pat. No. 3,862,885 discloses a process for uric acid determination by generating hydrogen peroxide with a microbe-originated uricase and a catalase-inhibitor (buffered to pH 5.5-7.0) and measuring the peroxide generated in the presence of an anionic surface active agent, a chromogen and peroxidase (pH 4.0-7.0). 
     Gochman and Schmitz have reported using 3-methyl-2-benzothiazolinone hydrazone hydrochloride with N,N-dimethylaniline to form an azo dye indicator in automated determinations of uric acid, Clin. Chem 17: 1154 (1971). 
     Notwithstanding the contributions by prior workers in the field, these procedures have had the disadvantage of requiring a series of separate operations, usually carried out in liquid phase. Coupled reactions using animal uricase and peroxidase simultaneously have been considered impossible because of the competition between uric acid and the chromogen, both being oxidized by H 2  O 2  in the presence of peroxidase, and of the drastic difference in the optimum pH of the two enzymes used. The prior art systems where both reactions are carried out at or near the same pH have the further drawback that certain reaction component candidates, such as animal-originated uricase, which exhibit superior performance characteristics cannot be used therein because they are inoperable as a component of the peroxidase-generating system within the pH range required by the reaction. 
     Thus, incorporation of means for uric acid determination in a stabilized, unitary reagent test has heretofore been impossible. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a unitized test means, such as a composition or device, for determination of uric acid in body fluids. 
     It is another object of the present invention to provide a uric acid test composition and device of a type which is particularly convenient for testing of large numbers of samples. 
     Another object of the invention is to provide a single operation uric acid test device which is stable over an extended time. 
     An additional object of the invention is to provide an improved test for the detection of uric acid wherein the above-identified advantages are achieved through incorporation therein of a novel purified, animal-originated, uricase-active substance, free of pH sensitive contaminants. 
     Other objects and a fuller understanding of the invention will be had by referring to the following description and claims drawn to preferred embodiments thereof. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there are provided unitary test means, such as a composition or device, purified uricase for use therein, method of making the test device and process for determination of uric acid therewith. More particularly, there is provided in test means for the detection of uric acid comprising a uricase-active substance, at least one chromogen, and a peroxidatively active substance, the improvement wherein the uricase-active substance is animal-originated uricase which is free of pH sensitive contaminants having a molecular weight of less than about 6000. The test means can take the form of a composition which can further include at least one coupling agent and at least one stabilizing agent. The compositions can optionally be incorporated with a carrier, such as a tablet or matrix, to provide a test device. The uricase-active substance is purified by fractionation of standard animal uricase preparations to remove low molecular weight contaminants and is stable over a heretofore unattainable pH range, permitting a unitized test. 
     The purified, animal-originated, uricase-active substance, a critical component of the above described composition, is prepared by fractionation purification of commercially available animal uricase preparations, such as are available from Miles Research Products, Miles Laboratories, Inc., Elkhart, Indiana 46514. Contaminants which are sensitive to variation in pH and are of relatively low molecular weight, less than about 6000, are removed. Surprisingly, by this seemingly simple purification process, the uricase-active substance produced is made stable over a pH range of from about 6.8 to about 7.5, thus enabling the preparation of a unitized, enzyme-catalyzed uric acid test composition having a plurality of reaction systems functional under a single set of reaction parameters. 
     Animal uricase is a cuproprotein (a conjugated metalloprotein), and the Cu ++  is believed to be a part of the catalytic site of the enzyme. The structure of the copper containing moiety of the active site may be schematically represented by the following: ##STR1## 
     The cuproprotein nature of the enzyme causes it to be rapidly, but reversibly, inhibited by a variety of agents which are capable of simultaneous reduction of Cu ++  and complexation of Cu +  ions. The uricase is therefore preferably purified by dialysis against a buffer with a low metal binding constant. 
     In a preferred embodiment, the enzyme is dialyzed against a solution containing buffers having a low metal binding constant, such as Tris (hydroxymethyl)-aminomethane (TRIS), piperazine-N,N&#39;-bis (2-ethanesulfonic acid (PIPES), N-tris-(hydroxymethyl) methyl-2-aminoethanesulfonic acid (TES) and phosphate buffer. 
     Uric acid is oxidized enzymatically by uricase to allantoin and hydrogen peroxide at about pH 7. The stoichiometry of the enzyme reaction proper, regardless of buffer or pH, is given by equation (1), i.e., the transfer of an electron pair from the urate monoanion to oxygen, yielding an unstable acid intermediate (1-carboxy-2,4,6,8-tetraazabicycl [3,3,0]-octa-4-en-3,7-dione), and hydrogen peroxide. 
     
         C.sub.5 N.sub.4 O.sub.3 H.sub.3.sup.- +O.sub.2 +H.sub.2 O=C.sub.5 N.sub.4 O.sub.4 H.sub.3.sup.- +H.sub.2 O.sub.2                    ( 1) 
    
     The intermediate product is a stronger acid (lower pK) than uric acid [pK=4.5, versus pK=5.75 for urate; pK 2  =11.5 versus pK 2  =10.3 for urate] and is capable of stabilization by forming metal chelates with copper (Cu ++ ) and cobalt (Co ++ ). In the presence of a highly purified enzyme at pH 7.0, stoichiometric amounts of allantoin, hydrogen peroxide, and carbon dioxide are formed. The hydrogen peroxide is then reacted with the chromogens in the presence of peroxidase to give a red complex. The overall chemical reactions of the test are as follows: ##EQU1## 
     In the present system the difficulties of competition between uric acid and the chromogen and of drastic difference in pH optimum of the enzymes are further overcome by employing a strong reducing agent, a hydrazone, and an extremely sensitive coupling agent which, in addition, serves as an activator of uricase. The chromgenic reactions using, for example, 3-methyl-2-benzothiazolinone hydrazone (MBTH) and primaquine diphosphate (PDP) are schematically represented in Diagram A as follows: ##STR2## 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular embodiments of the invention selected for exemplary illustration, and are not intended to define or limit the scope of the invention. 
     The commercial uricase preparation is purified by a process which, in a preferred embodiment, comprises fractioning the uricase preparation to remove molecules having less than about 6,000 molecular weight. Such is done by various techniques including dialysis, column chromatography, gel sedimentation and the like. 
     The hydrazones are condensation products of a hydrazine with an aldehyde or ketone and contain the grouping C═NNH 2 . Many hydrazones are capable of oxidatively coupling with certain aromatic amines or hydroxynapthalensulfonates to form a colored entity. Such include, among others, 3-methyl-2-benzothiazolinone hydrazone, 2-hydrazinobenzothiazole, N-methyl-pyridone-4-hydrazone, N-methyl-pyridone-2-hydrazone, N-methyl-quinolinone-2-hydrazone, methyl-quinolinone-4-hydrazone, N-methyl-2-benzothiazolinone hydrazone, N-methyl-thiazolinone-2-hydrazone, N-methyl-4-phenylthiazolinone-2-hydrazone, N-methyl-oxazolinone-2-hydrazone, N-methyl-benzoxazolinone-2-hydrazone and 1,3-dimethylbenzimidazolinone-2-hydrazone. 
     In a preferred embodiment of the composition, a 3-(C 1  -C 4  alkyl)-2-benzothiazolinone hydrazone chromogen, such as 3-methyl-2-benzothiazolinone hydrazone (MBTH), is used as the chromogen. Such hydrazones are strong reducing agents. The hydrazone is used in benzene solution at concentrations from about 0.05 mg% to about 0.2 mg%. 
     Exemplary of the coupling agents which can be used in combination with the chromogenic hydrazone are the following: 
     1. compounds having the general formula: ##STR3## wherein X is C, N, or S; R 1  is H, OH, amino, alkylenediamine, or aminoalkanol or combines with R 2 , where R 2  is H, to become NHCH 2  CHOHCH 2  ; R 2  and R 3  are the same or different and are H or SO 3  H; R 4  is H, OH, NHCH(CH 3 ) CH 2  CH 2  CH 2  NH 2 , or SO 3  H; R 5  is H, SO 3  H or acetamino; R 6  is H or OCH 3  ; and R 7  is H, OH or NH 2  ; and the acid addition salts, such as the phosphates, thereof. 
     2. compounds having the general formula R--CH 2  --R wherein each R is dimethylaniline, hydroxyphenyl, benzothiazole, or benzophenone; 
     3. Thiamine or its acid addition salt; 
     4. methylphenylpropanediamine; and 
     5. phenothiazine. 
     Preferred coupling agents include primaquine diphosphate (PDP), chromotropic acid, and 4, 4&#39;-methylene bis (N,N&#39; dimethylaniline)(MBDMA). The coupling agent is used in aqueous solutions of from about 1.0 mg% to about 5.0 mg%. 
     The composition can further include stabilizing agents, carboxymethylcellulose (CMC) and polyoxyethylene ethers of fatty alcohols (BRIJ® made by ICI United States Inc., Wilmington, Del. 19897) being advantageously selected. These are present in aqueous solutions in total concentrations of from about 0.5 mg% to about 5.0 mg%. 
     A solution containing composition according to the invention can be used to detect uric acid by adding it to a body fluid specimen such as urine. Formation of the chromophoric complex with resultant color change is effected. However, the composition is more advantageously used in the form of a solid preparation, rather than as a solution, and preferably in devices designed for convenience and reliability. 
     As an aspect of the invention, a device is prepared which comprises an inert carrier and, incorporated therewith, a composition according to the invention as described above and illustrated in the examples. The inert carrier can take the form of a bibulous or non-bibulous carrier matrix or can be in a tablet or other conventional form. 
     The term carrier matrix refers to bibulous and non-bibulous matrices which are insoluble in and maintain their structural integrity when exposed to physiological or other liquids. Suitable bibulous matrices which can be used include paper, cellulose, wood, synthetic resin fleeces, glass fiber, non-woven and woven fabrics and the like. Non-bibulous matrices include organo-plastic materials like polypropylene. For convenience the matrix can be associated with an insoluble support member such as one made of polystyrene. 
     Alternatively, the inert carrier can be embodied in the form of a pressed or molded tablet containing conventional carrier materials like disintegration agents, filling materials, and lubricants. 
     The test device of the present invention demonstrates a high degree of stability, as shown in the examples. This stability can be even further improved by separating components of the composition on a substrate and by suitably packaging the finished product in foil, a desiccant containing container or the like. The physical separation of components can be accomplished by printing, by encapsulation of one or more component, by use of a separate matrix or substrate for different components, by placement of incompatible components on opposite sides of a substrate, or the like. 
     As a further aspect of the invention there is also provided a method for preparing a uric acid test device which comprises impregnating, printing or otherwise contacting an inert carrier with a composition of the invention. If the carrier is impregnated with a composition in liquid form, the carrier is then subjected to the step of drying. 
     The invention further provides a method for using the composition or test device for the determination of uric acid in a liquid sample. The sample is tested by contacting the composition or device with the sample and observing any change in color. When a device of the type having a bibulous carrier matrix is used, the sample enters the matrix, and the color change is observed thereon. In addition to visual comparison, various instrumental methods can be employed to determine any color change developed, thus increasing the accuracy of the test by obviating the subjective determination of color by the human eye. 
     The activity of the enzyme preparation is measured by the number of units of activity per milligram of dry weight. The Commission on Enzymes of the International Union of Biochemistry has defined an International Unit (I.U.) of enzyme activity as 1 micromole (μmol) of substrate utilized per minute under specified conditions of pH and temperature control. 
     The relationship between percent reflectance (%R), reported in the examples, and the concentration of the absorbing species (uric acid) is given by the Kubelka-Monk equation which is provided, along with a detailed discussion of reflectance spectrophotometry in Kortumi, G., Reflectance Spectroscopy, Springer-Verlag New York Inc., 1969. In the relationship defined by the Kubelka-Monk equation the %R value decreases as the uric acid concentration detected increases, and vice versa. Thus, the readings taken inversely correlate, according to the equation, with the concentration of uric acid detected. All readings were taken at 530 nanometers unless indicated otherwise. 
     Reflectance readings can be obtained from commercially available spectrophotometers such as Beckman DK-2 Spectrophotomer, Beckman Instruments, Inc., Fullerton, Calif. 92634 or Spectrocolorimeter SCF-1, Israel Electro-Optical Industry Ltd. (distributed in the U.S. by Broomer Research Corporation, Plainwell, Long Island, N.Y. 11803). 
    
    
     The examples shown are merely illustrative and are not to be construed as a limitation of the invention. One skilled in the art will be able to make such variations, substitutions and changes in the ingredients and parameters as may seem desirable. 
     EXAMPLE I 
     A purified, animal-originated, uricase-active substance of the invention was prepared as described below. 
     Five milliliters (ml) of uricase (Boehringer Mannheim GmbH, Mannheim, Federal Republic of Germany), activity 9 IU/ml, was enclosed in the dialysis bag formed of Spectrapor membrane (Spectrum Medical Industries, Los Angeles, Calif. 60916) which is permeable to molecules of less than 6,000 molecular weight. The uricase was dialyzed with stirring against 0.5 Molar (M) TRIS, at pH 7.0, for 18 hours at 4° Centigrade (C). 
     Studies were initiated to determine the minimum volume of buffer required for sufficient dialysis of the enzyme. One ml of uricase was dialyzed against 20 ml, 50 ml, 100 ml, and 1000 ml of buffer. The dialyzed uricases were all assayed, and the results indicated no significant difference in activities in the various buffer volumes (Table I). 
     
                       TABLE I______________________________________Activity of Uricase Dialyzed in DifferentBuffer Volumes           Activity of DialyzedBuffer Volume   Uricase I.U./ml______________________________________ 20 ml          6.5 50 ml          6.2 100 ml         6.41000 ml         6.5______________________________________ 
    
     Thus, it appears from this that an end-point of substantial purity is reached for removal of contaminants even with the use of as little as 20 ml of buffer. 
     EXAMPLE II 
     Strips were prepared in a 2-dip process using uricase dialyzed against the various buffers indicated on Table II. 
     The first dip solution was formed by combining 0.5 ml uricase (3.0 I.U./ml), dialyzed against the respective indicated buffer, 0.5 ml of stabilizing agent (1.5 mg% CMC+0.8 mg% polyoxyethylene ether of aliphatic alcohol (BRIJ 35), 0.06 ml horseradish peroxidase, and 0.1 ml (3.75 mg%) of coupling agent PDP all in distilled water. The second dip solution was prepared by dissolving 5.0 milligrams (mg) of the chromogen MBTH in 10 ml of benzene. 
     Sheets of Whatman ET 31 filter paper (Whatman Inc., Clifton, N.J. 07014) 2.54 cm×10.16 cm in size were impregnated to saturation with the respective first dip solution, dried for 30 minutes at 50° C., saturated with the second dip solution, dried again, and cut to 5.1 mm×10.2 mm to provide devices of the invention. 
     The devices so prepared were then separated into three groups. Two of these groups were subjected to heat stress of 60° C. for periods of 24 and 72 hours, respectively. The third group was not subjected to heat stress at all (0) and served as a control. 
     These devices were then tested by delivering 0.03 ml aliquots thereon of serum samples containing the amounts of uric acid shown on Table II, and observing any resultant color change. Readings of %R were taken at 120 seconds with freshly prepared strips. 
     
                       TABLE II______________________________________TIME         URIC ACID (mg %)(hours)      2       4       6     8     10______________________________________    0       38.6    30.7  28.9  23.0  25.0TRIS    24       44.8    35.6  30.0  30.0  26.3   72       43.4    40.1  34.7  32.3  27.9   0        39.0    33.7  31.1  28.5  26.5PIPES   24       38.1    34.5  30.3  27.2  24.4   72       35.4    35.6  31.0  29.2  28.8   0        33.8    31.2  29.1  25.7  25.6TES     24       34.3    31.0  28.5  27.1  24.2   72       33.2    35.0  29.1  26.6  26.3______________________________________ 
    
     The results in Table II show an incremental decrease in percent reflectance (%R) as the uric acid concentration increases, the relationship being defined by the Kubelka-Monk equation. This indicates that each of the buffers tested is advantageously useful in preparing the purified, uricase-active substance. 
     EXAMPLE III 
     The effect on ultimate test sensitivity of pH variation of the dialysis buffer was examined. 
     Devices were prepared as in Example II, with the uricase having been dialyzed against phosphate buffer adjusted to the various pH levels shown in Table III. The data obtained, reported in %R, is set forth in Table III. 
     
                       TABLE III______________________________________URIC ACID (mg %)pH      2.0      4.0      6.0    8.0    10.0______________________________________6.8     19.4     13.8     12.4   10.7   9.37.0     19.5     14.3     11.5   10.2   9.27.5     19.0     13.5      8.1   10.3   9.2______________________________________ 
    
     The results show an incremental decline with the increase of uric acid concentration. This indicates that uricase dialyzed in buffer having a pH from at least about pH 6.8 to at least about pH 7.5 gave high sensitivity in detection of uric acid. 
     Results obtained using uricase dialyzed against TRIS, PIPES and TES buffers indicate a pH stability of the uricase over the same range. 
     EXAMPLE IV 
     Devices were prepared according to the following procedure in which the coupling agents shown in Table IV were used instead of PDP. 
     MBTH, 0.2 gram (g), was dissolved in a mixture of 50 ml methanol and 10 ml H 2  O. To 6 ml of this MBTH solution there was added 0.02 g of the selected coupling agent, 0.5 ml of 200 mg% peroxidase, and 0.1 ml dialyzed uricase (6 IU/ml) to form a solution containing the composition of the present invention. 
     The reaction solution so formed was tested by combining 0.2 ml thereof with 0.01 ml uric acid. The reactions observed with the various coupling agents when tested with 5 mg% and 10 mg% concentration uric acid are reported in Table IV. 
     
                       TABLE IV______________________________________           URIC           ACID      REACTIONCOUPLING AGENT  (mg %)    OBSERVED______________________________________chromotropic acid           5         purple           10        purplep-d-methylaminobenzaldehyde    5         no reaction           10        no reactionMBDMA           5         pale blue           10        pale bluebis (4-hydroxyphenyl)methane         5         no reaction           10        no reactionprimaquinediphosphate     5         bright red color           10        bright red coloriminodibenzyl   5         no reaction           10        no reaction______________________________________ 
    
     Thus, Table IV shows that chromotropic acid, MBDMA and primaquine diphosphate are effective when used as coupling agents in the composition of the present invention, whereas certain other compounds, namely para-d-methylamino benzaldehyde, bis (4-hydroxyphenyl) methane and iminodibenzyl, are not. 
     Other compounds which also are effective coupling agents include naphthylethylenediamine, naphthylaminoethanol, hydroxytetrahydrobenzoquinoline, phenothiazine, H-acid (8-amino-1-naphthol-6-sulfonic acid), 1-hydroxy-2-napthalenesulfonic acid, 1-hydroxy-3-napthalene-sulfonic acid, 1-amino-2-naphthalene-sulfonic acid, and 6-acetamino-1-hydroxynapthalene-sulfonic acid. 
     EXAMPLE V 
     Twenty clear serum samples of unknown uric acid content were anlyzed with devices prepared in a one-dip procedure as described below. Results were compared with test done by the standard phosphotungstate procedure according to Carroll et al., Clinical Chemistry17: 158 (1971). 
     Ten milliliters (ml) of distilled water containing 0.3 mg% of 3-methyl-2-benzothiazolinone hydrazone (MBTH), 0.15 mg% peroxidase, 0.075 mg% primaquine diphosphate (PDP), and 1.0 mg% carboxymethylcellulose were combined with 10 ml of 0.5 Molar (M) TRIS at pH 7.0 containing the purified animal-originated, uricase-active substance of the present invention having an activity of 3.0 IU/ml. 
     Sheets of Whatman filter paper 2.54 cm×10.16 cm in size were impregnated with 1 ml each of the above solution, dried for 30 minutes at 50° C., and cut to 5.1 mm×10.2 mm to provide devices of the invention. These devices were affixed by double-faced adhesive tape to elongated polystyrene support member for convenience. 
     The devices were tested by delivering 0.05 ml of each serum sample onto a respective device and reading any color developed after ten seconds and again after five minutes. All the readings were generated on an Ames Reflectance Meter (ARM) (Ames Company, Division of Miles Laboratories, Inc., Elkhart, Indiana 46514) with a yellow green filter (Edmund Scientific Co., Barrington, New Jersey 08007). The difference (Δ) in ARM units between the five minutes and ten seconds readings was used, to minimize variability in response of hand-made devices, for comparison with results of testing each sample by the standard phosphotungstate procedure, each of which is stated for the respective samples in Table V. 
     
                       TABLE V______________________________________    Ref. ValueSample   (mg% uric acid)  ARM units Δ______________________________________1        2.5              372        3.4              413        3.5              484        3.9              445        4.2              496        4.3              567        4.5              508        4.7              579        4.8              5010       5.5              6011       5.6              5912       5.6              5013       5.8              5414       6.0              7015       7.0              7116       8.3              7017       8.6              9118       9.0              8819       9.5              8020       10.0             86______________________________________ 
    
     The reading in ARM units varies according to a linear relationship with the concentration in mg% of uric acid present. The result obtained indicated that the tests described correlated to the reference assay procedure and were highly sensitive to differences in uric acid level. This sensitivity is evident from the distinct increase in readings between samples varying only slightly in uric acid concentration. 
     EXAMPLE VI 
     Uric acid test devices were prepared, according to various formulations in a two-dip procedure as follows. 
     A first dip solution was formed by combining 0.5 ml of the dialyzed, uricase-active substance (1.3 IU/ml), with an aqueous solution prepared of 0.1 ml (3.75 mg%) PDP, 0.06 ml (1.5 mg%) peroxidase, and 0.5 ml (1.5 mg% CMC and 0.8 mg% BRIJ). 
     A sheet of Whatman filter paper was impregnated to saturation with the above first dip solution and dried for 10 minutes at 60° C. The sheet was then impregnated to saturation with 10 ml of 0.05 mg% MBTH in benzene, dried for 10 minutes at 60° C., and cut to 5.1 mm×10.2 mm to provide devices of the invention. 
     The devices were tested by delivering 0.03 ml of sample containing uric acid concentrations of 2, 4, 6, 8 and 10 mg% thereto and observing the reflectance after 120 seconds. Readings of 39.3, 34.7, 29.4, 26.9 and 25.8% R were obtained. These readings confirm the inverse relationship defined by the Kubelka-Monk equation referred to earlier and show good sensitivity to the varying concentration of uric acid. 
     Devices were then made following the above procedure but using PDP concentrations of 1.0, 2.0, and 5.0 mg% instead of 3.75 mg%. Devices so prepared, were tested as above. The %R readings observed at 120 seconds indicated no significant variation in observed sensitivity as a result of the different concentrations of the coupling agent PDP used in the devices. 
     Likewise, when devices were prepared as described above but using concentrations of the stabilizing agent CMC of 0.5 and 2.4 mg% rather than 1.5 mg%. The devices thus prepared were tested as above and were observed to also detect uric acid with a high level of sensitivity. 
     Devices were prepared as described above but using the chromogen BMTH at concentrations of 0.01, 0.10 and 0.20 mg% instead of 0.05 mg%. Devices thus prepared were tested as above and likewise demonstrated excellent sensitivity to uric acid concentration. 
     Devices were prepared as above using peroxidase concentrations of 0.5, 1.0, 5.0 and 20 mg% instead of 1.5 mg%. When tested as above, each device so prepared distinguished uric acid concentrations with the same sensitivity as in the device having 1.5 mg% peroxidase concentration. 
     Devices were prepared as above using uricase-active substances having activities of 0.8 and 1.8 IU/ml rather than 1.3 IU/ml. When tested as above, no significant variation in sensitivity in the dtection of uric acid was observed as a result of the use of peroxidase of the various indicated activities. 
     Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes may be resorted to without departing from the spirit and scope of the invention.