- Email: [email protected]
Observations on diagnostic kits for the determination of uric acid
Clin. Biochem. 4, 279-286 (1971)
OBSERVATIONS ON DIAGNOSTIC KITS FOR THE DETERMINATION OF URIC ACID EDWARD K. KIM, LESLIE D. WADDELL, MARY L. E. SUNDERLAND AND JAMES E. LOGAN Clinical Laboratories, Canadian Communicable Disease Centre, Department of National Health and Welfare, Ottawa, Ontario, Canada, K1A OK9 (Received July 26, 1971)
SUMMARY
1. Diagnostic kits or reagent sets from eight manufacturers for the deterruination of uric acid were assessed. 2. T h e kits were assessed for adequacy of literature supplied, the reproducibility of the procedures, the accuracy of results compared with those obtained by reference method, and for the recovery of uric acid added to sera. 3. Only two kits were completely acceptable according to our criteria for a good laboratory test procedure. One other kit gave a satisfactory laboratory performance but did not contain adequate packaged instructions. T w o kits yielded d a t a for patient sera which could lead to incorrect clinical interpretation. 4. T h e cost analysis of various kits shows no correlation between the price and the kit performance.
IN OUR 1968 SURVEY ON THE USE OF DIAGNOSTIC KITS and reagents in clinical laboratories (1), kits were found to be extensively used either on a routine or s t a n d b y basis for the determination of uric acid. Approximately 18% of these laboratories measure uric acid by the use of kits. T h u s information on the reliability of the kits available on the m a r k e t should be useful. T h e results of an assessment of eight kits tested in this laboratory are presented. ~ [ETHODS
Tile uric acid kits which we have studied are listed as follows: 1. Medi-Chem Uric Acid T e s t ; .~Iedi-Chem, Inc., Santa Monica, Calif. 2. H y l a n d Uric Acid T e s t ; H y l a n d Div. Travenol Laboratories, Inc., Los Angeles, Calif. 3. Stanbio P L T for the determination of uric acid in serum or urine; Stanbio Laboratory, Inc., San Antonio, Texas. Correspondence: Dr. J. E. Logan, Clinical Laboratories, Canadian Communicable Disease Centre, Department of National Health and Welfare, Ottawa, Ontario, Canada. K1A OIZ9.
280
KIM et al.
4. Uri-Pak; Uni-Tech Chemical Manufacturing Co., Sun Valley, Calif. 5. Hycel Uric Acid; Hycel Inc., Houston, Texas. 6. Sigma Uric Acid Kit #680; Sigma Chemical Co., St. Louis, Mo. 7. Harleco Uric Acid; Harleco, Philadelphia, Pa. 8. Uri-Kit; Omni Tech Inc., Santa Monica, Calif. All these kits are advertised in various publications and sold in Canada. We purchased them from local sales representatives and distributors or directly from the manufacturers. The manufacturers' instructions were rigidly followed in using the kits after a sufficient period for familiarization. The adequacy of the literature supplied with the kits was assessed in accordance with the recommendations of the American Association of Clinical Chemists (~). We established an arbitrary rating system to analyze the supplied information in terms of points covered in these recommendations. One point was given for satisfactory compliance with and one-half point for partial compliance with a recommendation so that a total of 15 points indicated complete information has been provided to the user of the kits by the manufacturer. All the kit methods for uric acid are based on the reducing reaction of phosphotungstic acid in carbonate solution by uric acid and the measurement of the resulting tungsten blue colour intensity. All employ a precipitating reagent to remove protein except the Hycel kit which is a direct method without the deproteinization step. The Sigma kit method also requires a hydrolysis step with uricase to eliminate non-uric acid interference. An automated method, i.e., Crowley's phosphotungstate-carbonate procedure (3) was used as a reference procedure. For two years, which includes the evaluation period, we have participated as a reference laboratory in the National Reference Laboratory Network for the Proficiency Testing Program conducted by the Center for Disease Control, Atlanta, Ga. Throughout our participation our data by this automated reference procedure produced 'A' grade ratings. A method similar to that of Barnett (4) was followed in evaluating the kits. The day-to-day reproducibility of each was determined using lyophilized commercial control sera with three different levels of uric acid for a total of 10 days. The standard deviation and coefficient of variation were calculated and compared for acceptability by precision and accuracy criteria. In the patient sera study we compared results by the kits with those by the reference procedure. The bias of the kit method when compared to the reference procedure was established on 40 patient sera. Student's statistical 't' value was calculated from the collected data to determine the statistical significance of the bias results between the kit and reference method. Recovery experiments were performed after adding uric acid stock standard in three different levels to a low normal pool of patient sera whose value had been established by the reference method. Determinations were made in triplicate on the base serum pool and on sera with the three levels of added uric acid. The percentage recoveries were calculated in relation to the amount of uric acid added. The cost of the various kits and reagents were compared on a 100-test basis
DETERMINATION OF URIC ACID
281
to determine whether they bore any relation to the constituents supplied in the units or their performance.
RESULTS AND DISCUSSION The adequacy of the literature provided with the kits by the manufacturers was rated according to our arbitrary point system described above and is shown in Table I. Information as required in the A A C C recommendations could be obtained by field trialstudies and validation before the products are put on the market. The table indicates that no manufacturer provided a comparison study or justification of their product by statistical analysis. Adequate information could only be obtained by thorough evaluation studies either by their own staff or even better by engaging a reputable independent institution to validate the specificity, accuracy, precision and stability of their product. A scheme similar to that followed for the therapeutic pharmaceutical products should be practiced for diagnostic kits. The Standards Committee of the College of American Pathologists*, has set up an evaluation program so that manufacturers wishing to validate their products before they are put on the market m a y do so. The day-to-day reproducibility of the kit and reference procedures is shown in Table 2 for three different levels of concentration of uric acid in commercial lyophilized sera. The results are the mean values of ten determinations and coefficients of variation have been calculated. The criteria for acceptability of precision are the allowable limits of error (A.L.E.), suggested by Tonks (5). The data in Table 2 indicate that twice the coefficient of variation exceeds the A.L.E. for uric acid determinations, i.e., -4-10%, for Medi-Chem and Hyland at low levels, Stanbio and Sigma at low and intermediate levels, Uni-Tech at all three levels, Hycel at high levels and Harleco at low and high levels. Onmi Tech gave values which were all within acceptable limits of error. Accuracy and reproducibility are also assessed in comparison with the mean ± 3 S D of the reference procedure as suggested by Barnett (6). In Fig. 1, each vertical bar represents the ± S D confidence ranges of each kit, the mean of each kit being shown as the central line. The shaded band includes 99.7% of the reference values. From Table 2 and Fig. 1, it is obvious that certain kits perform very poorly. There is a disparity between some of the values obtained by the kits and those established by the reference method. In particular it is evident that the values obtained for commercial control sera by Hycel kits are much too high while those obtained by Uni-Tech and Harleco are too low. These also revealed poor reproducibility. For reasons not evident the Hycel kit produced lower results with patient sera. To solve this problem, we tried to find out whether this resulted from use of frozen or lyophilized sera rather than fresh samples. Our results did not bear this out. The manufacturer stated that a dialyzed sample may produce a disparity, but this was not a satisfactory explanation for our findings. Comparative studies of data on 40 patient sera have been carried out by each *Prudential Plaza, Suite 2115, Chicago, Illinois.
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DETERMINATION OF URIC ACID
283
TABLE 2 THE DAY-TO-DAY REPRODUCIBILITY OF THE K I T S AND REFERENCE PROCEDURE FOR THREE CONCENTRATION LEVELS OF U R I C ACID IN CONTROL SERA
Low Mean C.V. (mg/100 ml) (-4-%)
Kit or Method Harleco Hycel Hyland Medi-Chem Omni Teeh Sigma Stanbio Uni-Tech AA (Crowley)
2.76 4.04 2.76 3.12 3.03 2.85 2.91 2.79 2.96
5.96* 4.07 6.20* 5.40* 2.70 7.79* 6.96* 5.19" 2.34
Intermediate Mean C.V. (mg/100 ml) (4-%) 5.48 7.37 5.54 6.05 5.97 5.86 5.61 5.45 5.90
High Mean C.V. (mg/100 ml) (4-%)
4.45 3.44 3.62 4.06 3.06 6.97* 5.41" 7.25* 1.38
8.39 9.95 8.46 9.29 9.11 8.85 8.51 8.06 8.88
8.77* 6.11" 4.99 1.99 4.05 2.83 3.48 8.21" 0.71
*Outside the allowable limits of error. (mg/100)
t?Il-
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HIGH
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FIG. 1. The day-to-day reproducibility of the various procedures for three concentration levels of uric acid in control sera. of t h e a b o v e m e t h o d s over a period of one m o n t h . F i v e or six s a m p l e s of p a t i e n t sera were a n a l y z e d on each t e s t d a y . T h e m e a n of the results c o m p a r e d with those of the reference p r o c e d u r e are given in T a b l e 3. T h e d a t a i n d i c a t e t h a t s t a t i s t i c a l l y significant biases exist in all the kits a c c o r d i n g to S t u d e n t ' s s t a t i s t i c a l ' t ' test. I n general lower v a l u e s were o b t a i n e d b y m o s t of the kits e x c e p t the M e d i - C h e m , which g a v e higher results. As p r e v i o u s l y m e n t i o n e d the H y c e l k i t showed ]ow
KIM et al.
284
TABLE 3 BIAS OF KIT METHODS VERSUS REFERENCE PROCEDURE
No. of Discrepancies > 1 mg/lOO Kit or Method
IVlean mg/100
AA (Crowley) Harleco Hycel Hyland Medi-Chem Omni Tech Sigma Stanbio Uni-Tech
6.80 6.24 5.84 6.40 7.12 6.51 6.47 6.57 6.25
Bias mg/100 . 0.56 0.96 0.40 - 0.32 0.29 0.33 0.23 0.55
At the
"t" .
. 8.5 7.3 7.4 8.5 3.8 3.6 6.2 5.6
Total
Decision Level
6 15 2 0 3 I 0 9
I 6 1 0 0 0 0 6
.
values in patient sera in contrast to high ones found with control sera. Although we have not determined the cause of the lower trend of results shown by most of the kits, it is possible t h a t it might result from the simple precipitation step with modified Folin reagent or what is termed stabilized reagent by the manufacturers. Special order in the addition of the reagents and care should be exercised, as suggested by Folin et al. (7) in order to achieve full recovery of uric acid in the filtrate. Bornstein et al. (8) and Morris and MacLeod (9) stated t h a t there are two forms of uric acid present in the serum, only one of which is present in the filtrate following the addition of Folin tungstic acid reagent. Perhaps a n o t h e r factor contributing to the disparity of the kits would be inaccuracy and instability of the standards. T h e question arises as to how accurate a uric acid method should be for use in clinical diagnosis. Various studies have indicated t h a t the concentration of uric acid in the sera of normal individuals will be less than 6 m g / i 0 0 , male and female inclusive, and will be over 6 mg/100 for a s y m p t o m a t i c "essential" hyperuricemia, acute gouty arthritis, intercritical gout and chronic tophaceous gout, eclampsia, tissue breakdown, etc. (I1, 12, 18). Bishop and T a l b o t t (11, 12) studied one hundred patients with clinical gouty s y m p t o m s and found 989o of gouty subjects had uric acid values greater than 6 rag/100 ranging from 5.716.2 mg/100 and with a mean value of 8.8 rag/100. T h e S u b - c o m m i t t e e on Criteria of Medical Usefulness of the College of American Pathologists recommended the need for a standard deviation of 4-0.5 rag/100 at the medically significant decision level of 6 rag/100 for uric acid (10). According to the aforementioned criteria and to our findings, Uni-Tech and Hycel kits could yield false information to the physician for the m a n a g e m e n t of the g o u t y subjects. As shown in T a b l e 3, both of these kit methods produced a large bias from the reference mean and among the analyses of the 40 patients revealed numerous discrepancies greater than 1 mg/100 which is the criteria set by the aforementioned subcommittee. T h e Harleco kit also showed more discrepancies on this basis, b u t these were only found in the higher range, i.e.,
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286
KIM et al.
10 m g / 1 0 0 or over. T h u s these m a n u f a c t u r e r s should improve their products further. Recovery studies were carried out using low normal pooled sera in which the a m o u n t of uric acid was increased 50o-/0, 100% and 200% respectively from the starting base pool value of 3.0 rag/100. Pure stock uric acid standard (100 rag/ 100) was used to increase the concentration. Results are shown in T a b l e 4. I n general recoveries were favourable except for the Stanbio, Uni-Tech and Harleco kits which yielded some low values. No instability of reagents in a n y of the kits was noticed during the test period. T h e costs of the various kit reagents were compared on a 100-test basis. T h e r e is no a p p a r e n t explanation for the wide variation in cost per 100 tests. T h e cost for the eight kits for uric acid ranged from a b o u t $10 to $61 per 100 tests and there is no correlation between the price and the performance of the method b y those reagents. Therefore, some form of control in the marketing of diagnostic kits would seem desirable in view of quality differences and the m o u n t i n g costs of diagnostic medicine. REFERENCES 1. LOGAN, J. E., RENTON, H. M. & ALLEN, R.H. The use of diagnostic kits and reagents in the clinical laboratory in Canada. Clin. Biochem. 3, 81-89 (1970). 2. American Association of Clinical Chemists Committee on Standards and Controls. AACC policy regarding reagent sets and kits. Clin. Chem. 12, 43-44 (1966). 8. CROWLEY, L. V. & ALTON, F.I. Automated analysis of uric acid. Am. J. Clin. Path. 49, 285-288 (1968). 4. BARNETT, R.N. A scheme for the comparison of quantitative methods. Am. J. Clin. Path. 43, 562-569 (1965). 5. TONKS, D.B. Quality control in clinical laboratories, 1970 edition. Diagnostic Reagents Division, Warner-Chilcott Laboratories Co. Ltd., Scarborough, Ontario (1970). 6. BARNETT, R. N., CASH, A. D. & JUNGHANS, S. P. Performance of "kits" used for clinical chemical analysis of cholesterol. New Eng. J. Med. 279, 974-979 (1968). 7. FOLIN, O. A system of blood analysis, a revision of the method for determining uric acid. J. Biol. Chem. 54, 153-170 (1922). 8. BORNSTEIN, A. & GRIESBACH, W. l]ber das Verhalten der Harns~iure in fiberlebenden Menschenblut I. Biochem. Ztschr. 101, 184-196 (1920). 9. MORRIS, J. L. & MAcLEOD, A.G. The uric acid of human blood. J. Biol. Chem. $0, 65-75 (1922). 10. BARNETT, R.N. Medical significance of laboratory results. Am. J. Clin. Path. 50,671-676
(1968).
i 1 . TALBOTT, J.H.
Gout. Oxford University Press, New York (1943). 12. BISHOP C. & TALBOTT, J.H. Uric acid: Its role in biological processes and the influence upon it of physiological, pathological and pharmacological agents. Pharmacological Reviews. $, 231-273 (1953). 13. TALBOTT, J.H. Serum urate in relatives of gouty patients. J. Clin. Invest. 19, 645-648 (1940).
OBSERVATIONS ON DIAGNOSTIC KITS FOR THE DETERMINATION OF URIC ACID EDWARD K. KIM, LESLIE D. WADDELL, MARY L. E. SUNDERLAND AND JAMES E. LOGAN Clinical Laboratories, Canadian Communicable Disease Centre, Department of National Health and Welfare, Ottawa, Ontario, Canada, K1A OK9 (Received July 26, 1971)
SUMMARY
1. Diagnostic kits or reagent sets from eight manufacturers for the deterruination of uric acid were assessed. 2. T h e kits were assessed for adequacy of literature supplied, the reproducibility of the procedures, the accuracy of results compared with those obtained by reference method, and for the recovery of uric acid added to sera. 3. Only two kits were completely acceptable according to our criteria for a good laboratory test procedure. One other kit gave a satisfactory laboratory performance but did not contain adequate packaged instructions. T w o kits yielded d a t a for patient sera which could lead to incorrect clinical interpretation. 4. T h e cost analysis of various kits shows no correlation between the price and the kit performance.
IN OUR 1968 SURVEY ON THE USE OF DIAGNOSTIC KITS and reagents in clinical laboratories (1), kits were found to be extensively used either on a routine or s t a n d b y basis for the determination of uric acid. Approximately 18% of these laboratories measure uric acid by the use of kits. T h u s information on the reliability of the kits available on the m a r k e t should be useful. T h e results of an assessment of eight kits tested in this laboratory are presented. ~ [ETHODS
Tile uric acid kits which we have studied are listed as follows: 1. Medi-Chem Uric Acid T e s t ; .~Iedi-Chem, Inc., Santa Monica, Calif. 2. H y l a n d Uric Acid T e s t ; H y l a n d Div. Travenol Laboratories, Inc., Los Angeles, Calif. 3. Stanbio P L T for the determination of uric acid in serum or urine; Stanbio Laboratory, Inc., San Antonio, Texas. Correspondence: Dr. J. E. Logan, Clinical Laboratories, Canadian Communicable Disease Centre, Department of National Health and Welfare, Ottawa, Ontario, Canada. K1A OIZ9.
280
KIM et al.
4. Uri-Pak; Uni-Tech Chemical Manufacturing Co., Sun Valley, Calif. 5. Hycel Uric Acid; Hycel Inc., Houston, Texas. 6. Sigma Uric Acid Kit #680; Sigma Chemical Co., St. Louis, Mo. 7. Harleco Uric Acid; Harleco, Philadelphia, Pa. 8. Uri-Kit; Omni Tech Inc., Santa Monica, Calif. All these kits are advertised in various publications and sold in Canada. We purchased them from local sales representatives and distributors or directly from the manufacturers. The manufacturers' instructions were rigidly followed in using the kits after a sufficient period for familiarization. The adequacy of the literature supplied with the kits was assessed in accordance with the recommendations of the American Association of Clinical Chemists (~). We established an arbitrary rating system to analyze the supplied information in terms of points covered in these recommendations. One point was given for satisfactory compliance with and one-half point for partial compliance with a recommendation so that a total of 15 points indicated complete information has been provided to the user of the kits by the manufacturer. All the kit methods for uric acid are based on the reducing reaction of phosphotungstic acid in carbonate solution by uric acid and the measurement of the resulting tungsten blue colour intensity. All employ a precipitating reagent to remove protein except the Hycel kit which is a direct method without the deproteinization step. The Sigma kit method also requires a hydrolysis step with uricase to eliminate non-uric acid interference. An automated method, i.e., Crowley's phosphotungstate-carbonate procedure (3) was used as a reference procedure. For two years, which includes the evaluation period, we have participated as a reference laboratory in the National Reference Laboratory Network for the Proficiency Testing Program conducted by the Center for Disease Control, Atlanta, Ga. Throughout our participation our data by this automated reference procedure produced 'A' grade ratings. A method similar to that of Barnett (4) was followed in evaluating the kits. The day-to-day reproducibility of each was determined using lyophilized commercial control sera with three different levels of uric acid for a total of 10 days. The standard deviation and coefficient of variation were calculated and compared for acceptability by precision and accuracy criteria. In the patient sera study we compared results by the kits with those by the reference procedure. The bias of the kit method when compared to the reference procedure was established on 40 patient sera. Student's statistical 't' value was calculated from the collected data to determine the statistical significance of the bias results between the kit and reference method. Recovery experiments were performed after adding uric acid stock standard in three different levels to a low normal pool of patient sera whose value had been established by the reference method. Determinations were made in triplicate on the base serum pool and on sera with the three levels of added uric acid. The percentage recoveries were calculated in relation to the amount of uric acid added. The cost of the various kits and reagents were compared on a 100-test basis
DETERMINATION OF URIC ACID
281
to determine whether they bore any relation to the constituents supplied in the units or their performance.
RESULTS AND DISCUSSION The adequacy of the literature provided with the kits by the manufacturers was rated according to our arbitrary point system described above and is shown in Table I. Information as required in the A A C C recommendations could be obtained by field trialstudies and validation before the products are put on the market. The table indicates that no manufacturer provided a comparison study or justification of their product by statistical analysis. Adequate information could only be obtained by thorough evaluation studies either by their own staff or even better by engaging a reputable independent institution to validate the specificity, accuracy, precision and stability of their product. A scheme similar to that followed for the therapeutic pharmaceutical products should be practiced for diagnostic kits. The Standards Committee of the College of American Pathologists*, has set up an evaluation program so that manufacturers wishing to validate their products before they are put on the market m a y do so. The day-to-day reproducibility of the kit and reference procedures is shown in Table 2 for three different levels of concentration of uric acid in commercial lyophilized sera. The results are the mean values of ten determinations and coefficients of variation have been calculated. The criteria for acceptability of precision are the allowable limits of error (A.L.E.), suggested by Tonks (5). The data in Table 2 indicate that twice the coefficient of variation exceeds the A.L.E. for uric acid determinations, i.e., -4-10%, for Medi-Chem and Hyland at low levels, Stanbio and Sigma at low and intermediate levels, Uni-Tech at all three levels, Hycel at high levels and Harleco at low and high levels. Onmi Tech gave values which were all within acceptable limits of error. Accuracy and reproducibility are also assessed in comparison with the mean ± 3 S D of the reference procedure as suggested by Barnett (6). In Fig. 1, each vertical bar represents the ± S D confidence ranges of each kit, the mean of each kit being shown as the central line. The shaded band includes 99.7% of the reference values. From Table 2 and Fig. 1, it is obvious that certain kits perform very poorly. There is a disparity between some of the values obtained by the kits and those established by the reference method. In particular it is evident that the values obtained for commercial control sera by Hycel kits are much too high while those obtained by Uni-Tech and Harleco are too low. These also revealed poor reproducibility. For reasons not evident the Hycel kit produced lower results with patient sera. To solve this problem, we tried to find out whether this resulted from use of frozen or lyophilized sera rather than fresh samples. Our results did not bear this out. The manufacturer stated that a dialyzed sample may produce a disparity, but this was not a satisfactory explanation for our findings. Comparative studies of data on 40 patient sera have been carried out by each *Prudential Plaza, Suite 2115, Chicago, Illinois.
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DETERMINATION OF URIC ACID
283
TABLE 2 THE DAY-TO-DAY REPRODUCIBILITY OF THE K I T S AND REFERENCE PROCEDURE FOR THREE CONCENTRATION LEVELS OF U R I C ACID IN CONTROL SERA
Low Mean C.V. (mg/100 ml) (-4-%)
Kit or Method Harleco Hycel Hyland Medi-Chem Omni Teeh Sigma Stanbio Uni-Tech AA (Crowley)
2.76 4.04 2.76 3.12 3.03 2.85 2.91 2.79 2.96
5.96* 4.07 6.20* 5.40* 2.70 7.79* 6.96* 5.19" 2.34
Intermediate Mean C.V. (mg/100 ml) (4-%) 5.48 7.37 5.54 6.05 5.97 5.86 5.61 5.45 5.90
High Mean C.V. (mg/100 ml) (4-%)
4.45 3.44 3.62 4.06 3.06 6.97* 5.41" 7.25* 1.38
8.39 9.95 8.46 9.29 9.11 8.85 8.51 8.06 8.88
8.77* 6.11" 4.99 1.99 4.05 2.83 3.48 8.21" 0.71
*Outside the allowable limits of error. (mg/100)
t?Il-
MEDIUM
HIGH
l0" 9" 0
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I'lorleco 0--; Tech.
n
FIG. 1. The day-to-day reproducibility of the various procedures for three concentration levels of uric acid in control sera. of t h e a b o v e m e t h o d s over a period of one m o n t h . F i v e or six s a m p l e s of p a t i e n t sera were a n a l y z e d on each t e s t d a y . T h e m e a n of the results c o m p a r e d with those of the reference p r o c e d u r e are given in T a b l e 3. T h e d a t a i n d i c a t e t h a t s t a t i s t i c a l l y significant biases exist in all the kits a c c o r d i n g to S t u d e n t ' s s t a t i s t i c a l ' t ' test. I n general lower v a l u e s were o b t a i n e d b y m o s t of the kits e x c e p t the M e d i - C h e m , which g a v e higher results. As p r e v i o u s l y m e n t i o n e d the H y c e l k i t showed ]ow
KIM et al.
284
TABLE 3 BIAS OF KIT METHODS VERSUS REFERENCE PROCEDURE
No. of Discrepancies > 1 mg/lOO Kit or Method
IVlean mg/100
AA (Crowley) Harleco Hycel Hyland Medi-Chem Omni Tech Sigma Stanbio Uni-Tech
6.80 6.24 5.84 6.40 7.12 6.51 6.47 6.57 6.25
Bias mg/100 . 0.56 0.96 0.40 - 0.32 0.29 0.33 0.23 0.55
At the
"t" .
. 8.5 7.3 7.4 8.5 3.8 3.6 6.2 5.6
Total
Decision Level
6 15 2 0 3 I 0 9
I 6 1 0 0 0 0 6
.
values in patient sera in contrast to high ones found with control sera. Although we have not determined the cause of the lower trend of results shown by most of the kits, it is possible t h a t it might result from the simple precipitation step with modified Folin reagent or what is termed stabilized reagent by the manufacturers. Special order in the addition of the reagents and care should be exercised, as suggested by Folin et al. (7) in order to achieve full recovery of uric acid in the filtrate. Bornstein et al. (8) and Morris and MacLeod (9) stated t h a t there are two forms of uric acid present in the serum, only one of which is present in the filtrate following the addition of Folin tungstic acid reagent. Perhaps a n o t h e r factor contributing to the disparity of the kits would be inaccuracy and instability of the standards. T h e question arises as to how accurate a uric acid method should be for use in clinical diagnosis. Various studies have indicated t h a t the concentration of uric acid in the sera of normal individuals will be less than 6 m g / i 0 0 , male and female inclusive, and will be over 6 mg/100 for a s y m p t o m a t i c "essential" hyperuricemia, acute gouty arthritis, intercritical gout and chronic tophaceous gout, eclampsia, tissue breakdown, etc. (I1, 12, 18). Bishop and T a l b o t t (11, 12) studied one hundred patients with clinical gouty s y m p t o m s and found 989o of gouty subjects had uric acid values greater than 6 rag/100 ranging from 5.716.2 mg/100 and with a mean value of 8.8 rag/100. T h e S u b - c o m m i t t e e on Criteria of Medical Usefulness of the College of American Pathologists recommended the need for a standard deviation of 4-0.5 rag/100 at the medically significant decision level of 6 rag/100 for uric acid (10). According to the aforementioned criteria and to our findings, Uni-Tech and Hycel kits could yield false information to the physician for the m a n a g e m e n t of the g o u t y subjects. As shown in T a b l e 3, both of these kit methods produced a large bias from the reference mean and among the analyses of the 40 patients revealed numerous discrepancies greater than 1 mg/100 which is the criteria set by the aforementioned subcommittee. T h e Harleco kit also showed more discrepancies on this basis, b u t these were only found in the higher range, i.e.,
a
:
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10 m g / 1 0 0 or over. T h u s these m a n u f a c t u r e r s should improve their products further. Recovery studies were carried out using low normal pooled sera in which the a m o u n t of uric acid was increased 50o-/0, 100% and 200% respectively from the starting base pool value of 3.0 rag/100. Pure stock uric acid standard (100 rag/ 100) was used to increase the concentration. Results are shown in T a b l e 4. I n general recoveries were favourable except for the Stanbio, Uni-Tech and Harleco kits which yielded some low values. No instability of reagents in a n y of the kits was noticed during the test period. T h e costs of the various kit reagents were compared on a 100-test basis. T h e r e is no a p p a r e n t explanation for the wide variation in cost per 100 tests. T h e cost for the eight kits for uric acid ranged from a b o u t $10 to $61 per 100 tests and there is no correlation between the price and the performance of the method b y those reagents. Therefore, some form of control in the marketing of diagnostic kits would seem desirable in view of quality differences and the m o u n t i n g costs of diagnostic medicine. REFERENCES 1. LOGAN, J. E., RENTON, H. M. & ALLEN, R.H. The use of diagnostic kits and reagents in the clinical laboratory in Canada. Clin. Biochem. 3, 81-89 (1970). 2. American Association of Clinical Chemists Committee on Standards and Controls. AACC policy regarding reagent sets and kits. Clin. Chem. 12, 43-44 (1966). 8. CROWLEY, L. V. & ALTON, F.I. Automated analysis of uric acid. Am. J. Clin. Path. 49, 285-288 (1968). 4. BARNETT, R.N. A scheme for the comparison of quantitative methods. Am. J. Clin. Path. 43, 562-569 (1965). 5. TONKS, D.B. Quality control in clinical laboratories, 1970 edition. Diagnostic Reagents Division, Warner-Chilcott Laboratories Co. Ltd., Scarborough, Ontario (1970). 6. BARNETT, R. N., CASH, A. D. & JUNGHANS, S. P. Performance of "kits" used for clinical chemical analysis of cholesterol. New Eng. J. Med. 279, 974-979 (1968). 7. FOLIN, O. A system of blood analysis, a revision of the method for determining uric acid. J. Biol. Chem. 54, 153-170 (1922). 8. BORNSTEIN, A. & GRIESBACH, W. l]ber das Verhalten der Harns~iure in fiberlebenden Menschenblut I. Biochem. Ztschr. 101, 184-196 (1920). 9. MORRIS, J. L. & MAcLEOD, A.G. The uric acid of human blood. J. Biol. Chem. $0, 65-75 (1922). 10. BARNETT, R.N. Medical significance of laboratory results. Am. J. Clin. Path. 50,671-676
(1968).
i 1 . TALBOTT, J.H.
Gout. Oxford University Press, New York (1943). 12. BISHOP C. & TALBOTT, J.H. Uric acid: Its role in biological processes and the influence upon it of physiological, pathological and pharmacological agents. Pharmacological Reviews. $, 231-273 (1953). 13. TALBOTT, J.H. Serum urate in relatives of gouty patients. J. Clin. Invest. 19, 645-648 (1940).