Comparison of plasma and urinary concentrations of uric acid measured by the colorimetric phosphotungstate and enzymatic uricase methods

Abstract

SUBJECTS, METHODS AND RESULTSPlasma and 24-hour urine collections were obtained from unselected patients with normal creatinine clearance, over a six month period. All samples and standards were assayed for uric acid using all methods.Colorimetric phosphotungstate method. Uric acid added to phosphotungstate in alkaline solution produces an intensely blue colour caused by an uncharacterised chromophore. The colour yield is directly related to the concentration of uric acid. The automated method has high reproducibility and facilitates a high standard of quality control. Errors in this method are due to the activity of non-specific chromagens in the samples and from the loss of uric acid which precipitates with protein, which is then removed by dialysis. This results in non-linearity between colour yield and uric acid concentration especially in urine where non-specific substances-for example, cystine and thiols--can contribute to total measured colour.Enzymatic uricase methods. Uric acid is oxidised by uricase to allantoin. The disappearance of uric acid may be monitored by measuring the associated decrease in absorbance at 292 nmol/l in a spectrophotometer. This method has a high degree of specificity and sensitivity for the analysis of both blood and urine. Estimations of urinary excretion of uric acid by the phosphotungstate colorimetric method were a mean of 48% higher than by the manual uricase assay (n = 30; r = 0 840). This difference was not proportional to the concentration.There was an extremely good correlation of plasma and urinary concentrations of uric acid by the manual and automated uricase methods.COMMENT