Use of azure-D2 for the measurement of uric acid in serum.

It has been suggested that achieving a chromogenic endpoint with an absorbance read at 600 nm or greater will reduce the degree of spectral interference in many colorimetric methods. We have examined a uricase/peroxidase-based system utilising a novel oxygen acceptor (azure-D2) as chromogen which produces a chromophor with an absorbance which can be measured at 600 nm (Synermed). Results (median, range, mumol/l) obtained on patient sera (n = 113) using the Synermed method (297; 38-847) were lower than those obtained using a 293 nm uricase method (Du Pont Ltd., 312; 62-874) (p < 0.001, Synermed = -16.709 + 1.0065 Du Pont). Within- and between-batch CV's were < 3% in all cases. Results obtained in one external quality assessment scheme (WEQAS) were significantly lower (p < 0.001) than the method group mean (Synermed = -17.298 + 1.0056 WEQAS) but in a second scheme (NEQAS) results did not differ significantly (p > 0.05) from the method group mean (Synermed = -29.315 + 1.0570 NEQAS). Bilirubin had a negative effect (p < 0.0001; 300 mumol/l producing a 23 mumol/l reduction in uric acid) and haemoglobin had a small positive effect (p < 0.05; 5 g/l increasing uric acid by 8 mumol/l) on the assay. Lipaemia did not interfere (p > 0.05) but both ascorbic acid (100 mumol/l reducing uric acid by 68 mumol/l) and N-acetylcysteine (3 mmol/l reducing uric acid by 95 mumol/l) had significant negative effects (p < 0.0001 in both cases). Uraemic serum had no effect on the assay (p > 0.05) but serum storage for 72 hours at room temperature resulted in a significant (p < 0.0005) increase in measured uric acid. The Synermed method is a precise and accurate assay for serum uric acid. However, although generally showing low levels of spectral interference, chemical interferences in the assay from antioxidant components of serum may be problematic. This paper shows that the use of longer wavelengths of detection can reduce the significance of common spectral interferences.