Positive interference of catecholamine metabolites with quantitation of urinary uric acid by the direct acid ferric-reduction method.

We report the effect of major catecholamine metabolites such as homovanillic acid and vanilmandelic acid, normally present in urine, on quantitation of urinary uric acid by the direct acid ferric-reduction procedure [(Am. J. Clin. Pathol. 60, 691 (1973)]. When aqueous uric acid standards containing a mixture of these two metabolites, in concentrations comparable to those in a normal human 24-h urine, were added to Fe3+-phenanthroline or Fe3+-2,4,6-tripyridyl-s-triazine reaction mixture, the resulting absorbances at 505 or 593 nm considerably exceeded those of standards without such additions. Moreover, quantitation of urinary uric acid by the direct acid ferric-reduction procedure gave values that significantly exceeded those determined by the uricase method. Interference by the two metabolites was eliminated by extracting urine specimens with ethyl acetate before quantitation. Spectral scans revealed that homovanillic acid or vanilmandelic acid alone could also effectively reduce Fe3+-chelator complex and that the effect of these compounds on the analytical reaction was additive.