Interference of sodium azide with measurement of serum uric acid by the direct acid ferric reduction procedure.

We examined the effect of sodium azide on the quantitation of serum uric acid by the direct acid ferric reduction procedure. Ferric phenanthroline was used as redox indicator. Sodium azide, in a concentration commonly used as preservative (2 g/liter, 27.2 mmol/liter), increased the absorption at 505 nm and increased apparent uric acid values in specimens, as shown by calculations based on an azide-free standard. Spectral studies indicated that this interference was a result of the color produced by sodium azide in the reaction mixture. The mechanism for azide interference was the interaction of sodium azide and ferric ions to form ferric azide, which also absorbs extensively at 505 nm.