Enzymatic photometric test using TOOS (N-ethyl-N-(hydroxy-3- sulfopropyl)-m-toluidin).

Uric acid is oxidized to allantoin by uricase. The generated hydrogen peroxide reacts with 4-aminoantipyrine and N-ethyl-N-(hydroxy-3- sulfopropyl)-m-toluidin (TOOS) to a blue violet dye. Ascorbate oxidase avoids interference by ascorbic acid and other reducing substances.

Uric Acid + O2 + H2O       (Uricase)  →  Allantoin + CO2 + H2O2

4-AA + TOOS + 2 H2O2 (POD)→ Indamine + 3 H2O