Hyperxanthinemia interferes with serum uric acid determinations by the uricase method.

Serum xanthine concentrations as high as 148 mg/L were noted after treatment of a patient with Burkitt's lymphoma who was receiving allopurinol. These markedly above-normal values for xanthine led to spuriously low values for serum uric acid as measured by the uricase method. Rapid tumor lysis in patients who are receiving allopurinol may lead to marked hyperxanthinemia, which in turn may obscure hyperuricemia in such patients when the uricase method is used for uric acid analysis. In such situations, uric acid concentrations should be measured by the phosphotungstate colorimetric assay.