Interference of the chemotherapeutic agent etoposide with the direct phosphotungstic acid method for uric acid

A 62-year-old woman receiving chemotherapy with etoposide showed discrepant uric acid values as measured by a direct phosphotungstic acid (PTA) method (150 mg/L) compared with a uricase technique (40 mg/L). After ultrafiltration, the positive interference for the direct PTA method was retained in the protein fraction, but not in the filtrate. Adding exogenous etoposide to drug-free serum confirmed this interference for the direct PTA method, but not for the uricase procedure or a PTA technique preceded by dialysis. Decisions for aggressive patient management are often based on the magnitude of hyperuricemia. We do not recommend that the direct phosphotungstic acid method be used to measure uric acid in patients receiving etoposide.