Urine processing impacts uric acid level in HIV-infected adults: implications for diagnosing tenofovir-associated proximal tubulopathy.

Urine specimens are valuable to investigate kidney disease and antiretroviral nephrotoxicity. We observed large rust-colored pellets in some urine specimens after overnight storage at 4°C. Testing of supernatant under varying conditions demonstrated that this phenomenon reflects supersaturation and precipitation of uric acid in samples with high uric acid concentration. Delays in processing, even with refrigeration, may decrease the sensitivity of urine uric acid testing, with implications for the evaluation of proximal tubulopathy related to tenofovir.