A receiver operating curve analysis was performed to assess the predictive value of the urinary cystatin C to urinary creatinine ratio for the renal monitoring of tenofovir. Urinary cystatin C to urinary creatinine ratio was measured in 37 samples from patients referred for suspected tenofovir-induced Fanconi syndrome. The best threshold (14 microg/mmol) was associated with sensitivity, 90.9%; specificity, 88.5%; positive predictive value, 76.9%; and negative predictive value, 95.8%. Urinary cystatin C to urinary creatinine ratio allows to rule out a Fanconi syndrome in most cases; thus, it should be used for the safety follow-up of nucleotide reverse transcriptase inhibitor-treated patients.