Imaging characteristics of indinavir calculi

Purpose: Indinavir sulfate is an effective protease inhibitor of the human immunodeficiency virus type 1. Use is associated with a significant incidence of crystallization and stone formation in the urinary tract, and these calculi are not visible on plain radiographs. Previously all urinary stones, including uric acid and matrix, were believed to be radiodense on computerized tomography (CT). We conducted a retrospective study to evaluate the radiographic appearance of indinavir calculi. Materials and Methods: Retrospective chart review of 36 patients taking indinavir sulfate and presenting with renal colic was performed with attention to presentation, urinalysis, radiographic evaluation and management. Specifically, imaging characteristics on CT were addressed. Results: All patients complained of ipsilateral flank pain and 35 had nausea and/or vomiting. Of 30 patients with dysuria or urgency the majority had hematuria, and most had pyuria and/or proteinuria. No stones were visualized on abdominal radiography. Diagnosis was confirmed on 1 of 13 excretory urograms and 4 of 11 renal ultrasounds. None of 12 CT scans was diagnostic of renal lithiasis. Conclusions: Indinavir sulfate is a protease inhibitor with poor solubility and significant urinary excretion. Crystallization and stone formation are demonstrated in as many as 20% of patients taking the medication. Most patients present with flank pain, nausea or vomiting and hematuria. Previously CT was thought to identify all urinary calculi with clarity but it cannot reliably confirm the presence of indinavir calculi.