100% Uric Acid Stone Formers: What Makes Them Different?

OBJECTIVE To identify what risk factors on 24-hour urinalysis, if any, predispose patients to have higher percentages of uric acid (UA) stone composition in their stones, with specific emphasis on patients with pure UA stones. METHODS We retrospectively identified 308 patients from review of a kidney stone analysis database. Patients were grouped according to the percentage UA composition: 10%-20%, 30%-50%, 60%-90%, and 100% UA. Data were extracted from 24-hour urine collections and serum chemistries. Patients taking allopurinol, citrates, or thiazide diuretics were excluded. RESULTS The percentage UA stone composition increased as patients became older (P = .05) or heavier (P <. 001). Gender did not impact the percentage of UA in stones. Although a higher serum UA level was associated with higher UA stone composition (P <. 0006), urinary UA levels did not correlate (P = .1). In contrast, urinary pH correlated significantly with higher UA stone composition (P = .03). CONCLUSION Older and heavier patients with higher serum UA levels are more likely to have a pure UA stone. This information combined with traditional predictors (urine pH, radiopacity of stone, and Hounsfield units) may help identify those most likely to respond to dissolution therapy. (C) 2015 Elsevier Inc.