Stone composition independently predicts stone size in 18,029 spontaneously passed stones

Purpose To evaluate whether the size of spontaneously passed stones (SPS) may be associated with clinical parameters. Methods A search for SPS was conducted in our electronic stone database, comprising data on stones analyzed over the last 33 years at our institution. Adults with upper urinary tract stones were included. Cases with stenotic urinary tract disease or past history of anastomotic urinary tract surgery were excluded. Stone size expressed as maximal stone diameter (MSD) and stone volume (SV) was compared between groups by one-way ANOVA. Logistic regression analyses were performed to identify predictors of MSD ≥ 6 mm. Results Overall mean MSD and SV for 18,029 SPS was 4.1 mm and 11.5 mm 3 , respectively, and significantly differed between stone composition groups ( p < 0.001). The lowest mean MSD and SV were found for calcium oxalate monohydrate (3.6 mm and 9.0 mm 3 , respectively) and the highest mean MSD and SV were found for struvite (7.9 mm and 61.0 mm 3 , respectively). Stone composition and increasing age were found to be independent predictors of MSD ≥ 6 mm (both p < 0.001). Sex differentiation did not contribute as a predictor of MSD ≥ 6 mm. Conclusions Stone composition and—to a lesser extent—age serve as independent predictors of size of spontaneously passed stones. Of particular importance, large spontaneously passed stones of ≥ 6 mm may be frequently found in cystine, brushite or struvite stone formers, whereas a minority of all calcium oxalate stones exceed that cutoff. Future studies shall evaluate these parameters as possible predictors of spontaneous stone passage.