To describe the natural history of postureteroscopic renal stone fragments ≤4 mm based on computed tomography (CT) follow-up. The goal of ureteroscopy is to fragment stones, actively basket and remove fragments larger than 1 mm, and allow the remaining fragments to pass spontaneously. The reality is that smaller fragments may be difficult to extract or may be missed.

Patients treated with ureteroscopy and holmium laser lithotripsy for urolithiasis by a single surgeon from May 2001 to July 2008 at a tertiary referral center were identified. Patients with residual renal fragments measuring ≤4 mm on initial postoperative CT and at least one additional follow-up CT were included. Outcomes measured were fragment growth and location, stone event (emergency department visit, hospitalization, or additional intervention), and spontaneous fragment passage.

Of 330 ureteroscopies, 51 met inclusion criteria. For these patients, the mean follow-up duration was 18.9 months (1.6 years). Among 46 ureteroscopies for calcium-based stones, 9 patients (19.6%) experienced a stone event, 10 patients (21.7%) spontaneously passed their fragments, and the remaining 27 patients (58.7%) retained asymptomatic residual fragments. Among this asymptomatic group, mean fragment sizes were similar at 2.7, 3.3, 3.5, and 3.0 mm at mean follow-up durations of 2.8, 10.2, 16.8, and 33.0 months, respectively.

This study suggests that among patients with postureteroscopic renal stone fragments ≤4 mm, approximately one in five (or 19.6%) will experience a stone event over the following 1.6 years. The remaining patients will either become stone-free via spontaneous passage or retain asymptomatic stable-sized fragments.