Predictors of Residual Stone Fragments After Ureteroscopic Lithotripsy for Upper and Mid-ureteric Calculi: A Prospective Observational Study.

Introduction Residual stone fragments following ureteroscopic lithotripsy (URSL) contribute to stone recurrence, UTI, and the need for repeat procedures. Identifying preoperative predictors of incomplete stone clearance is essential for optimizing surgical planning and patient counselling. We aimed to evaluate clinical, stone-related, and operative predictors of residual fragments after URSL for upper and mid-ureteric calculi. Methods This prospective observational study enrolled 112 patients undergoing URSL for upper and mid-ureteric calculi at a single tertiary care centre between April 2024 and October 2025. A residual stone fragment was defined as any fragment >2 mm on non-contrast CT of the kidneys, ureters, and bladder (NCCT-KUB) at one month. Univariate analysis was performed using Student's t-test, the Mann-Whitney U test, and the chi-square test or Fisher's exact test. Receiver operating characteristic (ROC) curve analysis with Youden index optimization was performed for continuous predictors. Results Stone-free status was achieved in 102 of 112 patients (91.1%), while residual fragments were identified in 10 patients (8.9%). Hounsfield unit (HU) density was the strongest predictor of residual fragments (AUC, 0.920; 95% CI, 0.802-1.000; optimal cutoff, HU ≥1329; sensitivity, 100%; specificity, 82.4%; negative predictive value (NPV), 100%). All 10 residual cases occurred exclusively in patients with HU ≥1300; no patient with HU <1300 had a residual fragment. Stone size was a significant secondary predictor (13.4 ± 2.8 mm vs. 10.9 ± 2.8 mm; p = 0.009; AUC, 0.712). Stone location, anti-retropulsion device type, sex, age, and comorbidities were not significantly associated with residual fragment formation. Conclusion Preoperative HU density on NCCT-KUB is the strongest predictor of residual stone fragments after URSL, with a threshold of HU ≥1300 identifying all patients at risk with 100% sensitivity and an NPV of 100%. A stone size of ≥13 mm is a significant secondary predictor. Routine reporting of HU density may guide preoperative counselling, surgical planning, and the intensity of postoperative follow-up.