Association of a 0.85 Ratio of Endoscope–Sheath Diameter Threshold with Infectious Complications after Retrograde Intrarenal Surgery: A Two-Institution Retrospective Cohort Study

Purpose: To evaluate the clinical impact of the ratio of endoscope–sheath diameter (RESD) on postoperative infectious complications after retrograde intrarenal surgery (RIRS) and to assess whether a prespecified RESD threshold of 0.85 provides better risk discrimination than the conventional target (<0.75). Materials and Methods: We retrospectively evaluated consecutive adults who underwent RIRS at two academic institutions. The RESD was defined as the outer diameter of the ureteroscope divided by the inner diameter of the ureteral access sheath (UAS). The primary outcome was postoperative infectious complications graded according to the Clavien–Dindo classification. Infection rates were assessed across ordered RESD categories (<0.75, 0.75 to <0.85, and >0.85), with a p for trend from ordinal logistic regression, and compared using prespecified dichotomous thresholds (0.75 and 0.85). Predictors of infectious complications were assessed using univariable and prespecified multivariable Firth’s penalized logistic regression. Results: Among the 243 patients, 20 (8.2%) experienced infectious complications. Infection rates increased stepwise across RESD categories (<0.75, 0.75 to <0.85, and >0.85; 2.6%, 4.6%, and 14.6%, respectively; p for trend = 0.009). Using the prespecified 0.85 threshold, infection occurred in 14.6% (14/96) of patients with RESD >0.85 vs 4.1% (6/147) with RESD ≤0.85 ( p = 0.007), whereas dichotomization at 0.75 did not significantly discriminate infection risk (2.6% [1/39] vs 9.3% [19/204]; p = 0.214). In multivariable Firth’s penalized logistic regression, both RESD >0.85 (adjusted odds ratio [aOR] 3.73, 95% confidence interval [CI] 1.41–9.88; p = 0.008) and positive stone culture (aOR 3.50, 95% CI 1.33–9.22; p = 0.011) remained independently associated with infectious complications. Conclusions: RESD >0.85 was independently associated with post-RIRS infectious complications. A threshold of 0.85 provided more clinically useful risk stratification than the conventional RESD <0.75 target, while highlighting the importance of infected stone assessment.