Clinical Impact of the Ratio of Endoscope-to-Sheath Diameter on Infectious Complications after Retrograde Intrarenal Surgery

This study aimed to evaluate the safety of retrograde intrarenal surgery (RIRS) and to investigate whether the ratio of endoscope-to-sheath diameter (RESD), a surrogate marker of outflow resistance, is associated with postoperative infectious complications. We retrospectively reviewed 155 patients who underwent RIRS using a disposable flexible ureteroscope. RESD was calculated by dividing the outer diameter of the ureteroscope by the inner diameter of the access sheath, and dichotomized at a threshold of 0.85 (≤ 0.85 vs. >0.85). Univariate and multivariate analyses were performed to identify the factors associated with infectious complications. Postoperative infectious complications occurred in 10.3% of the patients. All patients were successfully managed with intravenous antibiotics alone, and none of them progressed to septic shock. RESD > 0.85 was significantly associated with higher infection rates (p = 0.041) and remained an independent risk factor in the multivariate analysis (odds ratio 3.38; 95% confidence interval, 1.07–10.71; p = 0.038). Other clinical variables, including body mass index, diabetes mellitus, operative time, and preoperative urine culture, were not significant predictors. RESD was independently associated with postoperative infectious complications after RIRS. A threshold of > 0.85 was identified as clinically relevant, which suggests that maintaining RESD at or below 0.85 may help reduce the risk of infection. Therefore, the RESD may serve as a simple and practical indicator of outflow resistance, aiding preoperative planning and intraoperative risk assessment.