Preoperative Prediction of Impacted Ureteric Stones Using the Impacted Stone Formula

Objective: To evaluate the diagnostic performance of the Impacted Stone Formula (ISF) in a South Asian cohort undergoing semirigid ureteroscopy (URS). Study Design: An analytical study. Place and Duration of the Study: Department of Urology, Aga Khan University Hospital, Karachi, Pakistan, from March to September 2025. Methodology: This study included 120 adult patients who underwent semirigid URS for a single, unilateral, radio-opaque ureteric stone at Aga Khan University Hospital, Karachi, Pakistan. Preoperative non-contrast computed tomography (NCCT) parameters, including stone characteristics, ureteric wall thickness (UWT), hydronephrosis grade, and Hounsfield unit (HU) measurements proximal and distal to the stone, were recorded and analysed using SPSS. ISF was calculated using the validated formula: ISF = UWT × (HU below/HU above) × (1 + hydronephrosis grade). Stone impaction was confirmed intraoperatively by failure to pass a guidewire beyond the stone during initial ureteral catheterisation. The cut-off value for ISF was determined using the Youden Index. Receiver operating characteristic (ROC) analysis and multivariable logistic regression were performed to assess predictive accuracy. Results: Among 120 patients, 57 (47.5%) had impacted stones. Impacted stones were associated with significantly higher UWT, hydronephrosis grades, stone density, and ISF values. ROC analysis demonstrated excellent discriminatory performance of the ISF with an area under the curve of 0.96. At a cut-off value of 15.5, ISF showed 75% sensitivity and 93.7% specificity. Impacted stones were associated with longer operative times, increased intraoperative bleeding, longer length of stay, and higher postoperative stent placement rates. Conclusion: The ISF is a reliable and practical preoperative predictor of ureteric stone impaction. Incorporation of ISF in routine practice, along with NCCT assessment, may improve surgical planning, risk stratification, and patient counselling before the procedure.