Estimating Minimally Important Differences for the Bladder Cancer Index Using Distribution- and Anchor-Based Approaches

Abstract

PurposeThe Bladder Cancer Index is a validated condition-specific health questionnaire assessing urinary, bowel and sexual function and quality of life among bladder cancer patients. We aimed to establish minimally important difference score thresholds that signal clinical importance. MethodsA prospective cohort of 150 patients who underwent radical cystectomy between 2013 and 2016 were followed for one year after surgery. Usable data from 138 patients was analyzed. Bladder Cancer Index and Medical Outcomes Study Short Form (SF-36) questionnaires were completed prior to cystectomy and at 3, 6 and 12 months postoperatively. Distribution-based minimally important differences were estimated at 1/3 and 1/2 standard deviations for each index domain across time points. Changes in index domain scores anchored to changes in a SF-36 overall health assessment question estimated anchor-based minimally important differences. Pooled averages between time points and methods were reported. ResultsDistribution-based minimally important difference for 1/3 standard deviations for urinary, bowel and sexual domains ranged between 5.3-7.3, 4.6-5.6, and 6.0-8.9, respectively. Ranges for 1/2 standard deviations were 8.8-10.9, 6.8-8.4, and 8.9-13.5, respectively. The anchor-based approach resulted in minimally important difference estimates of 6.2, 7.3, and 6.8, respectively. Aggregated results across the two approaches resulted minimally important differences of 6-9, 5-8 and 7-11 points for urinary, bowel and sexual domains, respectively. ConclusionsThrough two independent approaches, we established the first minimally important difference estimates for the Bladder Cancer Index. Defining patient reported outcome thresholds is important for interpreting changes or differences in Bladder Cancer Index scores.