Irritable bowel syndrome clinical trials: getting the IBS Severity Scoring System responder rate right

The Irritable Bowel Syndrome Severity Scoring System (IBS-SSS) is frequently used to characterize treatment effects in IBS patients. Calculation of a responder rate using IBS-SSS data commonly uses a cut-off value of -50 points.

To analyze data from a recently published IBS trial to evaluate whether this cut-off value is appropriate or needs to be revised.

Data from 201 patients were used to establish a receiver operating characteristic (ROC) curve for IBS-SSS change data using IBS-Adequate Relief scale data as a binary classifier. The area under the curve (AUC) and its 95% CI for the ROC were determined. Curves plotting accuracy rates and Youden's J index values against changes on the IBS-SSS used as cut-off values were employed to determine an optimal cut-off value.

The AUC of the ROC curve was 0.9738 (95% CI from 0.9565 to 0.9912). The curve of accuracy rates had two maxima (-122 and -133 points), while that of the Youden's J index values had only one maximum at -122 points, considered to be the optimal cut-off value.

The frequently used -50 points cut-off value might not be optimal for determining clinically meaningful responder rates from IBS-SSS measurements. At least for the IBS-SSS data analyzed in the present study, a change of -122 points appears to be more appropriate. Interpretation of IBS-SSS responder rates from past or future clinical trials should carefully check for the cut-off values applied for their calculation.