Responsiveness and minimal important change of the Quebec Back Pain Disability Scale in Italian patients with chronic low back pain undergoing multidisciplinary rehabilitation

Background: There is still a lack of information concerning MIC of the QBPDS, that limits its use for clinical and research purposes. Aim: Evaluating responsiveness and minimal important change (MIC) of the Quebec Back Pain Disability Scale (QBPDS) in Italians with chronic low back pain (LBP). Design: Methodological research based on an observational study. Setting: Outpatient rehabilitation hospital. Population: Two hundred and one patients with chronic LBP. Methods: At the beginning and end of a multidisciplinary rehabilitation programme, patients completed the QBPDS. At the end of treatment, they completed a 7-level global perceived effect (GPE) scale, which was split to obtain a dichotomous outcome (improved vs. stable). Responsiveness was calculated by distribution-based [effect size (ES); standardised response mean (SRM); minimum detectable change (MDC95)] and anchor-based methods [Receiver Operating Characteristics (ROC) curves]. ROC curves were also used to compute the MIC (based on QBPDS change score, both absolute and expressed as percentage). Correlations between the change score of the QBPDS and GPE were calculated. Results: The ES was 0.29, the SRM was 0.43, and the MDC95 was 12 points. ROC analysis of the absolute change scores showed a MIC value of 6 points, with an area under the curve (AUC), sensitivity, and specificity of 0.83 (95%C.I. 0.77-0.90), 77.7% and 80.8%, respectively. ROC analysis based on the percent change score from baseline revealed a MIC of 18% with an AUC, sensitivity and specificity of 0.85 (95%C.I. 0.79-0.91), 80.6% and 80.8%, respectively. Correlation between change score of the QBPDS and GPE was ρ=-0.67. Conclusions: The QBPDS score change (expressed in both absolute value and percentage from baseline) was sensitive in detecting clinical changes in Italian subjects with chronic LBP undergoing multidisciplinary rehabilitation. In clinical practice, we recommend -where absolute change is lower than MDC- to rely on the MIC taking into account the percentage change from baseline condition. Clinical rehabilitation impact: The present study investigated the responsiveness and MIC of the QBPDS in a group of patients with chronic LBP. Our findings showed that the QBPDS score may classify with good to excellent discriminatory accuracy subjects who consider themselves as improved. Where examining change, we recommend to consider both MICs we provided (expressing score change both in absolute value and as a percentage from baseline), and disregard values lower than MDC95, not being discernible from measurement error.