Study Design. A prospective cohort study with a 6-week follow-up of patients with chronic low back pain undergoing physiotherapy.
Objective. To examine the responsiveness of the Portuguese version of the Quebec Back Pain Disability Scale (QBPDS-PT), and to determine the minimal clinically important difference, minimal detectable change (MDC), and floor/ceiling effects.
Summary of Background Data. Measuring change over time is critical to assess the effectiveness of a physiotherapy intervention or to distinguish individual differences in response to treatment.
Methods. One hundred thirty-two patients were recruited from 16 outpatient clinics in 7 different regions of Portugal. A final sample of 120 patients completed the QBPDS-PT twice: at the baseline and after 6 weeks of physiotherapy treatment. The patient global impression of change scale was used as an external criterion measure to distinguish between improved or nonimproved patients' scores between baseline and follow-up. The responsiveness of the QBPDS-PT was assessed through correlation coefficient and receiver operating characteristics curves. The minimal clinically important difference was estimated by the receiver operating characteristics curve method and the MDC through the standard error of measurement.
Results. The scale revealed moderate responsiveness (ρ = 0.426 and area under the curve = 0.741; 95% confidence interval: 0.645–0.837). The MDC achieved 19 points, whereas the minimal clinically important difference was found to be 6.5 points (area under the curve = 0.741, sensitivity = 72%, specificity = 71%). A floor effect was founded with 15.8% of the participants reporting values within the MDC at the lower end of the available range of scores. For the highest baseline scores of QBPDS-PT (≥34 points) the optimal cutoff point was found to be 10.5 points (area under the curve = 0.738, sensitivity = 73%, specificity = 67%).
Conclusion. The QBPDS-PT demonstrated moderate levels of responsiveness, and is recommended to measure change in disability in patients with chronic low back pain after physiotherapy intervention.
Level of Evidence: N/A