Responsiveness and longitudinal validity of the Persian version of COMI to physiotherapy in patients with non-specific chronic low back pain.

Background and aims The Core Outcome Measures Index (COMI) is a short, self-reported questionnaire for assessing important outcomes in patients with low back pain (LBP). The present study was conducted to explore the responsiveness and longitudinal validity of the Persian COMI (COMI-P) in patients with non-specific chronic LBP. Methods In this prospective cohort study of patients with non-specific chronic LBP receiving physiotherapy, patients completed a booklet containing the COMI-P, Persian Functional Rating Index (FRI-P), and a visual analogue scale (VAS) for pain before and after the end of ten-sessions of physiotherapy. Patients also completed a global rating of change scale (GRCS) at the end of the physiotherapy. Responsiveness was examined by means of internal responsiveness methods [t-test, standard effect size (SES); standardized response mean (SRM), and Guyatt responsiveness index (GRI)] and external responsiveness methods [correlation with external criteria and receiver operating characteristics (ROC) curve]. Results Fifty patients with a mean age of 50.62 ± 13.8 years participated. The paired t-test showed significant changes in COMI-P scores (p < 0.001). The effect sizes for COMI-P were large (range 0.96-1.23). The score changes for the COMI-P revealed significant correlations with FRI-P (r = 0.67, p < 0.001), the VAS (r = 0.65, p < 0.001), and the GRCS (r = 0.34, p = 0.02). The COMI-P change scores showed excellent correlation with the dichotomized smallest detectable change (SDC) criterion (r = 0.83, p < 0.001). The ROC area under the curve for the COMI-P based on the dichotomized SDC criterion was perfect. The minimal clinically important change was estimated 2.15 points (sensitivity 94% and specificity 100%). Conclusions The COMI-P appears to have responsiveness and longitudinal validity in detecting changes after physiotherapy for non-specific chronic LBP. An improvement of 2.15 points in COMI-P total score is required to be interpreted as minimally clinically important change in individual patients.