PURPOSE: The Core Outcome Measure Index (COMI) is a multidimensional questionnaire that investigates five dimensions in low back pain (LBP) patients, but does not address the psychological dimension. As the biopsychosocial perspective is recognized as important to capture the entire clinical picture of these patients, this multicenter prospective cohort study was designed to investigate the psychometric properties of a modified version of the COMI (COMIAD) which included 2 additional items, exploring anxiety and depression, respectively.
METHODS: 168 subacute or chronic LBP patients recruited in spine clinics completed a set of questionnaires before and after treatment (follow-up at 6 months). Construct validity was explored by comparing each item of the COMIAD to validated full-length questionnaires. Thus two additional questionnaires were included to assess the construct validity of the anxiety and depression measures. The psychometric properties of the COMI and COMIAD were then compared.
RESULTS: The two new items showed good internal consistency, high correlations with the corresponding full-length questionnaires, no floor or ceiling effect and good reproducibility (test-retest agreement kappa 0.68 for anxiety, 0.62 for depression). The addition of the 2 items did not alter internal validity (Cronbach's alpha = 0.88 and 0.87, respectively). The smallest detectable difference, the Minimal Clinically Important Improvement and the Patient Acceptable Symptom State were only minimally affected by the changes.
CONCLUSION: The questions exploring anxiety and depression have good intrinsic and psychometric capacities (i.e., no floor or ceiling effects and high correlations with full-length scales) and did not significantly modify the psychometrics of the original COMI questionnaire. The COMIAD offers the possibility to include the psychological dimension in the multidimensional evaluation without significantly affecting questionnaire length.