Background Comorbidities can importantly influence the results of clinical studies on health outcomes. The generic self-administered comorbidity questionnaire (SCQ) is a frequently used instrument to assess common comorbidities which might impact functioning but has never been validated for use in ankylosing spondylitis (AS).
Objectives To evaluate aspects of validity of the SCQ in patients with AS.
Methods The SCQ (range 0-45) asks about the presence, treatment and functional limitations of 12 common comorbidities and three possible other but not pre-specified medical problems. The SCQ, demographics and indices of disease activity (BASDAI, ASDAS-CRP); physical function (BASFI) and health-related quality of life measures (HRQoL; SF-36, ASQoL, EuroQoL-VAS, work status), were administered to 98 patients with AS who were followed in the Outcome in Ankylosing Spondylitis International Study (OASIS). The agreement between the SCQ-items and comorbidities retrieved from medical records was calculated. Truth was assessed by correlating the SCQ with the Charlson-index (predicting mortality) and Michaud/Wolfe-index (predicting health outcomes); by testing the hypothesis that a valid comorbidity index should correlate with age, function and overall HRQoL but not with disease activity; and by exploring the contribution of comorbidity to these outcomes while adjusting for clinical-demographic characteristics. Furthermore, a modified version of the SCQ (mSCQ), in which musculoskeletal conditions were removed was evaluated for the same aspects of truth.
Results The median SCQ-score was 5 (range 0-19) and the median mSCQ-score was 2 (range 0-13). Frequently reported non-rheumatic comorbidities were hypertension (27.6%), inflammatory bowel disease (10.2%) and depression (9.2%). Agreement between self-report and medical records was moderate to perfect for all diseases included in the SCQ (kappa 0.47-1.00), except for stomach disease, depression, and osteoarthritis (kappa 0.14-0.15). The correlations of the SCQ with the Michaud/Wolfe index and the Charlson index were 0.39 and 0.24 respectively, and of the mSCQ with both indices 0.53 and 0.36 respectively. The SCQ correlated weakly with age (r=0.24) and disease activity (BASDAI r=0.27), and moderately with function (r=0.43), HRQoL (SF-36 physical r=-0.45; ASQoL r=0.43) and was associated with work status (OR 1.31, 95% CI 1.13-1.53). The mSCQ correlated weakly with age (r=0.28), moderately with function (r=0.41), HRQoL (SF-36 physical r=-0.41, ASQoL r=0.32) and with work status (OR 1.48, 95% CI 1.20-1.81), but not did not correlate with measures of disease activity. In multivariable analysis, both SCQ and mSCQ contributed independently to physical function, HRQoL and work disability, while the Michaud/Wolfe- and Charlson-index did not.
Conclusions The SCQ is a promising instrument to determine comorbidities and to understand the impact on health outcomes in patients with AS. Excluding rheumatic conditions from the SCQ (mSCQ) improved truth.
Disclosure of Interest None Declared