Objectives: To evaluate criterion and construct validity of the self-administered comorbidity questionnaire (SCQ) in patients with AS.
Methods: The SCQ and indices of disease activity, physical function, health-related quality of life (HRQoL) and work disability were administered to 98 patients with AS. Criterion validity was assessed by the agreement between the SCQ answers and comorbidities identified in medical records. Construct validity was assessed by correlating the SCQ with the Charlson index and MichaudWolfe index; by correlating the SCQ with demographics, physical function, HRQoL and AS-related 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) was evaluated for the same aspects of validity, after removing rheumatic conditions.
Results: Agreement was moderate to perfect for most conditions (k 0.471.00), except for ulcer disease, depression and OA (k 0.140.15). The correlation between the SCQ and Charlson and MichaudWolfe indices was 0.24 and 0.39 respectively, and between the mSCQ and both indices 0.36 and 0.53. Both SCQ and mSCQ correlated weakly to moderately with age, physical function and HRQoL (0.240.45). The SCQ also correlated weakly with disease activity (0.27) while the mSCQ did not (0.17). In multivariable analysis, both SCQ and mSCQ contributed independently to physical function, HRQoL and work disability, while the MichaudWolfe and Charlson indices did not.
Conclusion: 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 validity.