Objective: To assess the construct validity of the Self-Administered Comorbidity Questionnaire (SCQ) in patients with systemic sclerosis (SSc) and systemic lupus erythematosus (SLE).
Methods: The SCQ was modified for the SSc cohort to emphasize the objective of recording problems other than the patients' scleroderma. It was administered to 406 SSc and 147 SLE patients. Construct validity of the SCQ was evaluated separately in the SSc and SLE cohorts by testing the hypotheses that a valid comorbidity index should correlate with age and health-related quality of life (Medical Outcomes Trust Short Form 36 [SF 36]) but not with disease-specific variables.
Results: The SCQ score correlated with age in the SSc patients only (Tau B=0.37, P<0.001) and not in the SLE patients. It correlated with the SF 36 in both SSc and SLE. However, it also correlated with several disease-related variables. There was significant overlap between reports of comorbidities and disease-related problems in the SSc cohort.
Conclusion: Patients with systemic autoimmune diseases cannot distinguish true comorbidities from conditions related to their index disease and, as such, a self-administered comorbidity questionnaire does not appear useful in these diseases.