Construct validity of the Short Form-36 Health Survey and its relationship with BMI in obese outpatients

Objective: To investigate the construct validity of the Short Form-36 (SF-36) Health Survey questionnaire in obese patients. Research Methods and Procedures: Our series consisted of 1735 obese patients (age, 44.7 +/- 11.0 years; 1346 women) consecutively enrolled in the QUOVADIS study, an observational multicenter study of obese treatment-seeking outpatients. The construct validity of the SF-36 was assessed by main component analysis. Age-, gender-, and education-adjusted general linear models were used to investigate the relationship between BMI and SF-36 domains or factors identified by main component analysis. Results: BMI was significantly associated with poor health-related quality of life in all eight SF-36 domains, and the strongest association was observed with physical activity. Main components analysis generated a six-factor solution explaining 59% of the observed variance. BMI was strongly associated with factors based on the loading of items regarding the physical activity domain and factors based on role-physical and role-emotional items or general health and bodily pain items. In contrast, mental health-, vitality-, and social functioning-based factors were not related to BMI. Discussion: In obese treatment-seeking outpatients, the clustering of SF-36 items in main components is not significantly different from the domain-based approach generally used, thus confirming the robustness of such a generic questionnaire in this specific condition. However, the peculiar clustering of some SF-36 items and their relationship with BMI suggest that the health-related quality of life profile of subjects belonging to that population may be better described with alternative aggregations of the SF-36 items or with disease-tailored questionnaires.