Validation of self-reported weight and body mass index of the participants of a cohort of university graduates

Abstract

Objectives: To assess the validity of self-reported weight and Body Mass Index (BMI) in the participants of a multi-purpose prospective cohort. Methods The SUN Study is a cohort of university graduates, recruited and followed-up using mailed questionnaires. We compared the self-reported weight and height in the baseline questionnaire with the objectively measured weight and height in a medical visit after the participants filled the questionnaire. Participants did not know that the self-reported weight and height would be compared with the measured weight and height. Results: The average relative error was -1.45% (95% CI:-2.03% to -0.86%) for the weight and -2.64% (95% CI: -3.70% to -1.60%) for the BMI. Self-reported BMI showed values for sensitivity of 90%, for specificity of 100%, for positive and negative predictive values of 100% and 93% respectively, to detect overweight/obesity (BMIε 25 kg/m 2). The kappa index was 0.91 (95% CI: 0.81 to 0.99). The correlation coefficient between self-reported and measured weight was 0.991 (95% CI: 0.986 to 0.994) and it was 0.944 (95% CI: 0.911 to 0.965) for the BMI. Validity of self-reported weight was poorer among participants with higher weight (p= 0.021) and lower height (p= 0.034). Conclusions: Self-reported weight and BMI in university graduates is good enough to be used in epidemiological studies as a continuous outcome variable or to adjust for it as a potential confounder.