Reported BMI – new concerns!

Bias in hazard ratios arising from misclassification by self-reported weight and height in observational studies of body mass index and mortality by Flegal K, Kit BK and Graubard BI in the Am J Epi, 2017 (Epub ahead of print)
Abstract
Misclassification of body mass index (BMI) categories arising from self-reported weight and height can bias hazard ratios (HRs) in studies of BMI and mortality. We examined the effects on HRs of such misclassification using national US survey data for 1976 through 2010 that had both measured and self-reported weight and height along with mortality follow-up for 48,763 adults and a subset of 17,405 healthy never-smokers. BMI was categorized as < 22.5, 22.5-24.9 (reference), 25-29.9 (overweight), 30-34.9 (Class I obesity) and ≥35 (Class II/III obesity). Misreporting at higher BMI categories tended to bias HRs upwards for those categories, but that effect was augmented, counter-balanced or even reversed by misreporting in other BMI categories, in particular those that affected the reference category. For example, among healthy male never-smokers, misclassifications affecting the overweight and the reference categories changed the HR for overweight from 0.85 with measured data to 1.24 with self-reported data. Both the magnitude and direction of bias varied according to the underlying HRs in measured data, showing that findings on bias from one study should not be extrapolated to a study with different underlying HRs. Because of misclassification effects, self-reported weight and height cannot reliably indicate the lowest risk BMI category.