Validity of self-reported height and weight and derived body mass index in middle-aged and elderly individuals in Australia

Body mass index (BMI) is an important measure of adiposity. While BMI derived from self-reported data generally agrees well with that derived from measured values, evidence from Australia is limited, particularly for the elderly. We compared self-reported with measured height and weight in a random sample of 608 individuals aged ≥45 from the 45 and Up Study, an Australian population-based cohort study. We assessed degree of agreement and correlation between measures, and calculated sensitivity and specificity to quantify BMI category misclassification. On average, in males and females respectively, height was overestimated by 1.24cm (95% CI: 0.75-1.72) and 0.59cm (0.26-0.92); weight was underestimated by 1.68kg (-1.99-1.36) and 1.02kg (-1.24-0.80); and BMI based on self-reported measures was underestimated by 0.90kg/m2 (-1.09-0.70) and 0.60 kg/m2 (-0.75-0.45). Underestimation increased with increasing measured BMI. There were strong correlations between self-reported and measured height, weight and BMI (r=0.95, 0.99 and 0.95, respectively, p<0.001). While there was excellent agreement between BMI categories from self-reported and measured data (kappa=0.80), obesity prevalence was underestimated. Findings did not differ substantially between middleaged and elderly participants. Self-reported data on height and weight quantify body size appropriately in middle-aged and elderly individuals for relative measures, such as quantiles of BMI. However, caution is necessary when reporting on absolute BMI and standard BMI categories, based on self-reported data, particularly since use of such data is likely to result in underestimation of the prevalence of obesity.