Receiver operating characteristic analysis of body mass index to detect increased risk of functional morbidity in Vietnamese rural adults

Objective: To assess the performance of low BMI, and define optimal BMI cut-off values in order to detect fever-associated adult morbidity. Design: A cohort study of adults between 18 and 60 y in rural Vietnam, whose BMI and health status were assessed at baseline, and who were then monitored for illness events for 4 months. Nonparametric receiver operating characteristic (ROC) analysis was used to evaluate the performance of low BMI to detect the average number of restricted-days due to illness and to determine optimal cut-off values. Setting: A rural commune in the Red River Delta, northern Vietnam. Subjects: The study included 693 men and 739 women aged 18 - 60- y. Results: At baseline, 21% of the study participants had a BMI < 18.5 kg/m(2). As BMI progressively decreased, the percentage of participants experiencing morbidity with fever increased. The areas under the ROC curves for BMI were significantly greater than 0.5 for all levels of monthly average restricted-days of illness (MARDI) with fever, with best performance for >5 days/month. Excluding participants with acute or chronic disease at baseline improved the performance of BMI to detect MARDI with fever of 45 days ( area under ROC curve 0.95; 95% CI 0.92, 0.99). With increasing levels of MARDI with fever, BMI cut-offs fell to 17.9 kg/m(2) when MARDI with fever was >5 days. Conclusions: The ROC analysis demonstrates that low BMI performs well as a risk indicator of MARDI with fever of 45 days with an optimal BMI cut-off value of 17.9 kg/m(2). Sponsorship: Wellcome Trust Research Training Fellowship in Population Health.