Identification of the obese child: adequacy of the body mass index for clinical practice and epidemiology

OBJECTIVE: To assess the ability of simple definitions of BMI to successfully screen for children with high body fatness. DESIGN: We determined the sensitivity and specificity of the body mass index (BMI) by testing its ability to correctly identify children with high body fat percentage. Receiver operator characteristic (ROC) analyses were carried out using the top 5% of body fat percentage to define children as obese (true positives). SUBJECTS: Representative sample of 4175 7 y-old (88-92 month-old) children (2120 boys; 2055 girls) participating in the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC). RESULTS: The current obesity definition based on BMI (95th centile) had moderately high sensitivity (88%) and high specificity (94%). Sensitivity and specificity did not differ significantly between boys and girls. The ROC analysis showed that lower cut-offs applied to the BMI improved sensitivity with no marked loss of specificity: the optimum combination of sensitivity (92%) and specificity (92%) was at a BMI cut-off equivalent to the 92nd centile, Sensitivity of BMI using the new International Obesity Task Force (IOTF) cut-oft for obesity was much lower, and differed significantly (P <0.001) between boys (46%) and girls (72%). CONCLUSIONS: Screening for childhood obesity using the BMI is specific, and can have moderately high sensitivity if an appropriate cut-off is chosen. New recommendations based on the IOTF approach to defining childhood obesity are associated with lower sensitivity, and sensitivity differs between boys and girls.