Expected Body Weight in Adolescents: Comparison Between Weight-for-Stature and BMI Methods

OBJECTIVE: To test the hypothesis that the weight-for-stature (WFS) and BMI methods are not equivalent in determining expected body weight (EBW) in adolescents with eating disorders and to determine the sensitivity, specificity, and positive predictive value of each method to detect those <75% EBW. We hypothesized that differences in EBW would be greatest at the extremes of height.

METHODS: EBW was determined for 12 047 individual adolescents aged 12 to 19 years by the WFS and BMI methods by utilizing the same National Center for Health Statistics data sets. Absolute difference between the 2 methods for each individual was calculated and plotted against height by using a generalized additive model. The number of individuals whose weights were <75% EBW was determined by each method.

RESULTS: For girls, EBW was 3.52 ± 3.13% higher when using the WFS method compared with the BMI method. For boys, EBW_WFS was 3.45 ± 2.72% higher than EBW_BMI. Among adolescent girls, 65% had EBW_WFS higher than EBW_BMI. By using the EBW_WFS method as the gold standard, specificity of the EBW_BMI method to detect those <75% EBW was 0.999, but sensitivity was only 0.329. Absolute differences in EBW were most pronounced at the extremes of height.

CONCLUSIONS: The WFS and BMI methods are not equivalent in determining EBW in adolescents and are not interchangeable. EBW_WFS was ∼3.5% higher than EBW_BMI. In adolescents with eating disorders, use of the BMI method will underestimate the degree of malnutrition compared with the WFS method. Which method better predicts meaningful clinical outcomes remains to be determined.