According to the Harmonized Training package (HTP) the WHO recommends BMI for age, expressed as z-scores than compared to growth reference curves for that age group from the WHO standards. I have no personal experience using this in the field, however. From the HTP Module 6: Individual Assessment: Measuring malnutrition in older children and adolescents WHO recommends that acute malnutrition among children and adolescents 5-19 years be assessed by calculating Body Mass Index (BMI), and then adjusting for age to generate BMI-for-age. BMI is calculated based on the weight (in kg) divided by the square of the height (in m) of the individual. BMI-for-age should be presented as Z-scores based on the 2007 WHO Growth Reference (WHO GR) for children and adolescents 5 to 19 years of age. Classification of children and adolescents 5-19 years of age BMI-for-age Z-score Severe thinness: <-3SD Thinness: = -3 SD & < -2 SD Overweight: >+1SD & =+2 SD Obesity: >+2SD