Predictive value of weight-for-age to identify overweight children

Objective: The objective was to assess the predictive value of weight-for-age to identify overweight children and adolescents in the unusual research or public health situations where height is not available to calculate BMI. Research Methods and Procedures: Data from the National Health and Nutrition Examination Survey 1999 to 2004 were used to calculate the sensitivity, specificity, and positive and negative predictive values of selected weight-forage cut-off points to identify overweight children and adolescents (as defined by BMI >= 95th percentile). Positive and negative predictive values are dependent on prevalence and are reported here for this study population only. Results: The 50th and 75th weight-for-age percentiles had good sensitivity (100% and 99.6%, respectively), but poor positive predictive value (23.7% and 37.0%, respectively), while the 95th and 97th percentiles had reasonable positive predictive value (80.3% and 91.5%, respectively), but limited sensitivity (82.0% and 66.7%, respectively) to identify overweight subjects. The properties of weight-for-age percentiles to identify overweight subjects differed between sex, age, and race/ethnicity but remain within a relatively narrow range. Discussion: No single weight-for-age cut-off point was found to identify overweight children and adolescents with acceptable values for all properties and, therefore, cannot be used in the clinical setting. Furthermore, the positive predictive values reported here may be lower in populations with a lower prevalence of obesity. However, in unusual research or public health situations where height is not available, such as existing databases, weight-for-age percentiles may be useful to target limited resources to groups more likely to include overweight children and adolescents than the general population.