How good are BMI charts for monitoring children's attempts at obesity reduction?

Introduction: Body mass index (BMI) is the pragmatic measure to assess children's obesity clinically and BMI charts are widely used for counselling families about children's weight management over time. Aims: To explore the variability in clinicians'interpretation of BMI patterns and to ascertain the diagnostic accuracy of their judgement by relating it to change in body composition by dual-emission x-ray absorptiometry (DXA). Methods: Data from 70 children who participated in a trial of a weight management programme for obese children were analysed. BMI was plotted on UK 1990 charts at baseline, 6 months and 12 months, and four clinicians experienced in obesity management independently scored the charts on a five-point scale for how successful children were in tackling their obesity over a 6-month period. Scores were compared with change in BMI, fat mass and lean mass z-scores as measured by DXA. Results: 54 children (aged 8-15 years; BMI z-score 2.93 (SD 0.48)) had simultaneous BMI and DXA scans performed, giving 104 pairs of measurements 6 months apart. There was good consistency between clinicians'scores for weight management and these related well to change in BMI and fat mass z-scores, but not lean mass z-score. They reported that measurement proximity to centile lines and crossing of lines influenced their confidence in making a decision and change in severe obesity was harder to judge as higher centile lines are so far apart. Conclusions: BMI charts are useful for assessing children's attempts at weight management, and provide a reasonably accurate indication of change in body fat. Recommendations are made regarding BMI chart design and guidance in interpreting measurements.