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

Abstract

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-off 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.

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Acknowledgements

Ahmad Dorosty was sponsored by the Iranian Ministry of Health and Medical Education. We thank Dr Linda Hunt (Institute of Child Health, University of Bristol) and Dr John Norrie (Robertson Centre for Biostatistics, University of Glasgow) for statistical advice. We are extremely grateful to all the parents and children who took part in the study and to the midwives for their help in recruiting them. We acknowledge the dedicated work of the ALSPAC study team, including measurers, interviewers, computer technicians, clerical workers, research scientists, volunteers and managers. The ALSPAC study is supported by a variety of funders including, the Medical Research Council, The Wellcome Trust, the Department of Health, MAFF, other companies and medical charities. ALSPAC is part of the WHO-initiated European Longitudinal Study of Pregnancy and Childhood. Professor Tim Cole kindly provided the IOTF reference data in advance of publication and advised as to their use.

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Reilly, J., Dorosty, A., Emmett, P. et al. Identification of the obese child: adequacy of the body mass index for clinical practice and epidemiology. Int J Obes 24, 1623–1627 (2000). https://doi.org/10.1038/sj.ijo.0801436

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