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Sensitivity and specificity of classification systems for fatness in adolescents^1,2,3

¹ From the Obesity Unit, Karolinska Institutet, Karolinska University Hospital, Stockholm, Sweden

Background: Various body mass index (BMI) standards have been proposed for defining overweight in adolescence, but few studies have evaluated their diagnostic accuracy.

Objective: We compared the sensitivity and specificity of BMI-based classification systems for detecting excess fatness in adolescents.

Design: A cross-sectional analysis of 474 adolescents aged 17 y was used. Body composition was measured by using densitometry. The international BMI-based systems recommended by the International Obesity Task Force and the World Health Organization were evaluated on the basis of their sensitivity and specificity for detecting excess body fat. Receiver operating characteristic analysis was performed to derive cutoffs to maximize the sum of sensitivity and specificity. True positives were defined by using the percentage body fat cutoffs proposed by Williams et al (Am J Public Health 1992;82:358-63).

Results: For both classification systems, the specificity for overweight was high for both sexes (0.95–1.00). The sensitivity was fairly high for the males (0.72–0.84) but was very low for the females (0.22–0.25). For the males, a BMI cutoff equal to the 85th percentile on a Swedish BMI reference chart maximized the sum of sensitivity and specificity while having both high sensitivity (0.92) and high specificity (0.92). For the females, larger tradeoffs in specificity were needed to improve sensitivity. The mean (±SE) areas under the receiver operating characteristic curves for the males and the females were 0.97 ± 0.02 and 0.85 ± 0.02, respectively.

Conclusions: Recommended international classification systems have very high specificity, which results in few cases of nonoverweight adolescents being mislabeled as overweight. However, the sensitivity is very low in female adolescents. Thus, many overweight female adolescents could be missed in intervention programs that use the proposed international BMI cutoffs as selection criteria.

Key Words: Adolescent overweight • body mass index • classification systems • percentage body fat • diagnostic accuracy • sensitivity • specificity • receiver operating characteristic analysis

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