Sensitivity and specificity of classification systems for fatness in adolescents

Nutrition Research Newsletter, Sept, 2004

Despite the growing concern about adiposity-related problems among the young, no universally accepted classification system for adolescent obesity exists. Although body mass index (BMI; in kg/[m.sup.2]) is widely used for classification of adult overweight and obesity, its use in adolescents is controversial. The limitations of BMI as a measure of adiposity in the pediatric population are larger than those in the adult population because BMI varies with age, sex, and maturation. The purpose of the present study was threefold: 1) to evaluate the sensitivity and specificity of recommended, international, BMI-based classification systems for detecting fatness, 2) to compare these systems with a national reference, and 3) to examine the influence on the analyses of the choice of reference values for excess fatness.

The subjects in the Stockholm Weight Development Study were 481 adolescents (n = 279 females and 202 males). Body-composition data were available for 474 of the subjects. The adolescents were a subset of the offspring of 1423 women who participated in the Stockholm Pregnancy and Weight Development Study in 1984-1985. Subjects' weight was measured to the nearest 0.1 kg, standing height was measured and BMI was determined as Quetelet's index (kg/[m.sup.2]). ¿ was measured by using air-displacement plethysmography with the BodPod. The IOTF/Cole system consists of sex-specific BMI percentile curves that at age 18 yr pass through the BMI cutoffs for adult overweight and obesity of 25 and 30, respectively. The definitions of adolescent overweight and obesity are thereby linked to adult risk. The percentile curves were produced from large-survey data from the United Kingdom, the United States, Holland, Singapore, Hong Kong, and Brazil (n = 97 876 males and 94 851 females). The reference is recommended by the IOTF and is widely used.

The mean BMI values did not differ significantly between the sexes, whereas the mean ¿ was significantly higher in the females than in the males. The mean ¿ for the males was almost 9% points lower than the recommended 25¿ cutoff for overweight in males, whereas the mean ¿ for the females was nearly equivalent to the proposed cutoff of 30%. This explains the high prevalence of true positives for overweight among the females, in comparison with the prevalence of overweight as defined by various BMI-based references. ¿ and BMI were significantly correlated in both the males and the females. High correlational validity, however, does not guarantee clinical validity of classification systems. Therefore, the nature and extent of misclassifications were evaluated by ROC analysis.

The IOTF/Cole and WHO/MDD classification systems were highly specific for both sexes, but their sensitivity was very low for the females. Thus, almost all adolescents labeled as overweight were truly overweight, whereas ~75% of the truly overweight females were mislabeled as normal-weight. The result was similar for the widely used adult BMI cutoff of 25, which will be applied when the adolescents become 18 y of age.

In conclusion, the tradeoff between sensitivity and specificity should be analyzed in detail before making general recommendations about classification systems for overweight. The diagnostic demands on a classification system intended for use in clinical practice are different from those on systems intended for public health use or monitoring. Therefore, recommendations should be explicit regarding the setting in which suggested systems should be used. A multipurpose system may be the easiest to implement but would not suit the varying demands of public health, clinical practice, and monitoring. An international reference is a compromise to obtain acceptable, comparable prevalence estimates at the global level. At the national level, given the probable population differences in relative risks at certain BMI values, the seriousness of the adolescent obesity problem, and its character as a major cost driver through obesity-related illnesses, customized systems derived from national data are likely to be more efficient. Such systems should therefore be developed.

M Neovius, Y Linne, B Barkeling, S Rossner. Sensitivity and specificity of classification systems for fatness in adolescents. Am J Clin Nutr 80(3):597-603 (September 2004) [Correspondence: MG Neovius, Obesity Unit, Karolinska Institutet, Karolinska University Hospital SE-141 86 Stockholm, Sweden. E-mail: martin.neovius@medhs.ki.se.]