The primary weight criterion for a diagnosis of anorexia nervosa is a weight less than 85% of what is considered normal for that person’s age and height (DSM-IV and ICD-10). According to DSM-IV, a body mass index less than or equal to 17.5 kg/m², which originated from the ICD-10 diagnostic criteria for research, is an alternative and somewhat stricter guideline. However, this alternative criterion is not adjusted for age and sex. Because anorexia nervosa typically begins in late childhood, adolescence, or early adulthood, it is crucial to consider age in making a diagnosis because the relation of weight to height changes substantially during this age span. This is highlighted by the fact that with increasing age, the proportion of individuals with a body mass index less than or equal to 17.5 drops dramatically from 57% at 10 years to below 1% at age 35 in the German female population (similar percentages apply to the U.S. population) (1).

From a clinical perspective, the body mass index of a patient can only be interpreted appropriately when the age- and sex-specific distribution of the body mass index is known. Hence, the use of sex-specific age percentiles for body mass index has been proposed in order to assess the degree of underweight in acute anorexia nervosa, to determine target weight, and to assess weight outcome (1, 2). Independent of age and sex, the main DSM-IV weight criterion (less than 85% of expected body weight) corresponds to a body mass index between the fifth and 10th percentiles of the body mass index in both the U.S. and German populations (1). To address the issue of age- and sex-dependent distributions of the body mass index and to introduce a convenient and epidemiologically based definition of the weight criterion for anorexia nervosa, we suggest the use of the 10th percentile of the body mass index as a cutoff for underweight in industrialized countries.

On the basis of these considerations, we warn against indiscriminate use of the two weight criteria, both in clinical practice and research, because misclassifications with potentially serious sequelae can ensue. Thus, a body mass index of 17.5 in a 14-year-old girl is by no means indicative of anorexia nervosa. It is obvious that epidemiological studies cannot readily be compared if they are based on these different weight criteria. It should be realized that a body mass index of 17.5 is a strict weight cutoff only for individuals over age 20. For children, and to a lesser extent adolescents, the body mass index cutoff is less strict than the primary DSM-IV weight criterion.