The primary objective was to test 3 methods used to calculate EBW for adolescents with eating disorders: BMI, McLaren, and Moore. Specifically, we were interested in the extent to which these methods brought about agreement or disagreement on cut points for hospitalization (75% EBW), diagnosis (85% EBW), and healthy weight (100% EBW). Our secondary goal was to determine whether our findings would allow for clearer guidelines regarding the determination of EBW for this patient population.
Overall, there was moderate agreement between the 3 methods, with pairwise total classification accuracy at each cut point ranging from 84% to 98%. The 3 methods largely agree on %EBW in terms of clinically significant cut points with the exception of the discrepant calculations for a relatively small number of cases (2.5% -15.8%). Correlations were lowest for BMI and Moore (0.88) and highest for Moore and McLaren (0.96). The most discrepant calculations were observed among the taller patients (>75th percentile), shorter patients (<20th percentile), and those >16 years of age. Many of these most discrepant cases, when comparing the BMI and Moore methods, fell above and below 85% EBW. For instance, this discrepancy not only indicated disagreement on the weight criterion for possible diagnosis of AN, but also the same individual (see , first case) would warrant hospitalization given the Moore method (ie, 55% EBW) while simultaneously being considered close to normal weight given the BMI method (ie, 107% EBW). The evidence for agreement was not as striking for gender and menstrual status as it was for height and age. The McLaren method presented with the most significant limitation in that it cannot be used for boys >176 cm or girls >163 cm (median height for girls aged ≥14 years), which limited our original sample by >40%. In addition, the Moore method is challenging at extremes of height and weight. Therefore, our study demonstrates, even prior to the analyses, the importance of the BMI method as a methodology that can apply to children and adolescents at all ages, heights, and weights.
These discrepant calculations underscore the implications when using one method rather than another for the assessment of adolescents with eating disorders who are outside the norm for height or >16 years of age. This consideration is especially important for research endeavors when study inclusion is contingent upon a diagnosis that is arrived at via EBW calculations. For example, it is fair to say that for very tall adolescents, the BMI method will calculate higher %EBW than the Moore method. If clinicians are uncertain about the diagnosis of AN, they should consider the trade-offs of making a false-positive versus a false-negative diagnosis when choosing one method over another. However, in the interest of advancing a shared language among clinicians and researchers, we suggest that the BMI method be used as it may pose the fewest obstacles (ease of calculation) or exceptions (height and age). We acknowledge that in some instances clinical decision-making will be complex and require a more flexible approach. However, a uniform adherence to 1 method to calculate EBW will strengthen clinical and research practice.
Some limitations and strengths to our study should be considered. We did not know a priori that the shortcomings for the McLaren and Moore methods would result in these methods not being feasible for EBW calculations in a subset of our sample. However, only upon attempting comparisons of these methods did we learn that >40% of our sample could not be compared in this way. To date, it has not been well established that eating disorder patients would present this many outliers, nor has such a finding been presented in an empirical manner. Thus, our study shows that there is little utility for the McLaren method in an adolescent eating disorder sample. A limitation of all 3 methods involves the inability to account for stunted growth in pediatric subjects with eating disorders (ie, height stunting will affect calculation of EBW and will underestimate it in all likelihood). It is for practitioners to take this limitation into consideration when growth stunting is suspected clinically, based on genetic potential as evidenced by parental height, or on prior growth records showing a clear slowing of linear growth. As a result, clinicians should anticipate perhaps having to aim for higher treatment goal weights or adjusting EBW once linear growth returns to normal. Second, it is crucial to acknowledge that the cut points studied here, although commonly used in clinical practice, are arbitrary and should not be seen as absolute indicators of illness or health. For example, hospitalization is not indicated only when weight is below 75% EBW, and the DSM-IVTR cut point of 85% EBW was initially intended as an example but is often mistakenly reified into a concrete cut point. The DSM-5 Eating Disorder Workgroup specifically noted that for clinical purposes, it would be undesirable to settle on a “specific numerical standard” for weight for AN.16
Finally, we considered 3 methods for EBW calculation, whereas others still in use (see, eg, refs 17
) were not included in this comparison.