Transforming percentage of median body mass index (%mBMI) scores into corresponding BMI z- scores yields discrepancies for age and sex: Implications for pediatric eating disorder researchers and clinicians

Anorexia nervosa (AN) and atypical AN are distinguished by the presence or absence of a significantly low body weight. In children and adolescents, the population-based age- and sex-adjusted weight characteristics, BMI centile, BMI z-score (BMIz), and percentage of median BMI (%mBMI) are used to diagnose underweight. Whereas BMI centile and BMIz are interconvertible, %mBMI is not. We extend previous analyses to illustrate a) age- and sex-dependent discrepancies between %mBMI and BMIz and b) the non-linear relationship between BMIz and absolute body weight. Based on Centers for Disease Control growth charts, BMIz corresponding to the 65, 75, and 85%mBMI were determined for males and females aged 10 to 18 years. Body weights (in kg) corresponding to BMIz ranging from 0 to -7 (based on average heights) were also calculated and compared. Throughout the age span, BMIz corresponding to 65%mBMI ranged from − 5.0 to -6.9 in males and − 4.5 to -5.2 in females; at 75%mBMI, from − 2.9 to -3.6 in males and − 2.6 to -2.9 in females; and at 85%mBMI, from − 1.4 to -1.7 in males and − 1.3 to -1.4 in females. Body weight increments (in kg) per 1 BMIz were non-linear and dependent on reference BMIz. Transformations between %mBMI and BMIz are not linear, and these characteristics cannot be used interchangeably. Use of both BMIz and %mBMI in eating disorder research limits meta-analyses. Field consensus for universal weight-related assessment in adolescents is necessary.