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, 85, 115, 125, and 135%mBMI were determined for males and females aged 10 to 18 years at diverse height centiles. Body weights (in kg) corresponding to BMIz (ranging from - 7 to + 3) at respective 50th height centiles were also calculated and compared. Age-and-sex comparisons demonstrated the greatest divergence of %mBMI at the lowest and highest BMIz: for low BMIz the divergence was strongest at age 10 particularly in males, whereas for BMIz + 3 the corresponding %mBMI were 200.5 and 216.4 in males and females, respectively. Height centiles were minimally associated with %mBMI-to-BMIz transformations. Per 1 BMIz, increments in body weight (in kg) were non-linear and dependent on the reference BMIz. Findings illustrate non-linear transformations between %mBMI and BMIz. Use of both characteristics in eating disorder research limits meta-analyses. Field consensus for universal weight-related assessment in children and adolescents is necessary.