The Limitations of Transforming Very High Body Mass Indexes into z-Scores among 8.7 Million 2- to 4-Year-Old Children

J Pediatr. 2017 Sep;188:50-56.e1. doi: 10.1016/j.jpeds.2017.03.039. Epub 2017 Apr 19.


Objective: To examine the associations among several body mass index (BMI) metrics (z-scores, percent of the 95th percentile (%BMIp95) and BMI minus 95th percentile (ΔBMIp95) as calculated in the growth charts from the Centers for Disease Control and Prevention (CDC). It is known that the widely used BMI z-scores (BMIz) and percentiles calculated from the growth charts can differ substantially from those that directly observed in the data for BMIs above the 97th percentile (z = 1.88).

Study design: Cross-sectional analyses of 8.7 million 2- to 4-year-old children who were examined from 2008 through 2011 in the CDC's Pediatric Nutrition Surveillance System.

Results: Because of the transformation used to calculate z-scores, the theoretical maximum BMIz varied by >3-fold across ages. This results in the conversion of very high BMIs into a narrow range of z-scores that varied by sex and age. Among children with severe obesity, levels of BMIz were only moderately correlated (r ~ 0.5) with %BMIp95 and ΔBMIp95. Among these children with severe obesity, BMIz levels could differ by more than 1 SD among children who had very similar levels of BMI, %BMIp95 and ΔBMIp95 due to differences in age or sex.

Conclusions: The effective upper limit of BMIz values calculated from the CDC growth charts, which varies by sex and age, strongly influences the calculation of z-scores for children with severe obesity. Expressing these very high BMIs relative to the CDC 95th percentile, either as a difference or percentage, would be preferable to using BMI-for-age, particularly when assessing the effectiveness of interventions.

Keywords: children; obesity.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Body Mass Index*
  • Child, Preschool
  • Cross-Sectional Studies
  • Female
  • Growth Charts
  • Humans
  • Male
  • Nutrition Surveys
  • Overweight / diagnosis*
  • Overweight / epidemiology
  • Pediatric Obesity / diagnosis*
  • Pediatric Obesity / epidemiology