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

Abstract

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.

Conflict of interest statement

None of the authors has any potential, perceived or real, conflict of interest.

Published by Elsevier Inc.

Figures

Figure 1

Levels in the CDC growth charts of the 50th percentile, the 95th percentile, and the theoretical maximum value of BMIz. Levels of the observed maximum BMI in the current study are shown in the lower right panel. Note that the scales and range of values on the y-axes differ across the 4 characteristics.

Figure 2

BMIz values associated with various levels of ΔBMIp95 by sex and age

Figure 3

Violin plots showing the distribution of levels of BMI and various transformation across categories of %BMIp95. The width of each violin is proportional to the density of the sample at that value, and the horizontal lines represent the 10th, 50th, and 90th percentiles. Values below the 1st percentile or greater than the 99th percentile have been set to equal these percentiles.

Figure 4

Violin plots showing the distribution of levels of BMIz and %BMIp95 across categories of BMI, by sex and year of age. None of the 3-year-old boys had a BMI ≥ 28.0 kg/m2.