Distance and percentage distance from median BMI as alternatives to BMI z score

Abstract

BMI z (BMIz) score based on the Centers for Disease Control and Prevention growth charts is widely used, but it is inaccurate above the 97th percentile. We explored the performance of alternative metrics based on the absolute distance or % distance of a child's BMI from the median BMI for sex and age. We used longitudinal data from 5628 children who were first examined <12 years to compare the tracking of three BMI metrics: distance from median, % distance from median and % distance from median on a log scale. We also explored the effects of adjusting these metrics for age differences in the distribution of BMI. The intraclass correlation coefficient (ICC) was used to compare tracking of the metrics. Metrics based on % distance (whether on the original or log scale) yielded higher ICCs compared with distance from median. The ICCs of the age-adjusted metrics were higher than that of the unadjusted metrics, particularly among children who were (1) overweight or had obesity, (2) younger and (3) followed for >3 years. The ICCs of the age-adjusted metrics were also higher compared with that of BMIz among children who were overweight or obese. Unlike BMIz, these alternative metrics do not have an upper limit and can be used for assessing BMI in all children, even those with very high BMIs. The age-adjusted % from median (on a log or linear scale) works well for all ages, while unadjusted % from median is better limited to older children or short follow-up periods.

Keywords: BMI; Children; Metrics; Obesity.

Figure 1.

BMI (A) and BMI z-score (B) by age for girls who have adjusted BMI distances (solid lines) from the median of 60%, 110% and 160%. These values correspond to BMIs of approximately 35, 45, and 55 kg/m² at age 20 years. The dashed lines in (A) represent the corresponding unadjusted % distance. The three points in the left panel represent the BMIs of a girl at age 3, 10, and 18 years who has a BMI that is 140% of the 95th percentile.

Figure 2.

Intraclass correlation coefficients for unadjusted and age-adjusted BMI metrics by age at first examination. The points represent the mean age at first examination in each group.

Figure 3.

Intraclass correlation coefficients for unadjusted and age-adjusted BMI metrics by the interval between the first and last examinations. The points represent the mean interval in each group.