Better BMI z-score chart for tracking growth in obese, underweight kids

By Will Boggs MD

NEW YORK (Reuters Health) - A modified body-mass index (BMI) z-score chart based on the Centers for Disease Control and Prevention (CDC) 2000 age- and sex-specific percentile growth charts better tracks growth in severely obese or underweight children, as well as in normal children, researchers report.

"Inability to track growth changes, especially among the obese and severely obese, is an important problem that pediatric health providers encounter every day,” Dr. Madhumita Sinha from the National Institute of Diabetes and Digestive and Kidney Diseases' Diabetes Epidemiology and Clinical Research Section, Phoenix, Arizona, told Reuters Health by email. “The last clinical growth charts were published by CDC over 15 years ago; since then we have witnessed a childhood obesity epidemic. . . . These children need medical interventions; however, the current clinical growth charts do not allow growth tracking in children above a BMI of 36 kg/m2, so this is an urgent clinical problem.”

Dr. Sinha and colleagues constructed BMI-for-age z-score growth charts using the CDC’s modified z-score software program, which allows extreme BMI values to be quantified. Their findings were published online November 7 in Pediatrics.

“By using this method, the BMI of an individual is expressed relative to the median BMI in units of 1/2 the distance between 0 and +2 z-scores,” the researchers note. “The advantage of this method is that assumptions are not made beyond the limits of observed values in the reference sample, and values can be tracked outside of the -3 to +3 SD range.”

Severely obese children could not be plotted on the standard CDC BMI percentile chart, because their BMI points were above the chart cutoff. Conversely, children with low BMI were difficult to track on the standard percentile chart.

Adding the percentage of the 95th percentile curves to the standard BMI percentile chart allowed tracking of severely obese children, but it did not address severely underweight children and required a change of units within the chart when moving from normal to obese BMIs.

In contrast, the modified BMI z-score chart allowed uniform tracking of normal, underweight, and obese children.

"The only limitation we see to adopting the modified z-score is that users will not be used to it and may confuse it with the traditional growth charts based on percentiles or with the unmodified z score that results in a compression of high values of BMI," the researchers note.

“In this study, we propose a growth chart based on standardized z-scores,” Dr. Sinha said. “The advantages of this chart are: (It can) uniformly track growth in children both at the upper and lower extremes of growth; and (it) is color coded for easy interpretation; growth problems can be identified and tracked well. For example, crossing z-score lines and direction may alert a health provider and initiate an intervention. We feel once pediatric health providers get familiarized with the chart, it should be very user-friendly and can be incorporated into electronic health records.”

“We hope our article will generate enough enthusiasm and encourage CDC and the National Center for Health Statistics to publish new growth charts that will be useful for growth tracking of children at both extremes,” she added.

SOURCE: http://bit.ly/2zkGhVq

Pediatrics 2017.

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