A method for calculating BMI z-scores and percentiles above the 95 percentile of the CDC growth charts.

BACKGROUND

The 2000 CDC growth charts are based on national data collected between 1963 and 1994 and include a set of selected percentiles between the 3 and 97 and LMS parameters that can be used to obtain other percentiles and associated z-scores. Obesity is defined as a sex- and age-specific body mass index (BMI) at or above the 95 percentile. Extrapolating beyond the 97 percentile is not recommended and leads to compressed z-score values.

AIM

This study attempts to overcome this limitation by constructing a new method for calculating BMI distributions above the 95 percentile using an extended reference population.

SUBJECTS AND METHODS

Data from youth at or above the 95 percentile of BMI-for-age in national surveys between 1963 and 2016 were modelled as half-normal distributions. Scale parameters for these distributions were estimated at each sex-specific 6-month age-interval, from 24 to 239 months, and then smoothed as a function of age using regression procedures.

RESULTS

The modelled distributions above the 95 percentile can be used to calculate percentiles and non-compressed z-scores for extreme BMI values among youth.

CONCLUSION

This method can be used, in conjunction with the current CDC BMI-for-age growth charts, to track extreme values of BMI among youth.