Abstract
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 3rd and 97th 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 95th percentile. Extrapolating beyond the 97th 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 95th percentile using an extended reference population.
Subjects and methods
Data from youth at or above the 95th 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 95th 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.
Acknowledgement
The authors thank Dr. Yulei He for his assistance in checking statistical models/errors and giving suggestions in the draft.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Disclaimer
The findings and conclusions in this report are those of the authors and not necessarily the official position of the Centers for Disease Control and Prevention.