ADA indicates American Dietary Association; BMI IBW, body mass index ideal body weight.
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Moylan A, Appelbaum N, Clarke J, et al. Assessing the Agreement of 5 Ideal Body Weight Calculations for Selecting Medication Dosages for Children With Obesity. JAMA Pediatr. 2019;173(6):597–598. doi:10.1001/jamapediatrics.2019.0379
The prevalence of childhood obesity is increasing worldwide. There is little agreement on how prescribing clinicians should modify weight-based dosage regimens in children with obesity. The World Health Organization1 recommends the use of ideal body weight (IBW) for calculating dosages of all medications, whereas other formularies advise the use of IBW and IBW-derived measures, including adjusted body weight and lean body mass, depending on the lipid solubility of the medication.2 There are 5 methods to calculate IBW in children3:
McLaren: plot the child’s height for age and draw a horizontal line until the 50th percentile is crossed, and then draw a perpendicular line to the 50th percentile weight to obtain the IBW.
Moore: the IBW is the weight for age on the same percentile as height.
BMI50: the IBW is the 50th percentile body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) for age, multiplied by square of the height in meters.
American Dietary Association (ADA): the IBW is the 50th percentile weight for age.
for children shorter than 5 ft, the IBW in kilograms is calculated as ([height in centimeters]2 × 1.65)/1000.
for boys taller than 5 ft, the IBW in kilograms is calculated as 39 + (2.27 × [height in inches − 60]).
for girls taller than 5 ft, the IBW in kilograms is calculated as 42 + (2.27 × [height in inches − 60]).
No consensus exists as to which of these is most appropriate. We used data from the UK National Child Measurement Programme5 to assess the outcome of calculation of IBW by each of these methods on the dosage of medications prescribed to children with obesity.
The National Child Measurement Programme records the height and weight of 95% of children in reception (the final year of preschool; for children aged 4-5 years) and year 6 of school (for children aged 10-11 years) attending mainstream schools in England. The data were anonymized and provided publicly by National Health Services Digital. It is intended for use by analysts and researchers, and no specific ethical approval was required.
For children defined having obesity (with BMIs equal to or greater than the 95th percentile) in the 2015-2016 school year, we calculated the IBW for each child by each of the 5 methods using growth charts produced by the US Center for Disease Control and Prevention. The McLaren method was not used for children in year 6 because weight-for-height tables are limited to the height of 120 cm (47.2 in). The percentage of total body weight represented by IBW (pIBW) was calculated for each child and plotted against BMI for each method. This was used to calculate the median absolute difference between the pIBW for different methods, together with their approximate 95% CIs. Analysis was completed in September 2018 using Python version 3.6 (Python Software Foundation).
A total of 546 519 children in reception were included, of whom 58 493 (10.7%) were obese. In addition, 528 412 children in year 6 were included, of whom 61 638 (11.7%) were obese.
For the children in reception (Figure 1), the pIBW was highly similar via the Traub and Moore methods (median absolute difference, 3.49% [95% CI, 3.46%-3.52%]), and it was highly similar again via the ADA and McLaren methods (median absolute difference, 0.65% [95% CI, 0.64%-0.66%]). The greatest median disagreement in pIBW between methods was 13.7% (95% CI, 13.3%-14.1%) between the Moore and McLaren methods at a BMI of 21.5. For the children in year 6 (Figure 2), the only methods in agreement were Traub and BMI (median absolute difference, 2.67% [95% CI, 2.66%-2.69%]). The greatest disagreement in pIBW was 16.6% (95% CI, 15.6%-17.8%) between the Moore and ADA methods at a BMI of 31.5.
A child in reception with a BMI of 19.0 would have a pIBW of 75% by the McLaren method, whereas using the Moore method, a child with a BMI of 22.0 would be assigned the same proportion. A pIBW of 68% for a child in year 6 corresponds to a BMI of 24.0 according to the ADA method and a BMI of 29.5 by the Moore method.
Obesity changes the association between total body weight and fat-free mass. The volume of distribution of a medication is therefore variable, according to its lipid solubility, in patients with different BMIs. While alterations in the volume of distribution in individuals with obesity are the result of more complex interactions between drugs, protein binding, and tissue perfusion, this analysis shows that current methods of calculating IBW are an inconsistent surrogate for fat-free mass in children with obesity.
For an 11-year-old child with a BMI of 31.0, IBW will be calculated as between 51% and 65% of total body weight. When applied to the prescribing of gentamicin, a hydrophilic drug with a narrow therapeutic index, the starting dose will vary by 27%. Because the lower end of this range is likely to be an underestimation of the child’s weight,6 this is likely to result, at least initially, in the patient receiving subtherapeutic levels of medication.
To strive to provide safe, effective dosages of medication for every child, we should seek to develop models incorporating a compartmental approach using, for example, biological impedance. This would help clinicians provide medication dosages to children with obesity ideally.
Accepted for Publication: March 6, 2019.
Published Online: April 1, 2019. doi:10.1001/jamapediatrics.2019.0379
Corresponding Author: Alexander Moylan, MRCPCH, Paediatric Emergency Department, St Mary’s Hospital, Praed Street, London W2 1NY, United Kingdom (firstname.lastname@example.org).
Correction: This article was corrected on June 21, 2021, to fix the phrase “for children shorter than 5 ft, the IBW in kilograms is calculated as ([height in inches]2 × 1.65)/1000,” which should have read “for children shorter than 5 ft, the IBW in kilograms is calculated as ([height in centimeters]2 × 1.65)/1000.” The error has been corrected online. In addition, missing information on Author Contributions has been added online.
Author Contributions: The authors had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Concept and design: Moylan, Appelbaum, Clarke, Flahault Tairraz, Maconochie, Darzi.
Acquisition, analysis, or interpretation of data: Moylan, Appelbaum, Clarke, Feather.
Drafting of the manuscript: Moylan, Appelbaum, Clarke, Flahault Tairraz.
Critical revision of the manuscript for important intellectual content: Moylan, Appelbaum, Clarke, Feather, Maconochie, Darzi.
Statistical analysis: Appelbaum, Clarke.
Obtained funding: Appelbaum, Darzi.
Administrative, technical, or material support: Clarke, Maconochie.
Supervision: Appelbaum, Clarke, Maconochie, Darzi.
Other–Expert advise and discussion of relevant literature: Feather.
Conflict of Interest Disclosures: Dr Appelbaum reports grants from the National Institute of Health Research (NIHR) and the NIHR Imperial Biomedical Research Centre during the conduct of the study; in addition, Dr Appelbaum has a patent issued (WO2015025300A3) and a patent pending (WO2017125859A3); and the Helix Centre at Imperial College London, where Dr Appelbaum is the clinical lead, is running an effort in collaboration with the British National Formulary to develop digital tools in an attempt to improve paediatric medication safety. Dr Clarke reports grants from NIHR Patient Safety Translational Research Centre and NIHR Imperial Biomedical Research Centre during the conduct of the study. No other disclosures were reported.