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Different methods of classifying childhood body weight often gave very different answers, according to a British study.

In a research letter appearing in , Alexander Moylan, MRCPCH, of Imperial College Healthcare National Health Service Trust in London, and colleagues compared five different methods of determining ideal body weight in children: McLaren, Moore, BMI₅₀, America Dietary Association (ADA), and Traub methods.

They often disagreed, and the variability could pose an issue when determining ideal body weight for calculating children's drug dosages.

"There is little agreement on how prescribing clinicians should modify weight-based dosage regimens in children with obesity," Moylan and co-authors explained.

"The World Health Organization 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," they added, noting that currently there is no consensus between which method is preferred.

To compare these methods, the researchers drew upon records from Britain's National Child Measurement Programme, which included school children in their last year of preschool (ages 4-5), as well as kids in their sixth year of school (ages 10-11).

Moylan's group then calculated the percentage of total body weight represented by IBW (pIBW), according to the five methods, who were considered as having obesity – or in the 95th percentile. There were nearly 58,500 children ages 4-5 (10.7% of the population) and around 61,000 kids ages 10-11 (11.7% of the population) qualifying, the researchers said.

The five methods work as follows:

• McLaren: plot height for age, draw horizontal line until 50th percentile crossed, then draw perpendicular line to 50th percentile for weight
• Moore: weight for age on same percentile as height
• BMI₅₀: 50th percentile BMI for age
• ADA: 50th percentile weight for age
• Traub: IBW in kilograms is calculated in several ways depending on the child's height: for those under 5 ft, as [(height in centimeters)² × 1.65]/1,000; for boys taller than 5 ft, it's 39 + [2.27 × (height in inches - 60)]; and for girls taller than 5 ft, it's 42 + [2.27 x (height in inches - 60)]

When calculating ideal body weight for the young children in preschool, the ADA and McLaren methods were quite similar (median absolute difference 0.65%, 95% CI 0.64%-0.66%), as were the Traub and Moore methods (3.49%, 95% CI 3.46%-3.52%).

For these younger children, the Moore and McLaren methods had the largest divergence with a median difference of 13.7% for the percentage of ideal body weight for kids with a BMI of 21.5 (95% CI 13.3%-14.1%).

"A child in [preschool] 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," explained the researchers.

As for the older children, there was a little more variability between these methods. Only the Traub and BMI methods were relatively similar, with a difference of just 2.67% in percentage of ideal body weight calculations (95% CI 2.66%-2.69%). On the other hand, for kids who had a BMI of 31.5, there was a substantial disagreement between the ADA and Moore methods (16.6%, 95% CI 15.6%-17.8%).

"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," the group noted.

What these findings suggest is that these current methods of calculating ideal body weight in children are an "inconsistent surrogate for fat-free mass in children with obesity," Moylan and co-authors concluded.

When it comes to dosing medications for overweight children, as well as children with obesity, this degree of variability can lead to underdosing of medications.

"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," they posed as an example. "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, this is likely to result, at least initially, in the patient receiving subtherapeutic levels of medication."

To deal with this problem, Moylan's group recommended new methods that include other measures such as bioelectrical impedance to more accurately calculate ideal body weight, and thus the proper doses of medications.

Correction: The method for calculating Traub's IBW in children under 5 ft has been corrected to reflect that height in this group is calculated in centimeters.

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