Adjusted Body Weight Equation for Pediatric Obesity
The adjusted body weight (AdjBW) equation for pediatric patients with obesity is: AdjBW = IBW + 0.4 × (Total Body Weight - IBW), where IBW (ideal body weight) is calculated based on the 50th percentile BMI for the child's age and sex. 1
Rationale for Using Adjusted Body Weight
Medication dosing based solely on total body weight in pediatric patients with obesity (BMI ≥95th percentile) may lead to overdosing, as adipose tissue has different pharmacokinetic properties than lean tissue. 1
Ideal body weight (IBW) alone often underdoses medications, while adjusted body weight provides a middle ground that accounts for the increased lean body mass that accompanies obesity while avoiding excessive dosing from adipose tissue. 1
Implementation of IBW and AdjBW calculators in electronic health records, combined with evidence-based dosing protocols, significantly improved adherence to appropriate weight-based dosing from 1.2% to 24.2% in pediatric patients with obesity. 1
Calculating Ideal Body Weight (IBW)
IBW should be calculated using the 50th percentile BMI value for the child's age and sex from CDC growth charts, then solving for weight using the child's actual height: IBW = (50th percentile BMI) × (height in meters)². 1
For children aged 2 years and older, CDC growth charts should be used to determine the 50th percentile BMI value, as these charts are the standard for this age group. 2
Clinical Application and Medication Dosing
The AdjBW formula (IBW + 0.4 × [TBW - IBW]) should be used for most medications in pediatric obesity, particularly those with narrow therapeutic indices or significant toxicity profiles. 1
Specific medications requiring weight-based dosing adjustments include immune globulin, where using AdjBW instead of total body weight resulted in potential cost savings of $9,423 per patient without compromising efficacy. 1
Electronic health record integration of calculated IBW and AdjBW values, along with provider education on evidence-based dosing recommendations, is essential for consistent implementation in clinical practice. 1
Important Clinical Caveats
This equation applies specifically to children with obesity, defined as BMI ≥95th percentile for age and sex, not to children with normal weight or overweight (85th-94th percentile). 3
The 0.4 correction factor represents the estimated proportion of excess weight that contributes to drug distribution volume, though this may vary by medication class and should be verified against specific pharmacokinetic data when available. 1
For children with severe obesity (BMI ≥120% of 95th percentile or absolute BMI ≥35 kg/m²), which affects 4-6% of the pediatric population, careful monitoring and potential dose adjustments may be needed even when using AdjBW. 3