How do you calculate body surface area (BSA) for drug dosages?

Calculating Body Surface Area for Drug Dosages

The Mosteller formula is the recommended method for calculating body surface area (BSA) for drug dosages due to its simplicity and clinical applicability: BSA (m²) = √[(Height(cm) × Weight(kg))/3600]. 1, 2

Common BSA Formulas

• The Mosteller formula (√[(Height(cm) × Weight(kg))/3600]) is widely used due to its simplicity and suitability for both clinical and laboratory work in adults 2
• The DuBois formula (0.007184 × Weight(kg)^0.425 × Height(cm)^0.725) is historically common but was derived from only 9 subjects and may underestimate BSA in obese patients by 3-5% 2
• The Gehan and George formula (0.0235 × Height(cm)^0.42246 × Weight(kg)^0.51456) is particularly useful in pediatric populations 1
• A simplified formula BSA = 1/6(WH)^0.5 (where W is weight in kg and H is height in m) can also be used for rapid estimation 3

Clinical Applications of BSA

• BSA is essential for calculating doses of chemotherapeutic agents, which are frequently dosed based on m² to account for variations in drug metabolism and clearance 1, 4
• For medications with narrow therapeutic indices, precise BSA calculation is critical to avoid toxicity while maintaining efficacy 5
• In pediatric patients, BSA-based dosing is particularly important due to developmental differences in drug metabolism 1
• Many injectable medications like trastuzumab (8 mg/kg loading dose, 6 mg/kg maintenance) and ado-trastuzumab (3.6 mg/kg) use weight-based dosing that requires accurate calculations 1

Special Considerations

• For patients with abnormal body composition (obesity, edema, cachexia), standard BSA formulas may be less accurate 1, 2
• In obese patients, BSA calculated using the Mosteller formula is significantly higher than when using the DuBois formula (2.21 ± 0.22 m² vs. 2.14 ± 0.21 m²) 2
• For medications requiring renal clearance, creatinine clearance (often calculated using the Cockcroft-Gault formula) should be considered alongside BSA for appropriate dosing 5
• When using BSA for cardiac output indexing or other physiological parameters, consistency in the formula used is important for comparative assessments 1

BSA in Different Patient Populations

• Average BSA values differ significantly between normal weight (1.81 ± 0.19 m²), overweight (1.99 ± 0.16 m²), and obese (2.21 ± 0.22 m²) adults 2
• For pediatric patients, age-specific formulas may be more appropriate, such as the Mosteller or Gehan and George equations 1
• In patients with renal impairment, BSA calculations should be used alongside creatinine clearance for medication dosing 1, 5

Common Pitfalls to Avoid

• Using different BSA formulas for the same patient over time can lead to inconsistent dosing 6
• Failing to specify which BSA formula is being used in clinical documentation can create confusion in medication dosing 6
• Using BSA without considering organ function (particularly renal and hepatic) may lead to inappropriate dosing 5
• For medications requiring precise dosing due to narrow therapeutic ranges, relying solely on BSA without considering other patient factors may be insufficient 5