Body Surface Area (BSA) is used in medicine to calculate drug dosages, chemotherapy protocols, and physiological measures. Several formulas exist, each with different accuracy trade-offs.
Why BSA Is Used
BSA correlates better than body weight alone with:
- Kidney function (GFR)
- Cardiac output
- Drug distribution in oncology and paediatrics
Formula 1: Mosteller (Most Widely Used)
BSA (m²) = √(Height(cm) × Weight(kg) / 3600)
Example: 175 cm, 75 kg:
BSA = √(175 × 75 / 3600) = √(13125 / 3600) = √3.646 = 1.91 m²
Formula 2: DuBois and DuBois (Historical Standard)
BSA (m²) = 0.007184 × Height(cm)^0.725 × Weight(kg)^0.425
Formula 3: Boyd
BSA (m²) = 0.0003207 × Height(cm)^0.3 × Weight(grams)^(0.7285 − 0.0188 × log(Weight))
Comparison of Formulas
For a 175 cm, 75 kg adult:
| Formula | BSA |
|---|---|
| Mosteller | 1.912 m² |
| DuBois | 1.910 m² |
| Boyd | 1.905 m² |
| Haycock | 1.913 m² |
Differences are minimal for average adults. For children and obese patients, formulas diverge more.
Normal Adult BSA Values
| Group | Mean BSA |
|---|---|
| Adult male | 1.9 m² |
| Adult female | 1.6 m² |
| Children (2 years) | 0.5 m² |
| Children (10 years) | 1.14 m² |
| Newborn | 0.2–0.25 m² |
BSA-Based Drug Dosing Example
A chemotherapy drug is dosed at 85 mg/m². Patient BSA = 1.85 m²:
Dose = 85 × 1.85 = 157.25 mg
Doses are typically rounded to the nearest 5 or 10 mg.
BSA vs BMI
| Metric | Measures | Used for |
|---|---|---|
| BMI | Weight/height² | Screening obesity |
| BSA | Surface area | Drug dosing, physiology |
| Weight | Mass only | General dosing |
BSA is more physiologically meaningful than weight alone because organ function scales more closely with surface area.
Limitations
No BSA formula is perfectly accurate, especially at extremes:
- Very obese patients: actual drug distribution may not scale with BSA
- Neonates: different surface-to-volume ratio
- Amputees: standard formulae overestimate BSA
Many cancer protocols now use actual body weight (with capping) rather than BSA for some drugs.