BSA Formulas
Body surface area is estimated using height and weight. The most common formulas include:
Du Bois (1916)
BSA = 0.007184 × W0.425 × H0.725
Mosteller (1987)
BSA = √(W × H / 3600)
Haycock (1978)
BSA = 0.024265 × W0.5378 × H0.3964
Where W = weight in kg, H = height in cm. All formulas return BSA in m².
Clinical Uses of BSA
- Drug dosing: Chemotherapy and many medications are dosed per m² of BSA
- Cardiac index: Cardiac output divided by BSA normalizes heart function across body sizes
- Renal function: GFR is often normalized to 1.73 m² BSA
- Burn assessment: Percentage of BSA affected helps determine treatment
Average BSA Values
- Adult male: ~1.9 m²
- Adult female: ~1.6 m²
- Newborn: ~0.25 m²
- Child (10 years): ~1.14 m²
Why BSA Instead of Weight?
BSA correlates better than body weight with many physiological parameters including cardiac output, blood volume, and kidney function. Two people weighing the same but with different heights have different metabolic rates and drug distribution — BSA accounts for this. Many cancer treatment protocols and pediatric drug dosages are calculated per square meter of BSA rather than per kilogram.
The Rule of Nines (Burn Assessment)
In emergency medicine, BSA is used to estimate burn severity using the "Rule of Nines," which divides the adult body into regions each representing approximately 9% (or multiples) of total BSA:
- Head and neck: 9%
- Each arm: 9%
- Chest and abdomen (front): 18%
- Back: 18%
- Each leg: 18%
- Groin: 1%
Burns covering more than 20% BSA in adults (or 10% in children) typically require specialized burn center care.
Comparing BSA Formulas
| Formula | Year | Best For |
|---|---|---|
| Du Bois | 1916 | General clinical use, adults |
| Mosteller | 1987 | Quick calculations, widely accepted |
| Haycock | 1978 | Pediatric patients |
| Gehan & George | 1970 | Large sample validation |
| Boyd | 1935 | Broad range of body sizes |
| Fujimoto | 1968 | East Asian populations |
Most formulas give results within 5-10% of each other for average-sized adults. Differences become larger at extremes of height and weight. For pediatric dosing, the Haycock or Mosteller formulas are preferred because the original Du Bois study included few children.
BSA in Chemotherapy Dosing
Chemotherapy drugs have a narrow therapeutic window — too little is ineffective, too much is toxic. BSA-based dosing adjusts for body size, ensuring smaller patients don't receive overdoses and larger patients get enough drug. However, BSA-based dosing is imperfect: patients with the same BSA can metabolize drugs at different rates due to genetics, organ function, and body composition. Pharmacogenomic-guided dosing is increasingly used alongside BSA for certain drugs.