What it is
Body surface area (BSA) is the total external area of the body in m², and it is the dosing denominator for most cytotoxic chemotherapy and several other narrow-therapeutic-index drugs and fluid regimens. BSA correlates better than weight with metabolic rate, cardiac output, renal clearance and blood volume — the physiology that governs how a drug is handled.
The method
Doctaverse uses the Mosteller formula — the simplest and the one most cancer-care bodies mandate for consistency:
$$ BSA\ (m^2) = \sqrt{\frac{height\ (cm) \times weight\ (kg)}{3600}} $$
Give it height and weight and it returns BSA to two decimals. Several other formulas exist (Du Bois & Du Bois, Haycock, Boyd), and they disagree by a few percent — enough to nudge a dose across a rounding boundary — which is exactly why standardising on one equation matters more than the theoretical “best” one. Mosteller is validated across a wide age range and, because everyone uses the same equation, it removes the dose variability that creeps in when different formulas (Du Bois, Haycock, Boyd) are mixed within one institution.
When to use it
- Chemotherapy dosing (mg/m²) — the dominant use case.
- Fluid and electrolyte regimens that are surface-area based (e.g. some maintenance and burn protocols).
- Normalising physiologic measures — note GFR is reported per 1.73 m², the historical “standard adult” BSA.
Worked example
A child, height 100 cm, weight 16 kg:
$$ BSA = \sqrt{\frac{100 \times 16}{3600}} = \sqrt{0.444} = 0.67\ m^2 $$
A drug dosed at 75 mg/m² would give 75 × 0.67 ≈ 50 mg.
Pitfalls
- Garbage in, garbage out. A charting error in height or weight scales the dose directly — double-check measured (not stated) values, and re-measure children whose weight changes fast.
- Obesity and capping. Many centres historically capped BSA at 2.0 m² to avoid overdosing obese patients, but evidence does not support routine capping; follow your local chemotherapy protocol rather than an arbitrary ceiling.
- Formula consistency. Switching formulas mid-treatment shifts the calculated dose by a few percent — stick to one (Mosteller) across a patient’s course.
- Not for everyone. Some agents (e.g. carboplatin via Calvert/AUC dosing) and most neonatal drugs are dosed by weight or renal function, not BSA — don’t reflexively apply mg/m² where the protocol specifies otherwise.
- BSA estimates body size; it does not account for organ dysfunction. Renal or hepatic impairment still requires separate dose adjustment.
Run it: Body Surface Area (Mosteller)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.