BSA for Drug Dosing | Body Surface Area in Clinical Pharmacology
Understand how BSA is used to calculate safe and effective drug doses in oncology, burn care, and pediatrics, with dosing examples and capping guidelines. Free calculator.
How Body Surface Area Is Used in Drug Dosing
BSA-based dosing was introduced to reduce the inter-patient variability in drug clearance and toxicity. Organs responsible for drug metabolism and excretion — particularly the kidneys and liver — scale in proportion to body surface area rather than total body weight. Chemotherapy agents are the most common drugs dosed per m² of BSA, though BSA is also used in cardiology, nephrology, and pediatric medicine.
Common Drug Classes Dosed by BSA
| Drug Class | Examples | Dosing Basis |
|---|---|---|
| Cytotoxic chemotherapy | Doxorubicin, paclitaxel, cisplatin, 5-FU | mg/m² per cycle |
| Biologic agents | Some monoclonal antibodies (pediatric) | mg/m² or flat dose |
| Burn fluid resuscitation | Parkland Formula (Ringer's lactate) | 4 mL/kg/% TBSA burned |
| Cardiac output indexing | Cardiac Index = CO ÷ BSA | Reference: 2.5–4.0 L/min/m² |
| Renal function | GFR indexed to 1.73 m² BSA | Normalized glomerular filtration |
BSA Dosing Caps: When Not to Use Full BSA
Using actual BSA in obese patients can lead to toxic overdoses, particularly with chemotherapy. Many protocols impose a BSA cap:
| Scenario | Common Cap | Rationale |
|---|---|---|
| General adult oncology | BSA ≤ 2.0 m² | Prevents disproportionate dosing in obese patients |
| Cisplatin | Dose ≤ 200 mg per cycle | Nephrotoxicity and neurotoxicity risk |
| Doxorubicin | Cumulative lifetime ≤ 550 mg/m² | Cardiotoxicity (cardiomyopathy) risk |
| Pediatric weight-based switch | Switch to mg/kg if < 10 kg or BSA < 0.5 m² | Formulas less accurate at very low BSA |
Step-by-Step Drug Dose Calculation Using BSA
1. Measure height and weight accurately (without shoes, light clothing). 2. Calculate BSA using the Mosteller or DuBois formula. 3. Apply the protocol-specified cap if applicable. 4. Multiply: Dose = Drug Dose (mg/m²) × BSA (m²). 5. Round to the nearest practical dose (e.g., vial size). 6. Document the BSA used and the formula applied for the patient record.
Frequently Asked Questions
Should I use ideal body weight or actual body weight for BSA calculation?
Always use actual body weight for BSA calculation, not ideal or adjusted body weight. BSA formulas are validated with actual measurements. Protocols that are concerned about overdosing in obese patients address this with dosing caps, not by substituting ideal body weight.
How often should BSA be recalculated during a treatment course?
BSA should be recalculated at the start of each cycle if the patient's weight has changed by more than 5–10%. Significant weight loss during chemotherapy is common, and using outdated BSA can result in relative overdosing or underdosing as the patient's body composition changes.