Chemotherapy dose calculation and rounding: BSA, Calvert and dose caps
Chemotherapy doses are calculated, not simply transcribed, and small differences in method add up. Compounding software should use one method everywhere, show its working, and round only once — at the right point.
Body surface area (BSA)
Most cytotoxic doses are prescribed in mg/m². The two formulas in common use give slightly different results:
- Du Bois: BSA (m²) = 0.007184 × weight(kg)0.425 × height(cm)0.725
- Mosteller: BSA (m²) = √(height(cm) × weight(kg) / 3600)
Either is acceptable; what matters is that the same formula is used by prescribing, pharmacy and the label. A system that uses one formula on one screen and another elsewhere will produce doses that do not match.
Carboplatin and the Calvert formula
Carboplatin is dosed to a target area under the curve (AUC) rather than by BSA:
Dose (mg) = target AUC × (GFR + 25)
GFR is commonly estimated with the Cockcroft–Gault equation from the patient’s age, weight, sex and serum creatinine. Because estimated GFR can overstate real clearance, many institutions follow the recommendation to cap the estimated GFR at 125 mL/min — which limits the dose to AUC × 150 mg. The software should use the latest creatinine, show the estimated GFR and whether the cap was applied.
Dose caps
Some drugs carry a maximum dose regardless of BSA — vincristine is commonly capped at 2 mg in many protocols. Caps belong in the drug or protocol rules, applied automatically and shown on screen, not remembered by the person preparing.
Dose rounding
Rounding a calculated dose to a practical amount — often to the nearest vial size — reduces waste and preparation time. The Hematology/Oncology Pharmacy Association supports rounding within 10% of the ordered dose where appropriate. The software should apply your institution’s rounding policy consistently, record the percentage adjustment, and choose the least-waste combination of vials for the final dose.
Round once, at the end
A frequent source of label errors is rounding intermediate values. If a volume is rounded before the infusion rate is calculated, the rate on the label can be wrong by several mL/h — for example 180 instead of 175 mL/h. Volumes should never be truncated (255.97 mL is not “255 mL”). Good software carries full precision through the calculation and applies the unit’s display rules once, when the label is produced, with exceptions for products that need more precision.
Show the working
Pharmacists should be able to open any preparation and see each step: the formula, the inputs, the result and the rule applied. That is what makes a calculation checkable — and defensible.
This article is general information for pharmacy professionals and does not replace your institution’s protocols or clinical judgement.