The question
Work it with the BNF open before you show the answer. At exam pace this item has 1 min 12 s.
A 54-year-old woman attends the chemotherapy day unit for the first cycle of AC chemotherapy for early breast cancer. She is to be given doxorubicin hydrochloride 60 mg/m² by intravenous injection, followed by cyclophosphamide 600 mg/m² by intravenous infusion, with the cycle repeated every 21 days.
Her height is 1.62 m and her weight is 72 kg. Her body surface area is to be calculated from the formula printed on the day unit's prescribing worksheet:
body surface area (m²) = √(height (cm) × weight (kg) ÷ 3600)
Answer and marking
108 mg
Marks2
ExplanationStep 1: height in centimetres = 1.62 m × 100 = 162 cm. Step 2: 162 cm × 72 kg = 11 664, and 11 664 ÷ 3600 = 3.24. Step 3: body surface area = √3.24 = 1.8 m². Step 4: dose = 60 mg/m² × 1.8 m² = 108 mg. The figures resolve exactly, so no rounding instruction is given and the answer is marked all-or-nothing. Omitting the square root gives 194.4 mg and scores zero; entering the height as 1.62 rather than 162 gives 10.8 mg and scores zero; 1080 mg is the cyclophosphamide dose, not the doxorubicin dose.
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How to approach a calculation skills item
The repeatable method from the Calculation Skills section guide.
- Read the unitsIn the question, on the preparation, and in the answer box. Write them down before touching the calculator.
- Identify the targetOne dose? A daily dose? A volume? A rate per hour? The wrong target is the commonest error.
- Calculate the doseWeight times dose per kilogram, or the stated dose, in mass or units.
- Convert to what is givenDose divided by concentration gives volume; volume over time gives rate.
- Check the magnitudeWould a nurse draw up this volume? Would this rate empty the bag in a sensible time?
Before you submit
- Did I answer in the unit the box asks for?
- One dose or the daily dose?
- Micrograms and milligrams: did the thousand go the right way?
- Did I round only at the end?
- Is the magnitude plausible for a real patient?
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