Weight-based dosing, body-surface area and dose calculation — branching viva

Stage 1 — The weight and the height

Examiner: A nine-year-old boy is admitted with a cellulitis that needs intravenous therapy. The nurse is preparing the dose and asks you to confirm it. What is the first thing you want before you prescribe, and how do you get it? ^[2]

Candidate target: Ask for a measured weight in kilograms on a calibrated scale, not an estimate, and record it once on the chart in kilograms. Ask for the height in centimetres as well, because some drugs are dosed by body-surface area and you may need it. State that an estimated weight is a placeholder to be replaced by a measured weight the moment a scale is available, and that the single written weight in kilograms is the foundation of every dose. ^[2]

Examiner probe: The scale reads 30 kg and his height is 135 cm. Why does the height matter at all? ^[2]

Candidate target: Because some drugs, including several antimicrobials and all cytotoxic chemotherapy, are dosed by body-surface area rather than weight, and body-surface area is calculated from both height and weight by the Mosteller formula. Having the height means you can move between the two scalars without re-measuring. ^[1]