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]

Stage 2 — Choosing the scalar and the formula

Examiner: The drug you choose is dosed by body-surface area. Give me the formula you will use and work it for this child. [1]

Candidate target: State the Mosteller formula: body-surface area in square metres equals the square root of the height in centimetres multiplied by the weight in kilograms, divided by 3600. For 135 cm and 30 kg: 135 times 30 is 4050, divided by 3600 is 1.125, and the square root of 1.125 is 1.06 square metres. [1]

Examiner probe: Why body-surface area and not weight for this class of drug? [1]

Candidate target: Because drug clearance tracks metabolic body size, and body-surface area approximates metabolic mass across age bands by scaling with basal metabolic rate, cardiac output and organ blood flow. The empirical caution is that body-surface-area dosing is not always safer than weight-based dosing, as the valganciclovir example shows, so the scalar is the one the formulary specifies and not a habit. [1]

Stage 3 — The cap, the writing and the check

Examiner: The formulary gives the dose as 200 mg per square metre, with a maximum adult single dose of 250 mg. What do you prescribe? [1]

Candidate target: 1.06 square metres times 200 mg per square metre is 212 mg. The maximum adult single dose is 250 mg, so the cap does not bind and the dose is 212 mg. State the cap check explicitly, because the principle is that the weight-based or body-surface-area dose never exceeds the adult maximum single or daily dose, and in a larger adolescent the cap would bind. [1]

Examiner probe: Write the order. What rules govern how you write the number? [3]

Candidate target: Apply the leading-zero rule (write 0.5 mg, never .5 mg) and the no-trailing-zero rule (write 212 mg, never 212.0 mg), because both defend against a tenfold decimal error that is invisible to the eye and lethal in a small body. Then arrange an independent double check by a second clinician, who recalculates the dose, confirms the cap and confirms the units without referring to the original calculation. [3]

Stage 4 — The monitoring loop

Examiner: The drug is in fact vancomycin, not the body-surface-area drug I described. How does that change your plan? [7]

Candidate target: Vancomycin is dosed by weight, not body-surface area, and is monitored by level. The consensus target for a serious staphylococcal infection is an area-under-the-curve over twenty-four hours of 400 to 600 mg per litre per hour, assessed by a Bayesian approach or a trough drawn just before a dose at steady state. Write the dose time on every sample tube, because a trough drawn too early overestimates the level and a level without a recorded dose time is uninterpretable. Run the dose-adjust-measure loop: dose, draw the level at the correct time, interpret against the dose time, adjust toward the target, and re-check the calculation at each step. [7] [8]

Examiner probe: And if the child were obese rather than 30 kg? [7]

Candidate target: Choose the weight scalar deliberately: total body weight for drugs that distribute into fat, ideal body weight for water-soluble narrow-therapeutic-index drugs, adjusted body weight as a middle path. Defaulting to total body weight for every drug gives water-soluble narrow-therapeutic-index drugs at toxic doses, so the choice is guided by the drug's volume of distribution, its therapeutic index and the evidence, with the level monitored where evidence is limited. [7]