Weight-based dosing, body-surface area and dose calculation — formative SAQs

1. Body-surface-area formula and calculation

The bedside formula is 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 this child, height 160 cm and weight 52 kg gives 160 times 52 equals 8320, divided by 3600 equals 2.311, and the square root of 2.311 equals 1.52 square metres. ^[1]

2. Calculated dose and the adult cap

The calculated dose is 1.52 square metres times 50 mg per square metre, which equals 76 mg per dose. The maximum adult single dose stated by the formulary is 90 mg per dose, so the cap does not bind here and the prescribed dose is 76 mg once daily. The cap is checked explicitly, because the principle is that the weight-based or body-surface-area dose never exceeds the adult maximum single or daily dose; in a larger adolescent the cap would bind and the dose would stop at the adult ceiling. ^[1]

3. The two writing rules

The first rule is the leading zero: write 0.5 mg, never .5 mg, because a bare decimal can be misread as 5 mg and produce a tenfold overdose. The second rule is no trailing zero: write 5 mg, never 5.0 mg, because the trailing zero can be misread as 50 mg and also produce a tenfold overdose. Both rules exist because a misplaced decimal is invisible to the eye and lethal in a small body, and the independent double check is the second defence that catches what the writing rule prevents. ^[3]

4. Final step and safeguard

The final step before the drug is given is 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. The accompanying safeguard is to confirm the route, the frequency and the formulation are age-appropriate, and that the concentration of any oral liquid is one calculation assumed. ^{[2] [3]}