Weight-Based Dosing: mg/kg Practice Questions for the CBT

Weight-based questions are two-step questions pretending to be one-step. The core multiplication is easy. The mark is lost either side of it: misreading what the per-kg figure covers, or stopping before the question’s real finish line.

The base formula:

Dose = dose per kg × patient weight in kg.

Gentamicin 5 mg/kg for a 72 kg patient: 5 × 72 = 360 mg. Always kilograms; UK prescriptions and the CBT don’t deal in pounds or stones.

Single dose or daily total?

The notation decides everything. mg/kg is one dose. mg/kg/day is the whole day, and the prescription will split it: “40 mg/kg/day in four divided doses”.

Worked example. A 20 kg child, 40 mg/kg/day in four divided doses.

• Daily total: 40 × 20 = 800 mg.
• Per dose: 800 ÷ 4 = 200 mg.

Give 800 mg as a single dose and you’ve quadrupled it. The exam plants the daily total among the options every time this pattern appears. When you see /day, your answer needs a division before it’s finished.

The dose is often not the answer

The other extension: the question hands you a stock strength, which means it wants a volume, not a dose. A 3 mg/kg prescription for 70 kg with 50 mg/mL stock:

• Dose: 3 × 70 = 210 mg.
• Volume: 210 ÷ 50 = 4.2 mL.

Chained calculations like this are where Part A earns its reputation. Neither step is hard; the skill is noticing there are two. Before answering any weight-based question, check the last line of the stem: does it ask for mg, mL, or a rate? The 210 will be sitting in the options whether it’s the answer or not.

Sense-checks that work at speed

Adult single doses land in grams or hundreds of milligrams; a weight-based answer of 36 mg for a 72 kg adult on a 5 mg/kg drug means a lost zero. For children, the dose should be visibly smaller than an adult’s for the same drug. And decimal per-kg values (1.5 mg/kg, 7.5 mg/kg) multiply cleanly if you split them: 1.5 × 80 is 80 plus half of 80, so 120. Faster than typing it, and you’ll spot a slip because each part is trivially checkable.

Weight-based dosing is also where the CBT overlaps with clinical judgement (maximum doses, dose banding), which the main guide’s drug calculation formulas chapter touches on. For the exam, the arithmetic above covers it. Five questions below, then percentages and ratios.