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Loading Dose Calculation — FRCA Primary MCQ

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EasyPharmacology - General PrinciplesLoading Dose CalculationFRCA Primary

A drug with linear pharmacokinetics is to be given as a loading dose intended to rapidly achieve a specified target plasma concentration. Which expression gives the dose to be administered, where F is the systemic bioavailability of the route used?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: D — Loading dose = Volume of distribution × Target concentration / Bioavailability

Explanation lettering: D = shown as A · A = shown as B · B = shown as C · C = shown as D

C is correct. The amount of drug required in the body is the apparent volume of distribution multiplied by the target plasma concentration. Dividing by F corrects for incomplete systemic bioavailability; for intravenous administration, F = 1. Clearance determines drug elimination and therefore the maintenance dose rate, so D represents a maintenance infusion-rate calculation rather than a loading dose. In a one-compartment model, A equals 0.693 × Vd × target concentration and therefore underestimates the intravenous loading dose. B rearranges the half-life relationship to give Vd alone, which is a volume rather than a dose. E inverts the relationship and is dimensionally incorrect.

Reference: UKMi Quality and Risk Management Group. Medicines Information Enquiry Answering Guidelines, Pharmacokinetics section, page 84. Resources updated February 2022. https://www.wmic.wales.nhs.uk/wp-content/uploads/2022/10/UKMi-Enquiry-Answering-Guidelines-2022-update.pdf