Common Registration Assessment, Part 1

Vancomycin weight-based doses and infusion times for the GPhC assessment

Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.

Published 9 November 2026. Independent revision material, not GPhC questions, not for patient care.

An intravenous line inserted into a patient's arm
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The manufacturer's UK product information for vancomycin 1000 mg powder for concentrate for solution for infusion states that for patients aged 12 years and older the recommended intravenous dose is 15 to 20 mg/kg of body weight every 8 to 12 hours, not to exceed 2 g per dose, and that in seriously ill patients a loading dose of 25 to 30 mg/kg of body weight can be used.

The same information states that vancomycin shall only be administered as a slow intravenous infusion of at least one hour duration or at a maximum rate of 10 mg/min, whichever is longer, sufficiently diluted with at least 100 ml per 500 mg or at least 200 ml per 1000 mg.

It also states that at the time of use, 20 ml of water for injections is added to the 1000 mg vial, giving a solution of 50 mg per ml, and that subsequent dose adjustments should be based on serum concentrations, with renal function taken into consideration.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish a single dose from a daily total. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Multiply the stated mg/kg by the stated body weight for the dose in mg.
  2. Check the result against the stated 2 g per dose ceiling.
  3. Divide the dose in mg by 10 mg/min for the minimum infusion time, and compare it with one hour: use whichever is longer.
  4. Keep mg/kg, mg, g, ml and minutes separate; convert only when the question asks.

Three practice questions

Work each one on paper first, then open the answer. All patients and prescriptions are fictional.

Question 1

A fictional seriously ill patient weighs 80 kg and is prescribed a vancomycin loading dose of 25 mg/kg. What is the loading dose, and how many 1000 mg vials does it need?

Show the answer

Answer: 2000 mg; 2 vials

Working: 25 mg/kg x 80 kg = 2000 mg. 2000 mg / 1000 mg per vial = 2 vials, and 2 g does not exceed the 2 g per dose ceiling.

Question 2

A fictional patient weighs 70 kg and is prescribed vancomycin 15 mg/kg every 12 hours. What is each dose, and what is the total daily dose?

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Answer: 1050 mg per dose; 2100 mg daily

Working: 15 mg/kg x 70 kg = 1050 mg per dose. Every 12 hours means two doses daily: 1050 mg x 2 = 2100 mg daily.

Question 3

A fictional patient is prescribed a 1000 mg vancomycin infusion. Using the label maximum rate of 10 mg/min and the minimum duration of one hour, what is the minimum infusion time, and what is the minimum dilution volume?

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Answer: 100 minutes; at least 200 ml

Working: At 10 mg/min: 1000 mg / 10 mg/min = 100 minutes, which is longer than one hour, so 100 minutes applies. The label states at least 200 ml per 1000 mg.

eMC SmPC: Vancomycin 1000 mg powder for concentrate for solution for infusion

https://www.medicines.org.uk/emc/product/15737/smpc

Where marks are lost

  • Exceeding the 2 g per dose ceiling with a large weight-based dose instead of checking against it.
  • Reporting 60 minutes for a 1000 mg infusion; at the 10 mg/min maximum rate it takes 100 minutes, which is longer.
  • Multiplying 15 mg/kg by the weight and reporting it as the daily total when doses are given every 12 hours.

Frequently asked questions

Are these official assessment questions?

No. They are original revision exercises, not official GPhC questions and not for patient care.

Does this calculation decide a real vancomycin dose?

No. Follow the stated prescription in the exercise. Real dosing is adjusted to serum concentrations, with renal function taken into consideration.

How should I check the answer?

Compute each single dose from mg/kg and weight first, check it against the 2 g ceiling, then multiply by the stated number of daily doses.

Sources

The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.

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More calculation topics

See also the formula sheet, the eight sample questions and the approved calculator page.