Common Registration Assessment, Part 1
Top-up doses from measured drug levels
When a level comes back low, the dose to add is volume times the gap. Three original questions.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's article on pharmacokinetics explains that the loading dose relationship can also give a top-up dose if the drug is already present but the concentration is too low. The measured concentration is subtracted from the target before multiplying by the volume.
It is a short calculation, and the usual error is to use the target concentration alone and give a full loading dose on top of what is already there.
The method
- Find the volume of distribution in litres, using weight if V is per kg.
- Subtract the measured concentration from the target concentration.
- Multiply the difference by V to get the top-up dose.
- Check that the answer is smaller than a full loading dose.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient has a volume of distribution of 35 L. The target concentration is 20 mg/L and the measured level is 8 mg/L. What top-up dose is needed? These figures are made up for the exercise.
Show the answer
Answer: 420 mg
Working: Gap = 20 − 8 = 12 mg/L. Dose = 35 L × 12 mg/L = 420 mg.
Question 2
A drug has V = 0.7 L/kg. A patient weighing 80 kg has a measured level of 14 mg/L and the target is 20 mg/L. What top-up dose is needed? These figures are made up for the exercise.
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Answer: 336 mg
Working: V = 0.7 × 80 = 56 L. Gap = 20 − 14 = 6 mg/L. Dose = 56 × 6 = 336 mg.
Question 3
A patient weighing 50 kg has V = 0.6 L/kg, a measured level of 7.5 mg/L and a target of 18 mg/L. What top-up dose is needed? These figures are made up for the exercise.
Show the answer
Answer: 315 mg
Working: V = 0.6 × 50 = 30 L. Gap = 18 − 7.5 = 10.5 mg/L. Dose = 30 × 10.5 = 315 mg.
What the article says
The article gives the top-up form of the loading dose relationship, where the measured concentration is subtracted from the target. The drug values on this page are invented for practice.
Where marks are lost
- Using the target concentration instead of the gap.
- Forgetting to convert V per kg to litres.
- Not checking the units of the measured level.
Frequently asked questions
Why not give the full loading dose again?
Part of the target is already in the body, so the full dose would overshoot.
Do these questions need the half-life?
Not for a simple top-up. The half-life matters if time has passed since the level was taken.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
See also the formula sheet, the eight sample questions and the approved calculator page.