Common Registration Assessment · Topic deep-dive
GPhC Displacement Volume Questions
Displacement volume is one of the most consistently failed Part 1 patterns. The Board of Assessors has named it in both June 2025 and November 2025 feedback. The maths is simple once you understand the principle — but the principle catches candidates who skipped this topic in undergraduate teaching.
What the GPhC actually tests on this topic
- Understanding displacement volume: the volume the solid drug occupies in solution after reconstitution
- Calculating the final volume after reconstitution = diluent volume + displacement volume
- Calculating the final concentration based on the total final volume, not just the diluent volume
- Calculating the dose volume to withdraw based on the correct final concentration
- Recognising when displacement matters (it always matters for accurate dosing) and when it is small enough to be negligible in practice
Common pitfalls — from the Board of Assessors’ feedback
Using diluent volume instead of total volume for concentration
If a vial contains 1 g of powder with a displacement of 0.2 mL and you add 4.8 mL of diluent, the final volume is 5.0 mL — and the concentration is 1 g / 5.0 mL = 200 mg/mL, not 1 g / 4.8 mL. This is the single most common displacement error and is named in November 2025 feedback.
Adding diluent to make the total volume rather than reaching the total volume
Read the stem carefully. “Add 5 mL of diluent” vs “make up to a final volume of 5 mL” are different instructions. The first gives you total volume = 5 mL + displacement. The second gives you total volume = 5 mL (and diluent needed = 5 mL − displacement).
Ignoring displacement for paediatric doses
Small displacement values matter more for paediatric doses because the dose is smaller and the proportional error from using the wrong concentration is larger. A 5% concentration error in an adult IV dose is usually clinically tolerable; in a neonate it is not.
Sample calculation questions
CRA-style practice questions, not real exam questions. The Board of Assessors does not endorse third-party question banks.
A 62 kg patient with an eGFR of 8 mL/min/1.73m² is prescribed amikacin at 15 mg/kg once daily. The BNF recommends a maximum single dose of 1500 mg and the following renal dose adjustments: eGFR 0-14 mL/min/1.73m²: reduce to 25% of normal dose. Amikacin injection is available as 50 mg/mL in 2 mL vials. What is the correct amikacin dose for this patient?
- A. 232.5 mg
- B. 465 mg
- C. 58.12 mg
- D. 77.42 mg
- E. 2325 mg
Show worked answer
Correct answer: A
A patient is started on carbamazepine for epilepsy — initiation using the following titration schedule: 100 mg twice daily for 7 days; 200 mg twice daily for 7 days; 300 mg twice daily for 14 days; 400 mg twice daily for 28 days. Carbamazepine is available as 100 mg tablets. Tablets come in packs of 84. How many packs should be dispensed for the primary drug?
- A. 1.67 pack(s) of 84
- B. 5 pack(s) of 84
- C. 3.33 pack(s) of 84
- D. 10 pack(s) of 84
- E. 20 pack(s) of 84
Show worked answer
Correct answer: B
Displacement volumes — frequently asked
What is displacement volume?
The volume that the dry powder of a drug occupies when fully dissolved in diluent. For a vial of 1 g cefazolin with a displacement of 0.5 mL, if you add 4 mL of water the final volume is 4.5 mL, not 4 mL — and the concentration must be calculated on that 4.5 mL.
Where do I find displacement values?
The BNF, BNFC, and the Summary of Product Characteristics (SmPC) for each injection. The Medusa IV guide is the standard hospital reference. CRA questions will provide the displacement value in the stem — you do not need to memorise individual values, but you must know how to use them.
When is displacement small enough to ignore?
In practice, displacements under about 0.1 mL on a final volume above 10 mL introduce a <1% error and are often ignored in adult dosing. Paediatric and neonatal dosing demand displacement is always accounted for. CRA questions test the principle — assume displacement always matters in exam answers unless the stem explicitly says otherwise.
Practise displacement volumes questions in context
Across the full bank of GPhC exam questions — every format, with worked answers grounded in the Board of Assessors’ published feedback.