Common Registration Assessment · Topic deep-dive
GPhC IV Infusion Calculation Questions
IV infusion rate calculations are a Part 1 staple and a recurring Board of Assessors weak area — specifically named in November 2025 feedback as “IV infusion rate unit mismatches.” The patterns the GPhC tests are predictable; the candidates who fail this section are usually getting the unit conversions wrong, not the underlying maths.
What the GPhC actually tests on this topic
- Calculating infusion rate (mL/hour or drops/min) from a prescribed dose and concentration
- Calculating dose delivered per minute or per hour from a known infusion rate
- Converting between mg/mL, mcg/mL, mg/kg/min, mcg/kg/min
- Time-to-infuse calculations and remaining-volume safety checks
- IV bolus vs IV infusion — recognising which the question describes
- Realistic infusion rate sense-checks (typically 1–500 mL/hour for adult peripheral lines)
Common pitfalls — from the Board of Assessors’ feedback
Unit mismatch between dose and concentration
November 2025 feedback explicitly names this as a Part 1 weak area. If the dose is in mcg/kg/min and the concentration in mg/mL, you must convert before calculating rate. The most common error is leaving the units mismatched and ending up with an infusion rate three orders of magnitude wrong.
Forgetting the remaining-volume safety check
The June 2025 feedback names this: “As part of a safety check, trainee pharmacists should be able to calculate the remaining volume to ensure the infusion is being administered correctly.” If a patient has been on an infusion at a known rate for a known time, the remaining volume should match — if it does not, the infusion is being delivered at the wrong rate.
IV bolus vs infusion confusion
A bolus is a total dose given over a short fixed time. An infusion involves rate, duration, and a final volume. The Board of Assessors flagged this distinction in June 2025. Read the stem twice — the verbs and timings tell you which it is.
Missing the realistic-rate sense-check
An adult peripheral IV infusion typically runs at 1–500 mL/hour. A central line may run faster. If your answer is 5,000 mL/hour or 0.05 mL/hour, you have a unit error. Always sense-check the final number against a realistic clinical rate.
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
IV infusion calculations — frequently asked
How do I avoid unit mismatches in IV calculations?
Write down every unit in the stem before doing any maths: weight in kg, dose in mg or mcg, per kg or absolute, per minute or per hour, concentration in mg/mL or mcg/mL. Convert everything to consistent units first, then do the calculation. Most failed IV calc questions come from doing the maths before fixing the units.
What is a remaining-volume safety check?
A check that the volume left in the infusion bag matches what should remain given the prescribed rate and the time elapsed. If a 1,000 mL bag has been running at 100 mL/hour for 4 hours, 600 mL should remain. If the bag shows 400 mL remaining, the rate is wrong — either the pump is set incorrectly or the bag was hung incorrectly. The Board of Assessors expects you to do this check as part of every IV calculation.
How do I convert mcg/kg/min to mL/hour?
In four steps: (1) calculate the dose per minute = mcg/kg/min × weight in kg. (2) Convert to dose per hour = dose per minute × 60. (3) Convert dose units to match concentration (e.g. mcg to mg). (4) Calculate volume per hour = dose per hour ÷ concentration (mg/mL). Sense-check the final number is in the realistic 1–500 mL/hour range.
Practise iv infusion calculations questions in context
Across the full bank of GPhC exam questions — every format, with worked answers grounded in the Board of Assessors’ published feedback.