Common Registration Assessment, Part 1
Combined additives in one infusion
Three original questions on infusions with multiple additives, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Adding anything to an infusion bag changes the final volume, and the final volume is what sets the concentration and the pump rate. Questions of this type test whether you remember to count every additive.
The questions here are fictional and practise only the arithmetic. Add the volumes first, then work out concentration and rate from the total.
The method
- List every volume: the bag plus each additive.
- Add them for the final volume.
- Divide each drug mass by the final volume for its final concentration.
- Divide the final volume by the infusion time for the pump rate.
- Round the rate as the question directs.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional 500 mL bag has 20 mL of potassium concentrate added. The bag is to run over 6 hours. What pump rate in mL per hour is set, to one decimal place?
Show the answer
Answer: 86.7 mL/hour
Working: Final volume = 500 + 20 = 520 mL. 520 / 6 = 86.67, so 86.7 mL/hour.
Question 2
A fictional 100 mL bag receives drug A 1.2 g in 10 mL and drug B 500 mg in 10 mL. What is the final concentration of drug A in mg per mL?
Show the answer
Answer: 10 mg/mL
Working: Final volume = 100 + 10 + 10 = 120 mL. Drug A = 1,200 mg / 120 mL = 10 mg/mL.
Question 3
A fictional 250 mL bag has 5 mL of magnesium solution added and runs over 4 hours. What pump rate in mL per hour is set, rounded to the nearest whole number?
Show the answer
Answer: 64 mL/hour
Working: Final volume = 250 + 5 = 255 mL. 255 / 4 = 63.75, which rounds to 64 mL/hour.
Add volumes first
Before any rate or concentration, write down the final volume. Every other line of the working hangs off that number.
Where marks are lost
- Setting the rate from the bag volume alone and ignoring the additive volumes.
- Using each drug's own ampoule volume instead of the shared final volume for concentration.
- Adding the drug masses together when the question asks about only one of them.
Frequently asked questions
Do small additive volumes really matter?
For rates they can shift the answer by a few mL per hour, which is exactly what the assessment is testing. Always count them.
Which volume do I use for concentration?
The final combined volume after every additive is in the bag.
Sources
- The Pharmaceutical Journal, medication maths: infusion rates
- GPhC, 2026 assessment specification and permitted items
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
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
See also the formula sheet, the eight sample questions and the approved calculator page.