Common Registration Assessment, Part 1
Infusion line priming and dead space
Three original questions on line volumes and flushing, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
An infusion line holds its own volume of fluid, typically 15 to 25 mL. If the line is primed with the drug solution and not flushed at the end, that volume of drug stays in the line and the patient never receives it.
The calculations are about delivered volume: what leaves the bag minus what stays in the line, and how much flush is needed to push the drug through.
The method
- Note the bag volume, the line (dead space) volume and the flow rate.
- Without a flush, delivered volume = bag volume minus line volume.
- A flush equal to at least the line volume delivers the residual drug.
- Convert flush volume to time at the same rate: volume ÷ rate × 60 minutes.
- Add flush time to infusion time if the question asks for total time.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A 100 mL infusion runs at 200 mL/h through a line with a 20 mL dead space, and the line is flushed with 20 mL at the same rate. What is the total time from start to end of flush?
Show the answer
Answer: 36 minutes
Working: Infusion: 100 ÷ 200 × 60 = 30 minutes. Flush: 20 ÷ 200 × 60 = 6 minutes. Total = 36 minutes.
Question 2
A 50 mL bag is given through a line holding 22 mL, primed with the drug solution and not flushed. What volume and what percentage of the dose does the patient receive?
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Answer: 28 mL, which is 56% of the dose
Working: Delivered = 50 - 22 = 28 mL. 28 ÷ 50 × 100 = 56%.
Question 3
A line plus extension set has a dead space of 12 mL. Local guidance is to flush with at least twice the dead space volume. At 100 mL/h, how long does the minimum flush take, to the nearest minute?
Show the answer
Answer: 14 minutes
Working: Flush volume = 2 × 12 = 24 mL. 24 ÷ 100 × 60 = 14.4 minutes, so 14 minutes to the nearest minute.
How this is examined
Sketch the bag, the line and the patient. What leaves the bag minus what stays in the line is what the patient gets, and the flush closes the gap.
Where marks are lost
- Assuming the whole bag reaches the patient when the line is primed with drug and never flushed.
- Adding the line volume to the delivered dose instead of subtracting it.
- Forgetting the flush itself takes time at the same flow rate.
Frequently asked questions
Why prime the line with the drug solution at all?
Priming removes air. If you prime with drug, the dead space holds drug at the end, so a flush is needed to deliver it. Priming with saline and flushing after is another approach.
Is the flush volume part of the dose?
The flush carries no extra drug; it pushes the residual drug from the line into the patient. It does add to the total fluid given.
Sources
- BNF, guidance on prescribing
- 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
- 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
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
See also the formula sheet, the eight sample questions and the approved calculator page.