The PSA
Section 5 of 8
Calculation skills
Accurate dose, rate and unit calculations, conversions and infusion rates.
16 marks · 8% of the exam8 items × 2
What it tests
Free-text numeric answers. Careful arithmetic under time pressure — weight-based dosing, infusion-pump rates, drip rates and concentration conversions.
How to tackle it
- 1Write out the units and cancel them — most errors are unit conversions (g ↔ mg ↔ microgram)
- 2Weight-based dose = dose(mg/kg) × weight(kg)
- 3Infusion rate (mL/h) = volume(mL) ÷ time(h); drip rate (drops/min) = volume(mL) × drop factor ÷ time(min)
- 4Double-check the order of magnitude — does the answer make clinical sense?
Worked example
Give 1 L of saline over 8 hours through a standard giving set (20 drops/mL).
Answer
1000 × 20 ÷ 480 min ≈ 42 drops/min.
Pitfalls & high-yield
- Classic errors: mixing mg and micrograms, forgetting to convert hours→minutes, mis-reading concentration (e.g. 1:1000 = 1 mg/mL).
- Paediatric doses are weight-based — read the weight carefully.
How it’s marked
2 marks per item, 8 items = 16 of 200 marks. Standard-set per sitting (pass mark ≈ 60–63%, varies).