Pharmaceutical Calculations and Product Preparation
What this domain tests
Calculation stations test accuracy, unit discipline, and the ability to show your work clearly. Preparation stations test aseptic technique and the practical judgment around compounding and reconstitution. The examiner values a clean, checkable setup over a fast answer.
Core rules
- Write the equation with units before substituting numbers, and carry units through. Cancel units explicitly; if the units do not cancel to the target, the setup is wrong.
- Convert to a single unit system first. Common conversions: 1 g = 1000 mg, 1 mg = 1000 micrograms, 1 L = 1000 mL, 1% w/v = 1 g per 100 mL.
- For paediatric doses, calculate the dose per kilogram, then the volume, then compare against the maximum. State the maximum.
- For dilutions, use C1V1 = C2V2 and keep both concentrations in the same units.
- For compounding, record the formula, the lot numbers, the quantity prepared, and the beyond-use date.
- For aseptic preparation, maintain first-air and critical-site discipline, disinfect the septum, and avoid touching critical sites.
Worked example
A child weighing 14 kg is prescribed 15 mg/kg/dose of a suspension supplied as 250 mg/5 mL, to be given every 8 hours.
Reasoning: dose = 15 mg/kg x 14 kg = 210 mg per dose. Volume = 210 mg x (5 mL / 250 mg) = 210 x 0.02 = 4.2 mL per dose. Check the maximum: confirm the per-dose and per-day limits for the drug and compare 210 mg against them; if the calculated dose exceeds the maximum, contact the prescriber before dispensing. Daily total = 4.2 mL x 3 = 12.6 mL. State each step with units so the check is visible.
Traps that cost marks
- Mixing milligrams and micrograms, or grams and milligrams, mid-calculation.
- Decimal-point slips; recheck by estimating the answer's order of magnitude.
- Forgetting to compare the calculated dose against the maximum.
- Using the wrong concentration when a product comes in multiple strengths.
- Omitting the beyond-use date or lot numbers on a compounded preparation.