Pharmaceutical Calculations and Measurement
What the exam tests
This domain tests safe calculation: metric conversion, percentage concentration, dilution, dose by body weight, divided doses, IV flow rates, and quantity to dispense. The exam rewards showing units and checking plausibility, not just producing a number.
Core rules
- Convert to one unit system before calculating. 1 g = 1000 mg; 1 mg = 1000 micrograms; 1 L = 1000 mL.
- For percentage solutions, % w/v means grams per 100 mL. Thus 0.9% sodium chloride contains 0.9 g per 100 mL.
- Dose by weight: calculate total daily dose, divide by number of doses, then compare with the maximum. Pediatric doses are usually per kg per dose or per kg per day; read which one.
- Quantity to dispense: dose per day × days, then divide by units per container and round up to a whole pack.
- IV drip rate: drops per minute = (volume in mL × drop factor) ÷ time in minutes.
Worked example
A child weighs 12 kg. The order is 15 mg/kg/day divided every 8 hours. Total daily dose = 12 × 15 = 180 mg/day. Three doses per day means 180 ÷ 3 = 60 mg per dose. If the oral suspension is 250 mg/5 mL, each dose is 60 ÷ 250 × 5 = 1.2 mL. Check that 1.2 mL is measurable with the supplied syringe and that 60 mg does not exceed the maximum single dose.
Traps that cost marks
Mixing milligrams and micrograms; treating % w/v as % v/v; dividing by the wrong number of doses; rounding a dose down when a measurable volume is needed; and forgetting to check maximum dose, renal adjustment, or measurement device.