Pharmacology, Calculations, and Preparation Verification
Calculations
Candidates must handle dilution, concentration, ratio/proportion, alligation, and dosing calculations. Know how to convert units (mg to g, mL to L), calculate percentage concentrations, and use the formula: desired concentration × desired volume = stock concentration × stock volume. Understand milliequivalents, osmolarity, and flow-rate calculations where relevant.
Preparation Verification
Know the importance of verifying the drug, dose, base solution, and final volume before compounding, and of a second-person check for high-risk or high-alert medications. Understand compatibility, stability, and pH considerations.
Traps
Common errors: confusing percentage strength (w/v vs v/v), forgetting to account for the volume of added drug when calculating final volume, and misplacing decimal points. Another trap is answering with the amount of stock solution rather than the amount of diluent.
Worked Example
To prepare 500 mL of a 1:5000 solution from a 1:1000 stock: 1:5000 means 1 g per 5000 mL, or 0.2 g per 1000 mL. Required drug = 500 mL × (1 g/5000 mL) = 0.1 g. Stock concentration is 1 g/1000 mL, so stock volume = 0.1 g ÷ (1 g/1000 mL) = 100 mL. Diluent = 500 − 100 = 400 mL.