Pharmaceutical Calculations and Compounding
Core concepts
Accuracy and unit discipline are everything here. Know ratio strength, percentage concentrations (%w/w, %w/v, %v/v), dilution (C1V1 = C2V2), alligation, milliequivalents, and paediatric weight-based dosing. Understand compounding fundamentals: clean technique, weighing and measuring limits, the minimum weighable quantity, and the use of aliquot methods for small quantities. Know isotonicity concepts and sodium chloride equivalent calculations. Reconstitution and dilution of injectables, infusion rate calculations (mL/h, drops/min), and body surface area dosing are commonly tested.
Traps in questions
- Mixing %w/w and %w/v without accounting for density.
- Using total volume instead of solvent volume in dilution problems.
- Forgetting that a 1:1000 solution is 0.1%, not 1%.
- Rounding at intermediate steps rather than the end.
Worked example / rule of thumb
Prepare 500 mL of a 0.9% sodium chloride solution from a 20% stock. Required mass = 0.9% × 500 = 4.5 g. Volume of stock = 4.5 g / 0.20 = 22.5 mL, made up to 500 mL. Rule of thumb: convert every percentage to a fraction of 100 first, keep units consistent, and check that the answer is physically sensible.
Public Health, Health Promotion and Patient Counseling
Core concepts
This domain connects pharmacy to populations and people. Know the levels of prevention (primary, secondary, tertiary), vaccination principles and cold chain, screening concepts (sensitivity, specificity, positive predictive value), and health behaviour models at a conceptual level. Understand health literacy, teach-back, and plain-language communication. Counselling should cover the medicine's name and purpose, how and when to take it, expected benefits and side effects, what to do if a dose is missed, storage, and when to seek help. Understand antimicrobial stewardship and the pharmacist's role in promoting adherence and lifestyle change.
Traps in questions
- Confusing sensitivity (detects true cases) with positive predictive value (probability a positive result is true).
- Counselling in technical jargon and assuming understanding.
- Omitting the "what to do if you miss a dose" element.
- Equating adherence with obedience rather than shared decision-making.
Worked example / rule of thumb
A screening test has 90% sensitivity and 95% specificity in a population with 1% prevalence. Of 10,000 people, 100 have disease (90 detected), and 9,900 do not (495 false positives). PPV ≈ 90/(90+495) ≈ 15% - a positive result is far from a diagnosis. Rule of thumb: use teach-back ("tell me how you will take this") rather than "do you understand?" to confirm counselling.