Pharmaceutical Calculations, Compounding and Product Preparation
What this domain tests
Pharmaceutical calculations and compounding test accuracy under time pressure: unit conversion, ratio/proportion, dilution and alligation, percentage concentrations, milliequivalents, osmolarity, IV flow rates, creatinine clearance, pediatric weight-based dosing, and compounding calculations such as displacement volume, aliquot method and capsule filling.
Core rules
- Always convert to one unit system before calculating; write the equation with units and cancel them.
- Percentage concentrations: % w/v = g per 100 mL; % w/w = g per 100 g; % v/v = mL per 100 mL. They are not interchangeable without density.
- Dilution: C1V1 = C2V2 for the solute; the diluent volume is final volume minus stock volume.
- Alligation alternate gives parts of each strength; convert parts to grams or milliliters using the total quantity.
- Displacement volume: the volume occupied by the drug powder must be subtracted from the vehicle to reach the final volume.
- Creatinine clearance (Cockcroft–Gault): CrCl = [(140 − age) × weight (kg)] / (0.814 × SCr (µmol/L)), ×0.85 for females. Confirm the equation and units used by your program.
Worked example
Prepare 120 g of 2% hydrocortisone cream from 1% and 5% creams. Alligation: parts of 1% = 5 − 2 = 3; parts of 5% = 2 − 1 = 1; total parts = 4. Each part = 120 g / 4 = 30 g. So use 90 g of 1% and 30 g of 5%. Check: 90 g × 0.01 = 0.9 g; 30 g × 0.05 = 1.5 g; total 2.4 g in 120 g = 2% w/w. If the prescription were 2% w/w and the creams were labeled w/w, no density correction is needed; if one were w/v, density would be required.
Traps
Do not confuse mg and µg (a 1000-fold error); do not round intermediate values too early; do not ignore displacement volume in suppositories or IV admixtures; do not use alligation parts as absolute amounts; do not assume 1 mL = 1 g for all liquids; do not forget significant figures for a balance that reads to 0.1 mg.