Ansel's Pharmaceutical Calculations

What the book covers

10 high-yield topics. Each has the points most often examined and links to practise them.

1. 1Units and conversions

   • 1 g = 1,000 mg = 1,000,000 micrograms; write micrograms in full to avoid errors.
   • 1 kg = 2.2 lb; 1 fluid ounce is about 30 mL; 1 teaspoon is 5 mL.
   • Convert everything to the same unit before calculating.
   • Check if the answer is reasonable before choosing an option.
   Practise MCQs

2. 2Weight and body surface area dosing

   • Weight-based dose equals mg/kg × weight, then divide by the number of doses.
   • Body surface area (Mosteller) is the square root of height (cm) × weight (kg) divided by 3,600.
   • Chemotherapy is mostly dosed by body surface area.
   • Use ideal or adjusted body weight for some drugs in obesity, such as aminoglycosides.
   Practise MCQs

3. 3Percentage strength

   • w/v percent means grams per 100 mL; 5% dextrose is 5 g per 100 mL.
   • w/w percent is grams per 100 g, used for creams and ointments.
   • v/v percent is mL per 100 mL, used for alcohol solutions.
   • mg/mL × 0.1 gives the percent w/v.
   Practise MCQs

4. 4Ratio strength and ppm

   • 1:1,000 means 1 g in 1,000 mL, which is 1 mg/mL (adrenaline for IM use).
   • 1:10,000 adrenaline is 0.1 mg/mL.
   • Parts per million means mg per litre for dilute solutions.
   • Convert ratio to percent by dividing 100 by the ratio's second number.
   Practise MCQs

5. 5Dilution and concentration

   • C1 × V1 = C2 × V2 for simple dilutions.
   • Alligation mixes a strong and a weak preparation to make an intermediate strength.
   • Adding diluent changes concentration, not the amount of drug.
   • Stock solutions are made so a set volume gives the required dose.
   Practise MCQs

6. 6IV infusion rates

   • mL/hour equals total volume divided by hours.
   • Drops per minute equals volume (mL) × drop factor divided by minutes.
   • Dose-rate infusions: mL/hour = (dose per kg per minute × weight × 60) divided by concentration.
   • Standard drip sets give 20 drops/mL; microdrip sets give 60.
   Practise MCQsNotes: Sterile Compounding and IV Admixtures

7. 7Milliequivalents and osmolarity

   • mEq equals mg × valence divided by molecular weight.
   • Potassium chloride: about 13.4 mEq of potassium per gram.
   • Osmolarity adds the particles from each dissociated ion.
   • Normal saline is about 308 mOsm/L, close to plasma.
   Practise MCQs

8. 8Isotonicity

   • Eye drops and injections should be close to isotonic with body fluids.
   • Sodium chloride equivalents (E values) show how much NaCl a drug is worth.
   • 0.9% NaCl is isotonic; the freezing point method uses a depression of 0.52 °C.
   • Hypertonic solutions draw water out of cells; hypotonic ones cause swelling.
   Practise MCQsNotes: Sterile Products

9. 9Creatinine clearance and renal doses

   • Cockcroft-Gault: (140 − age) × weight divided by (72 × serum creatinine in mg/dL), × 0.85 for women.
   • Convert micromol/L creatinine to mg/dL by dividing by 88.4.
   • Use the result to pick the dose band in the product information.
   • Dosing interval can be extended instead of lowering each dose.
   Practise MCQsNotes: Therapeutic Drug MonitoringCheat sheet

10. 10Reconstitution and displacement

    • Powder in a vial takes up volume, so final volume is diluent plus displacement volume.
    • Ignoring displacement gives a weaker solution than intended, which matters most for small paediatric doses.
    • Final concentration equals the vial content divided by the final volume, not the diluent volume.
    • Check the product information for the stated displacement value and reconstitution volume.
    Practise MCQsNotes: Sterile Compounding and IV Admixtures