Medical Calculations and Pharmacology
Medical Calculations and Pharmacology
This domain is the calculation core of the exam: mg/kg dosing, dilutions, constant-rate infusions, fluid rates, and unit conversions, plus pharmacokinetics and drug classes.
Core rules
- Dose in mg = body weight (kg) × dose (mg/kg). Volume in mL = dose in mg ÷ concentration (mg/mL).
- CRI: total daily dose ÷ 24 gives dose per hour; divide by concentration to get mL/hr.
- Conversions: 1 kg = 2.2 lb; 1 g = 1,000 mg; 1 L = 1,000 mL; a 1% solution = 10 mg/mL.
- Half-life determines dosing interval, and steady state is reached after roughly 5 half-lives.
- First-pass hepatic metabolism lowers oral bioavailability; IV or transdermal routes avoid it.
Worked example
A 20 kg dog is prescribed 5 mg/kg of a drug supplied as 50 mg/mL. Dose = 20 × 5 = 100 mg. Volume = 100 ÷ 50 = 2 mL. If the order is q12h for 7 days, the total dispensed volume is 2 mL × 2 doses × 7 days = 28 mL, plus a small overage for accuracy.
Traps
- Rounding the dose before dividing by concentration.
- Confusing mg/kg with mg/lb, which overdoses by a factor of 2.2.
- Reading a 10% solution as 10 mg/mL instead of 100 mg/mL.
- Forgetting that insulin and some drugs are dosed in units, not milligrams.