Pharmacokinetics and Drug Calculations Flashcards

A pharmacist receives a prescription: 'Theophylline infusion 0.5 mg/kg/hr for a 60 kg patient. Available solution: 500 mg in 500 mL.' What is the infusion rate in mL/hr?

Answer: 30 mL/hr

Dose per hour = 0.5 mg/kg/hr × 60 kg = 30 mg/hr. Concentration = 500 mg / 500 mL = 1 mg/mL. Rate = 30 mg/hr ÷ 1 mg/mL = 30 mL/hr.

A patient weighing 70 kg requires a gentamicin dose of 7 mg/kg once daily. The available vial is 80 mg/2 mL (40 mg/mL). How many mL should be drawn up?

Answer: 12.25 mL

Total dose = 7 mg/kg × 70 kg = 490 mg. Volume = 490 mg ÷ 40 mg/mL = 12.25 mL. This volume would typically be further diluted in 50–100 mL of normal saline for IV infusion over 30–60 minutes.

A continuous infusion of norepinephrine is ordered at 0.1 mcg/kg/min for a patient weighing 75 kg. The bag contains 8 mg in 250 mL of D5W. What is the infusion rate in mL/hr?

Answer: 14.1 mL/hr

Dose per minute = 0.1 mcg/kg/min × 75 kg = 7.5 mcg/min. Concentration = 8 mg/250 mL = 0.032 mg/mL = 32 mcg/mL. Rate = 7.5 mcg/min ÷ 32 mcg/mL = 0.234 mL/min × 60 = 14.1 mL/hr.

A parenteral nutrition bag contains the following per day: dextrose 300 g, amino acids 70 g, lipids 50 g. What is the total caloric content?

Answer: 2,170 kcal

Dextrose: 300 g × 3.4 kcal/g = 1,020 kcal. Amino acids: 70 g × 4 kcal/g = 280 kcal. Lipids: 50 g × 9 kcal/g = 450 kcal. Total = 1,020 + 280 + 450 = 1,750 kcal. Note: IV dextrose provides 3.4 kcal/g (not 4 kcal/g as for oral carbohydrates) because it is the monohydrate form.

A pharmacist is adjusting vancomycin using Bayesian PK software. The measured trough is 8 mg/L (target 15–20 mg/L for serious MRSA infection). The current dose is 1,000 mg q12h. What initial dose adjustment is appropriate?

Answer: Increase dose to approximately 1,500 mg q12h or add a loading dose — trough significantly below target

A trough of 8 mg/L is significantly below the target of 15–20 mg/L for serious MRSA infections (e.g., bacteremia, endocarditis). Increasing the dose is necessary. AUC/MIC-guided dosing (target AUC/MIC 400–600) is the current standard, but trough-based adjustment is still used in some settings. A substantial dose increase is required.

A pharmacist calculates that a patient needs a daily dose of 450 mg of an extended-release tablet. The available tablets are 150 mg. How many tablets per dose if given twice daily?

Answer: 1.5 tablets — consider rounding to 2 tablets (300 mg) twice daily or 150 mg in the morning and 300 mg in the evening

450 mg/day ÷ 2 doses = 225 mg per dose. 225 mg ÷ 150 mg/tablet = 1.5 tablets. Since tablets should not usually be split for extended-release formulations, the pharmacist should discuss with the prescriber: options include 150 mg in the morning + 300 mg (2 tabs) in the evening = 450 mg/day, or rounding to 300 mg BD = 600 mg/day with monitoring.