Pharmaceutical Calculations Flashcards

A medication has a half-life of 6 hours. If a patient receives a 500 mg dose, approximately how much drug remains in the body after 24 hours?

Answer: 31.25 mg

24 hours = 4 half-lives; 500 × (1/2)^4 = 500 ÷ 16 = 31.25 mg.

A total parenteral nutrition (TPN) bag contains 500 mL of 50% dextrose and 500 mL of 10% amino acids. What is the final dextrose concentration in the combined 1000 mL solution?

Answer: 25%

Dextrose content = 500 mL × 0.50 = 250 g; final concentration = 250 g ÷ 1000 mL = 0.25 = 25%.

A physician orders vancomycin 15 mg/kg IV every 12 hours for a 70 kg patient. The pharmacy has vancomycin 500 mg/10 mL vials. How many mL are needed per dose?

Answer: 21 mL

Dose = 15 mg/kg × 70 kg = 1050 mg; concentration = 500 mg/10 mL = 50 mg/mL; volume = 1050 ÷ 50 = 21 mL.

An oral solution contains 125 mg of active ingredient per 5 mL. A patient is prescribed 0.375 g twice daily. How many mL should the patient take per dose?

Answer: 15 mL

0.375 g = 375 mg; volume = (375 mg ÷ 125 mg) × 5 mL = 15 mL per dose.

A pharmacy receives an order for potassium chloride 20 mEq IV in 100 mL NS. KCl is available as 2 mEq/mL. How many mL of KCl solution should be added to the bag?

Answer: 10 mL

Volume = 20 mEq ÷ 2 mEq/mL = 10 mL.

A 1:200 w/v solution contains how many mg of drug per mL?

Answer: 5 mg/mL

1:200 w/v = 1 g per 200 mL = 1000 mg ÷ 200 mL = 5 mg/mL.

A patient needs phenytoin 5 mg/kg/day in 2 divided doses. The patient weighs 154 lbs. Phenytoin suspension is 125 mg/5 mL. How many mL per dose?

Answer: 7 mL

Weight = 154 lbs ÷ 2.2 = 70 kg; daily dose = 5 × 70 = 350 mg; per dose = 175 mg; volume = (175/125) × 5 = 7 mL.