Pharmaceutical Calculations Flashcards

A patient requires an IV infusion of dopamine at 5 mcg/kg/min. The patient weighs 80 kg. The dopamine concentration available is 400 mg in 250 mL D5W. What is the infusion rate in mL/hr?

Answer: 15 mL/hr

Dose = 5 mcg/kg/min × 80 kg = 400 mcg/min = 24,000 mcg/hr = 24 mg/hr; concentration = 400 mg/250 mL = 1.6 mg/mL; rate = 24 mg/hr ÷ 1.6 mg/mL = 15 mL/hr.

A prescription calls for 2% hydrocortisone cream. You have 1% and 5% hydrocortisone creams in stock. Using the alligation method, what ratio of 1% to 5% cream should be mixed?

Answer: 3 parts 1% : 1 part 5%

By alligation: 1% gives (5−2)=3 parts and 5% gives (2−1)=1 part, so the ratio of 1%:5% is 3:1.

A child weighing 22 kg needs amoxicillin 40 mg/kg/day in 3 divided doses. The suspension available is 250 mg/5 mL. How many mL per dose should the child receive?

Answer: 5.9 mL

Total daily dose = 40 mg/kg × 22 kg = 880 mg/day; per dose = 880 ÷ 3 ≈ 293 mg; volume = (293 mg ÷ 250 mg) × 5 mL ≈ 5.9 mL.

How many grams of dextrose are contained in 500 mL of D10W?

Answer: 50 g

D10W means 10 g dextrose per 100 mL; 500 mL × (10 g/100 mL) = 50 g.

A pharmacist needs to prepare 240 mL of a 0.9% NaCl solution using water for injection and a 23.4% NaCl concentrate. How many mL of the 23.4% concentrate is needed?

Answer: 9.2 mL

Using C1V1 = C2V2: 23.4% × V1 = 0.9% × 240 mL; V1 = 216/23.4 ≈ 9.2 mL.

A heparin infusion is ordered at 1,200 units/hr. The bag contains 25,000 units in 250 mL NS. What is the infusion rate in mL/hr?

Answer: 12 mL/hr

Concentration = 25,000 units ÷ 250 mL = 100 units/mL; rate = 1,200 units/hr ÷ 100 units/mL = 12 mL/hr.

A prescription reads: 'Take 1 tablespoon three times daily for 10 days.' How many mL of medication must be dispensed to fill this prescription exactly?

Answer: 450 mL

1 tablespoon = 15 mL; 15 mL × 3 doses/day × 10 days = 450 mL.