PSI Pharmaceutical Calculations 3

Question 1

A drug has a half-life of 6 hours. What fraction of the original dose remains in the body after 24 hours?

Accepted Answer

1/16

Answer

24 hours equals 4 half-lives; (1/2)^4 = 1/16 of the original dose remains.

Question 2

A patient has a creatinine clearance of 30 mL/min (normal = 100 mL/min). A renally cleared drug is normally dosed every 8 hours. What is the appropriate adjusted dosing interval?

Accepted Answer

Approximately every 27 hours

Answer

Adjusted interval = normal interval × (normal CrCl ÷ patient CrCl) = 8 × (100/30) ≈ 27 hours.

Question 3

An aminoglycoside is to be dosed to achieve a peak concentration of 8 mg/L. The patient weighs 60 kg and the volume of distribution is 0.25 L/kg. What loading dose is required?

Accepted Answer

120 mg

Answer

Loading dose = target Cp × Vd = 8 mg/L × (0.25 L/kg × 60 kg) = 8 × 15 = 120 mg.

Question 4

A 500 mL bag of 5% dextrose for infusion contains how many grams of dextrose?

Accepted Answer

25 g

Answer

5% w/v = 5 g per 100 mL; 500 mL contains 5 × 5 = 25 g of dextrose.

Question 5

A topical preparation contains a drug at 0.1% w/v. How many milligrams of drug are present in 20 mL of the preparation?

Accepted Answer

20 mg

Answer

0.1% w/v = 0.1 g/100 mL = 1 mg/mL; 1 mg/mL × 20 mL = 20 mg.

PSI Pharmacy Practice Test

PSI Pharmacy Practice Test

PSI Pharmaceutical Calculations 3