Wattle Pharmacy Prep

Pharmacokinetic calculations
Practice questions with explanations

Pharmacokinetic calculations belongs to Pharmacokinetics and pharmacodynamics, which makes up about 10% of the Overseas Pharmacist Readiness Assessment (OPRA®). Answer each question yourself before opening the explanation.

1. A patient is switched from oral morphine 30 mg twice daily to a 24-hour subcutaneous infusion using a 2 to 1 conversion. What is the infusion dose over 24 hours?

  1. 60 mg
  2. 30 mg
  3. 120 mg
  4. 15 mg
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B. 30 mg

The total oral dose is 60 mg over 24 hours, and dividing by two for the parenteral route gives 30 mg subcutaneously. Conversion ratios vary between references, and the calculated dose is often reduced further for incomplete cross-tolerance and reviewed with breakthrough doses available at about one sixth of the daily total.

2. A drug has a therapeutic range of 5 to 15 mg/L and a half-life of 24 hours. What dosing interval is most appropriate?

  1. Every 4 hours
  2. Every week
  3. Once daily, since one half-life between doses produces a threefold peak to trough range that fits within the therapeutic range
  4. Every 72 hours
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C. Once daily, since one half-life between doses produces a threefold peak to trough range that fits within the therapeutic range

With a half-life of 24 hours, daily dosing gives a twofold variation, which sits comfortably within a threefold range. Longer intervals would let the trough fall below the effective concentration, and much shorter ones add inconvenience without benefit, which is why dosing intervals commonly approximate the half-life.

3. A drug has a clearance of 6 L/h and an oral bioavailability of 0.8. What oral dose given every 12 hours maintains an average steady-state concentration of 10 mg/L?

  1. 60 mg
  2. 900 mg
  3. 720 mg
  4. 576 mg
Show answer

B. 900 mg

Maintenance dose = CL x Css x tau / F = 6 L/h x 10 mg/L x 12 h / 0.8 = 720 / 0.8 = 900 mg. The bioavailability correction increases the oral dose, because only 80% of what is swallowed reaches the circulation. Forgetting to divide by F gives 720 mg and underdoses the patient by 20%.

These are 3 of the 60 questions on pharmacokinetic calculations in the app.

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More topics in Pharmacokinetics and pharmacodynamics

Absorption and bioavailability · Distribution and protein binding · Excretion and clearance · Metabolism and cytochrome P450 · Receptor pharmacodynamics · Dosing in special populations