Saudi Prometric Pharmacist Pharmacokinetics and Drug Calculations 2 — Questions and Answers
Question 1: A pharmacist is calculating a loading dose for lidocaine. The target plasma concentration is 2 mg/L, the Vd is 1.1 L/kg, and the patient weighs 70 kg. What is the loading dose?
- 154 mg (Correct answer)
- 110 mg
- 22 mg
- 1,540 mg
Correct answer: 154 mg
Vd (total) = 1.1 L/kg × 70 kg = 77 L. Loading dose = Target Cp × Vd = 2 mg/L × 77 L = 154 mg. This is the standard loading dose calculation. For IV lidocaine, this is typically given as 1–1.5 mg/kg (1 mg/kg × 70 = 70–100 mg, reflecting the fact that Vd decreases during distribution phase for lidocaine). The formula result is 154 mg.
Question 2: A maintenance infusion is required to maintain a steady-state plasma concentration of 15 mg/L for a drug with a clearance of 8 L/hr. What infusion rate (Ro) is needed?
- 120 mg/hr (Correct answer)
- 1.875 mg/hr
- 0.53 mg/hr
- 960 mg/hr
Correct answer: 120 mg/hr
At steady state: Infusion rate (Ro) = Target Css × Clearance = 15 mg/L × 8 L/hr = 120 mg/hr. This fundamental equation shows that steady-state concentration is determined by the ratio of infusion rate to clearance. Doubling clearance (e.g., enzyme induction) would require doubling the infusion rate to maintain the same Css.
Question 3: An IV infusion of 500 mg drug is administered over 2 hours. The drug has a clearance of 10 L/hr and a half-life of 6 hours. What is the Vd?
- 86.6 L (Correct answer)
- 60 L
- 100 L
- 20 L
Correct answer: 86.6 L
Vd = (CL × t½) / 0.693 = (10 L/hr × 6 hr) / 0.693 = 60 / 0.693 = 86.6 L. This derives from the relationship: t½ = (0.693 × Vd) / CL, rearranged to Vd = (CL × t½) / 0.693.
Question 4: A pharmacist needs to calculate the AUC (area under the concentration-time curve) for a drug given at 200 mg IV bolus. The clearance is 20 L/hr. What is the AUC?
- 10 mg·hr/L (Correct answer)
- 4,000 mg·hr/L
- 200 mg·hr/L
- 0.1 mg·hr/L
Correct answer: 10 mg·hr/L
AUC = Dose / Clearance = 200 mg / 20 L/hr = 10 mg·hr/L. The AUC represents total systemic drug exposure. Clearance is calculated from AUC: CL = Dose/AUC. This relationship is used in bioequivalence studies where AUC is measured and compared between formulations.
Question 5: A patient receives gentamicin and a peak concentration of 8 mg/L is measured 1 hour after a 30-minute IV infusion. The trough is 1 mg/L at 8 hours post-infusion. What is the approximate half-life?
- 2.3 hours (Correct answer)
- 4 hours
- 8 hours
- 1 hour
Correct answer: 2.3 hours
From peak to trough: 8 mg/L → 1 mg/L over a 7-hour period. This represents 3 half-lives (8→4→2→1 mg/L). Half-life = 7 hours / 3 = approximately 2.33 hours. Normal gentamicin t½ is 2–3 hours in patients with normal renal function.
Question 6: What does a two-compartment pharmacokinetic model describe that a one-compartment model does not?
- A rapid distribution phase (alpha phase) as drug distributes from blood to tissues, followed by a slower elimination phase (beta phase) (Correct answer)
- A single exponential decline in plasma concentration
- Nonlinear kinetics at high concentrations
- Only renal elimination without hepatic metabolism
Correct answer: A rapid distribution phase (alpha phase) as drug distributes from blood to tissues, followed by a slower elimination phase (beta phase)
In a two-compartment model, after IV bolus there is an initial rapid decline (alpha/distribution phase) as drug rapidly distributes into peripheral tissues, followed by a slower monoexponential decline (beta/elimination phase) as drug is cleared. This creates a bi-exponential plasma concentration curve. Many drugs (digoxin, aminoglycosides) follow two-compartment kinetics.
Saudi Prometric Pharmacist Pharmacokinetics and Drug Calculations 2
A pharmacist is calculating a loading dose for lidocaine.
The target plasma concentration is 2 mg/L, the Vd is 1.1 L/kg, and the patient weighs 70 kg.
What is the loading dose?