Saudi Prometric Pharmacist Pharmacokinetics and Drug Calculations 2

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?

Accepted Answer

154 mg

Answer

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?

Accepted Answer

120 mg/hr

Answer

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?

Accepted Answer

86.6 L

Answer

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?

Accepted Answer

10 mg·hr/L

Answer

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?

Accepted Answer

2.3 hours

Answer

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.

Saudi Prometric Pharmacist Practice Test

Saudi Prometric Pharmacist Practice Test

Saudi Prometric Pharmacist Pharmacokinetics and Drug Calculations 2