A drug is being administered by IV infusion at a rate of 60 mg/hr. Its half-life is 4 hours and volume of distribution is 20 L. What is the steady-state plasma concentration (Css)?

• A 4.33 mg/L
• B 17.3 mg/L ✓
• C 8.67 mg/L
• D 3.0 mg/L

Explanation

Css = Infusion rate / Clearance. First compute clearance: CL = 0.693 × Vd / t½ = 0.693 × 20 L / 4 hr = 3.465 L/hr. Then Css = Infusion rate / CL = 60 mg/hr / 3.465 L/hr ≈ 17.3 mg/L. At steady state, the rate of drug input equals the rate of elimination (Rate in = CL × Css), so this formula applies directly. A larger volume of distribution prolongs half-life and lowers clearance at a given t½, resulting in higher Css for the same infusion rate.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.