A patient receives a drug at a constant IV infusion rate (R0) of 50 mg/h. Its clearance is 10 L/h. Assuming first-order elimination, what is the steady-state plasma concentration (Css)?

• A 0.5 mg/L
• B 50 mg/L
• C 5 mg/L ✓
• D 500 mg/L

Explanation

At steady state: Css = R0 / CL = 50 mg/h / 10 L/h = 5 mg/L. This fundamental PK equation shows that steady-state concentration is determined solely by infusion rate and clearance (not by Vd or half-life). Doubling the infusion rate doubles Css; halving clearance (e.g., renal failure) doubles Css. The time to reach steady state is determined by half-life (approximately 4–5 half-lives), not by infusion rate.

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

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