A drug is administered by continuous intravenous infusion at a rate of 4 mg/h. Its total body clearance is 2 L/h. Assuming first-order kinetics, what steady-state plasma concentration will be achieved?

• A 0.5 mg/L
• B 2 mg/L ✓
• C 8 mg/L
• D 16 mg/L

Explanation

At steady state, the rate of input equals the rate of elimination, so Css equals infusion rate divided by clearance: 4 mg/h divided by 2 L/h = 2 mg/L. This relationship also underlies maintenance dosing, where dose rate equals target Css multiplied by clearance (adjusted for bioavailability with oral dosing). Option C comes from multiplying rather than dividing, and option A from inverting the ratio.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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