For intermittent IV infusion dosing, which equation correctly defines Css,avg?

• A. Css,avg = (F × Dose) / (CL × τ)
• B. Css,avg = (F × τ) / (CL × Dose)
• C. Css,avg = Dose / (F × CL × τ)
• D. Css,avg = (F × Dose × τ) / CL

Answer: (A)

The key idea is that the average steady-state concentration during intermittent IV infusion is set by the rate at which drug enters the system, divided by how quickly the body clears it. The input rate for each dosing interval is the amount given per dose divided by the infusion interval, scaled by bioavailability: r_in = F × Dose / τ. The average concentration then equals this input rate divided by clearance: Css,avg = r_in / CL = (F × Dose) / (CL × τ). When the route is IV (F = 1), this simplifies to Dose / (CL × τ). That’s why this form correctly captures how dosing amount, dosing interval, and clearance determine the average concentration over time. If you try to place τ in the numerator or swap Dose and τ, you’d end up with the wrong units and misrepresent the rate entering the body, which is why those alternatives aren’t correct.