Dosing adjustment in renal impairment: Which approach helps maintain Css?

• A. Decrease dose or extend interval to maintain Css
• B. Increase dose
• C. Stop drug
• D. Switch to IV

Answer: (A)

When a drug is given repeatedly, its steady-state concentration (Css) is governed by the balance between how much drug is entering the body per time and how quickly the body clears it. In renal impairment, clearance drops, so keeping the same dosing pattern without adjustment raises Css because the drug is eliminated more slowly.

Css is proportional to the dosing rate divided by clearance (and also depends on bioavailability and dosing interval). If clearance decreases, you need to reduce the dosing rate to keep Css the same. This can be done by lowering the dose or by extending the dosing interval (spacing out doses further). For example, if clearance halves, you could either halve the dose with the same interval or keep the same dose but double the interval.

That’s why decreasing the dose or extending the interval is the appropriate way to maintain Css in renal impairment. Increasing the dose would push Css higher, stopping the drug would eliminate Css, and switching to IV doesn’t inherently fix the Css without adjusting the dosing rate for the new clearance.