What is the impact of renal replacement therapy (dialysis) on drug PK?

• A. Dialysis does not affect PK
• B. Dialysis can remove drugs from plasma; may require dose adjustments
• C. Dialysis always removes all drugs
• D. Dialysis reduces bioavailability

Answer: (B)

Renal replacement therapy can directly remove drugs from the bloodstream during treatment, changing how much drug is available in the body over time. The amount removed depends on properties of the drug and the dialysis setup: small molecules that are not tightly bound to plasma proteins, with a relatively small volume of distribution and good water solubility are more readily cleared by dialysis. Because the drug is being removed in addition to the patient’s remaining kidney function, the total body clearance can increase, which can lower drug exposure if the dose isn’t adjusted. That’s why dose adjustments or timing changes are often needed when a patient is on dialysis—some drugs may require a larger or more frequent dose, or a post-dialysis supplemental dose to maintain therapeutic levels.

Drugs that are highly protein-bound, have a large volume of distribution, or distribute mainly into tissues are not significantly removed by dialysis, so their pharmacokinetics are less affected. Also, dialysis doesn’t change the inherent absorption of an oral dose into the systemic circulation (bioavailability)—it can only remove drug from the circulating plasma once it’s in the bloodstream.