Drugs are eliminated by the kidney to different extents. This can range from 100% (f = 1) when the drug is completely eliminated unchanged in urine to 0% (f = 0) when the drug is not excreted in urine at all.
The fraction of drug excreted unchanged in urine is determined from the magnitude of the relative to the .
Consider this group of drugs that are eliminated unchanged in urine to different extents (range from 0.95 to 0.1).
Assume that there is 50% decrease in kidney function and this will result in decreasing the of the drug by 50% without affecting the nonrenal clearance.
When the represents 10% of the and the decreases by 50%, the will decrease only by 5% .
On the other hand when the represents 95% of the and the decreases by 50%, the will decrease by 47.5%.
The pharmacokinetics of drugs that are eliminated unchanged in urine to a larger extent (i.e. larger f , or higher CLR relative to CLT) will be affected more by renal dysfunction.
