The Fraction of the Dose Excreted Unchanged in Urine

The fraction of the iv dose excreted unchanged in urine is equal to:

Fraction =

CLR

CLT
=

ke

k

When no drug is excreted in urine, the renal clearance and ke are equal to zero.

When all the dose is excreted in urine, the renal clearance is equal to the total body clearance and ke is equal to k.

After oral (extravascular) administration:

Aeāˆž =

CLR

CLT
F Dose =

ke

k
F Dose

and

Aeāˆž

F Dose
=

CLR

CLT
=

ke

k

This means that after oral administration, the fraction of the bioavailable dose (FD) excreted in urine is determined from the ratio of renal clearance to CLT or the ratio of ke to k.

This means that the fraction of the bioavailable dose excreted unchanged in urine is constant for the same drug in the same patient.

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