Example: Which of the following three antibiotics will require the largest reduction in dose in a patient whose kidney function is only 20% of normal function ?

a- Tobramycin (f = 0.98, half life 2.2 hr, Vd =16 L, and the dose is 150 mg q 8 hr)

b- Chloramphenicol (f = 0.05, half life 2.7 hr, Vd=60L, and the dose is 1 gm q 6 hr)

c- Ciprofloxacin (f = 0.5, half life = 4.2 hr, Vd = 120 L, and the dose is 500 mg q 12 hr)

Tobramycin (f = 0.98) has the largest difference in the steady state conc between normal and kidney failure patients.

Chloramphenicol (f = 0.05) steady state conc is not different in normal and kidney failure patients.

Ciproflosacin (f = 0.5) steady state conc is slightly different in normal and kidney failure patients.

Answer

The pharmacokinetics of drugs with larger fraction excreted unchanged in urine, will be affected more by the decrease in kidney function.

Tobramycin which is primarily excreted in urine (f = 0.98) will require the largest reduction of dose in patients with kidney failure.