Example: Oxytetracycline is a broad spectrum antibiotic used in the treatment of a variety of infections. It has a half life of 9 hr and Vd of 50 L. The average oxytetracylcine dose in patients with normal kidney function is 500 mg q 8 hr. What is the dose required in a patient with only 20% of the normal kidney function if 70% of oxytetracycline dose is eliminated unchanged in urine ?

Kidney function
The decrease in kidney function causes slower
rate of orytetracycline elimination and higher
steady state conc. Lower dose is needed in
patients with reduced kidney function.
f = 0.7
KF = 0.2

Answer:

Dosefailure = Dosenormal [ 1-f(1-KF) ]
Dosefailure = 500 mg [ 1-0.7(1-0.2) ]
Dosefailure = 220 mg q 8 hr
Kidney function
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