Example : Cefazoline is a first generation cephalosporin with activity against most Gram-positive aerobic organisms. A patient is receiving 500 mg q 8 hr cefazoline administered as a short infusion of 0.5 hr duration. Calculate the maximum and minimum cefazoline conc at steady state. (Cefazoline Vd is 12 L and k is 0.35 hr-1)

Cpmax ss =

K0( 1 - e-kt' )

k Vd( 1 - e-kτ )
Cp min ss = Cp max ss e-k(τ-t')
Dose = 500 mg
t' = 0.5 hr τ = 8 hr
Ko = Dose/t' = 1000 mg/hr
k = 0.35 hr-1 Vd = 30 L

Answer :

Maximum conc :

Cp max ss =

1000 mg/hr (1 - e-0.35 hr -1 0.5 hr)

0.35 hr-1 12L(1 - e-0.35 hr-1 8 hr ) = 40.7 mg/L

Minimum conc :

Cp min ss = 40.7 mg/L e-0.35 hr -1 (8hr - 0.5 hr) = 2.95 mg/L
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