Example:

After an iv dose of 300 mg erythromycin, the plasma erythromycin conc was 4.2 mg/L after 2 hours and 0.74 mg/L after 6 hours of drug administration.
Calculate erythromycin CLT.

a- Plot the plasma conc-time data.

b- Draw the best line that goes through the samples.

c- Starting at any point on the line, find out the half life of the drug.

d- The Y-intercept is the plasma concentration at time zero (Cp0).

CLT = k Vd

Cp0

Vd =

dose

Cp0

t1/2

k =

0.639

t1/2

The answer:

Half life : 1.6hr (Graphically)
k =

0.693

t1/2 =

0.693

1.6 hr = 0.433 hr -1
Cp0 = 10 mg/L (Graphically)
Vd =

Dose

Cp0 =

300mg

10 mg/L = 30 L
CL T = K Vd
CL T = 0.433 hr -1 x 30 L = 13 L/hr
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