b- If the patient is receiving aminoglycoside
but steady state is not achieved:

One plasma sample should be obtained just before drug administration. Then after administration of the new dose, three plasma samples are obtained over a period of time equivalent to the estimated half life.

i- The half life can be determined graphically

Or the elimination rate constant can be determined from the slope of the line.

slope= -k/2.303
t 1/2
Cpx
Cp =

K0

k Vd ( 1 - e-kt' )+ Cpx e-kt'

ii- The line that goes through the samples obtained after drug administration is back extrapolated to determine the plasma conc at the end of drug administration.

iii-The Vd can be determined from the expression that describes the plasma conc at the end of drug administration because all the other parameters are known

Cpx is the plasma conc obtained just before drug administration.

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