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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

Determination of absorption rate constant (Ka)

Feathering method = method of residuals = curve stripping

 The method allows the separation of the monoexponential constituents of a

biexponential plot of plasma concentration against time.

Method of residuals

 The value of ka is obtained by the following procedure:

1. Plot the drug concentration versus time on semilog paper with the concentration

values on the logarithmic axis.
2. Obtain the slope of the terminal phase by extrapolation.
3. Take any points on the upper part of the line and drop vertically to obtain

corresponding points on the curve.
4. Read the concentration values and plot the values of the differences at the
corresponding time points.

5. Estimate the absorption rate constant from the slope (-ka/2.3) of the residual line.

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