## absorption / elimination [PK / PD]

Hi VSP,

» Can we define the absorption phase and elimination phase with concentration time data/Plot?

There is no “absorption/elimination phase”. Both absorption / elimination start almost immediately with dosing. See here (ka > kel) and there (ka = kel; flip-flop PK).

As long as drug is released from the formulation and dissolved, absorption continues until
• the formulation leaves the GIT (hence it does not make sense to sample more than 72 hours for an IR formulation) or
• the formulation proceeded in the GIT beyond the absorption window of the drug.
If you are interested in λz (as an estimate of kel) consider the TTT-method (see this post). At the inflection point of the profile (2×tmax) absorption is practically (!) complete (i.e., 93.75%).

 i    % abs. 0.5   50.00 1     75.00 1.5   87.50 2     93.75 2.5   96.875 3     98.4375 3.5   99.21875 4     99.609375

where i is the multiplier of tmax. Once you have an estimate of elimination you can obtain absorption f.i. by the method of residuals or by Wagner-Nelson’s.

Dif-tor heh smusma 🖖
Helmut Schütz

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