PHARM/PCS V5 · Procedure 29
Pharmacokinetics IV: Plasma Concentration-Time Data
From an oral dose's plasma profile, get the absorption and elimination rate constants, the peak time, AUC, and F/Vd, by the method of residuals.
What it computes
Two-compartment model for oral (or other absorptive) administration. A multi-step algorithm: (1) fit the elimination tail to get Ke, (2) compute residuals Cp′ − Cp and fit their log to get Ka, (3) compute AUC by trapezoidal rule + tail extrapolation, and (4) derive F/Vd and the full model equation.
Inputs and outputs
You provide
- The oral dose
- Time and plasma concentration, starting at time 0
- How many sampling periods after the peak to begin the elimination fit (default 1)
PHARM/PCS reports
- Ke from the elimination tail and Ka from the residuals
- Time to peak
- AUC, by the trapezoidal rule with the tail extrapolated
- F/Vd and the fitted model equation
- A plasma concentration-time plot
When to use it
For oral or other absorbed doses when you want absorption and elimination rate constants and bioavailability-related terms from a plasma profile. It builds on AUC (Procedure 25) and the exponential-fitting logic behind Procedures 24 and 28.
Reading the results
The method of residuals assumes absorption is faster than elimination (Ka > Ke); when they're close, or reversed (flip-flop kinetics), the two constants can swap identities, so sanity-check that your fitted Ka really is the larger one. If your last sample is early, the extrapolated AUC tail can dominate F/Vd, so watch how much area comes from extrapolation.
The whole analysis in one step
Elimination fit, residuals, absorption fit, AUC, and the model equation all run together. You still choose where the elimination tail begins, as in the 1986 program, which lets you test how sensitive Ke and Ka are to that choice.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.