PHARM/PCS V5 · Procedure 28
Pharmacokinetics III: Volume of Distribution
Estimate the apparent volume of distribution from an IV bolus concentration-time profile, by back-extrapolating the elimination phase.
What it computes
After IV bolus administration, the plasma concentration shows an initial rapid drop (distribution phase) then a prolonged exponential decay (elimination). Linear regression of ln(Cp) vs. time over the elimination phase, extrapolated to t=0, gives C0; then Vd = Dose / C0.
Inputs and outputs
You provide
- The IV bolus dose
- The time at which the elimination phase begins
- Time and plasma concentration Cp, one pair per line
PHARM/PCS reports
- The regression of ln(Cp) on time for the elimination phase, with r
- C0, the concentration back-extrapolated to time zero
- Vd = dose / C0
- A plot of ln(Cp) against time
When to use it
To estimate the apparent volume of distribution from an IV-bolus concentration-time profile, a core parameter for scaling doses and interpreting the other PK procedures.
Reading the results
The method hinges on regressing only the terminal (elimination) phase and extrapolating back; include early distribution-phase points and you'll underestimate C0 and overestimate Vd. Choosing where distribution ends and elimination begins is the main source of variability, so pick the linear tail of the ln(Cp) vs. time plot deliberately.
You choose where elimination starts
The procedure asks when the elimination phase begins, just as the 1986 program did, because that choice drives the answer. The plot of ln(Cp) against time shows where the points fall into a straight line. Output matches the book to every printed digit.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.