In a two-compartment model, which phase corresponds to elimination?

• A. Beta phase
• B. Alpha phase
• C. Gamma phase
• D. Delta phase

Answer: (A)

In a two-compartment PK model, after an intravenous dose you see two distinct exponential declines in concentration over time: a fast distribution phase and a slower terminal phase. The fast phase is alpha and reflects drug moving quickly between the central and peripheral compartments. The slower, terminal decline is elimination, and that corresponds to the beta phase. On a log-concentration versus time plot, the beta phase is the straight-line portion with slope -beta, representing the overall elimination rate constant (and the corresponding half-life). Gamma and delta are terms associated with more complex multi-compartment models, not the two-compartment case.