I have a question about the calculation of accumulation ratio after multiple dosing in NCA.
If the study drug was given daily from Day 1 to 10 and the PK samples were collected on Day 1 and Day 10, I got two lambda_z from both Day 1 and Day 10. It looks like that the WinNonlin uses the lambda-z on Day 10 to calculate the accumulation ratio after multiple dosing: R=1/(1-e-lambda_z*24).
My question is which lambda_z should be used for the accumulation ratio calculation? Any input is appreciated!