Is “DOSE=0” accepted by NONMEM?

Dear NONMEM users, I have a group of patients, whose plasma drug levels are measured and known at time=0 (no further drug is given to the patients afterwards.). I try to know, after how long time the drug will be completely cleared out from these patients. Naturally I used NONMEM to fit the patients' V and CL, in order to get plasma concentration change profile. However seems NONMEM only accepts DOSE is a positive number. If DOSE=0, NONMEM can't perform the fitting. Can anyone kindly give me some suggestions to solve this problem? Thanks a lot in advance! PTID DV AMT time EVID 2 . 0 0 1 2 6.8 0 0 0 2 1.1 0 3 0 2 0 0 6 0 3 . 0 0 1 3 1.08 0 0 0 3 0.16 0 3 0 3 0.03 0 6 0 3 0 0 9 0 Zheng