Steady state dose

Dear NONMEM users, I have a question about NONMEM's calculation of steady-state doses. I am trying to model a system with two compartments. I want my t=0 boundary condition to be that the system is at steady-state, with a rate of cell death/clearance from one of the compartments which is higher than for all t>0. Unfortunately, I don't think there is an analytic solution for this steady state (see code copied below). From an old NMusers thread ( http://www.mail-archive.com/ [email protected] /msg00584.html ), I found out that NONMEM will find the steady state if you give it SS=1, RATE=0, AMT=0 in a t=0 record in the data file. When I tried this though, NONMEM found the wrong root of the steady-state equation (a negative one). Do you know if there is a way of making sure the steady state it finds is the positive one? For the moment, I have been initialising the compartments near to the steady state and at t=-(a lot), and letting them equilibrate, but I think this is slowing down my runs and would prefer to use another method. Thank you very much in advance for any advice you can offer. Joanna Lewis PhD student UCL Institute for Child Health 30 Guilford Street, London WC1N 1EH $PROBLEM hapuarachchi0_15sep_1 $INPUT ID BAGE AGE WEEK TIME CD DV AMT RATE SS TYPE CMT IG $DATA data.csv IGNORE=@ $SUBROUTINE ADVAN6 TOL=5 $MODEL COMP=(X) COMP=(Y) $PK ANO = THETA(1)*EXP(ETA(1)) EP = THETA(2)*EXP(ETA(2)) DEO = THETA(3)*EXP(ETA(3)) SIG = THETA(4)*EXP(ETA(4)) SCA = THETA(5)*EXP(ETA(5)) Q = THETA(6)*EXP(ETA(6)) THE = THETA(7)*EXP(ETA(7)) RHO = THETA(8)*EXP(ETA(8)) HIV = SS*THETA(9)*EXP(ETA(9)) $DES ALP = ANO*EXP(-EP*SCA*A(1)) DEL = DEO*EXP(SIG*SCA*A(1)) MU = Q*A(2) + HIV DADT(1) = THE + 2*RHO*A(2) - ALP*A(1) - DEL*A(1) DADT(2) = ALP*A(1) - RHO*A(2) - MU*A(2) $ERROR IPCD = A(1)+A(2) Y = IPCD + IPCD*EPS(1) $THETA ...etc.. $OMEGA ...etc... $SIGMA ...etc... $ESTIMATION MAXEVAL=0 METHOD=1 INTER SIGDIG=1 PRINT=1 $TABLE ...etc...