Re: Data Construct / ADVAN selection

From: Amir Abbas Tahami Monfared <tahamima@MAGELLAN.UMontreal.CA> Subject: Re: Data Construct / ADVAN selection Date: Tue, 10 Apr 2001 14:11:47 -0400 Dear NONMEM users, I used NONMEM to do the population modeling and I have a couple of questions. The data items are specified for the following single-dose infusion type (Inf. time=2). I used one and two-cmt models. The results and the goodness of fit satisfies the use of two-cmt model. However, I noticed that the estimate of VSS (VSS=theta(4)+eta(4)) for all individual is identical after POSTHOC estimation. zero order infusion with inf.time=2, data for one individual: Sub. Dose Time DV RATE 1 100 0 0 50 1 . 0 0 . 1 . 1 3.22 . 1 . 2 3.55 . 1 . 2.1 2.72 . 1 . 2.3 2.42 . 1 . 2.5 1.86 . 1 . 3 1.40 . 1 . 4 0.45 . 1 . 5 0.53 . 1 . 8 0.32 . I tried this control stream: $PROB Population PK ... $INPUT ID AMT TIME DV RATE $DATA Data $SUBS ADVAN3 TRANS3 $PK TVCL=THETA(1) CL=TVCL+ETA(1) TVV=THETA(2) V=TVV+ETA(2) TVQ=THETA(3) Q=TVQ+ETA(3) TVVSS=THETA(4) VSS=TVVSS+ETA(4) S1=V $ERROR Y=F+ERR(1) $THETA (0.1,30) (0.1,25) (0.1,50) (0.1,100) $OMEGA 0.25 0.25 0.25 0.25 $SIGMA 0.04 $EST SIGDIGITS=4 MAXEVAL=999 POSTHOC PRINT=5 $COV $TABLE ID TIME CL V Q VSS $SCAT TIME VS DV $SCAT ... I would also like to add that the estimates for volume of distribution in central cmt. and VSS are half of the mean estimate obtained by STS method. If someone has any advice, comments, on the above specification, or suggestions for how to model such data, I would really appreciate it. Thanks in advance. Amir A. Tahami