Cmax/Tmax in the DES block

Interesting experience concerning computation of Cmax and Tmax (and probably other stats) in the DES block. We used to use this way: http://cognigencorp.com/nonmem/current/2007-December/4125.html Specifically, reserved the place in the memory: $ABB COMRES=2 Set these values to zero for each new subject: $PK IF(NEWIND.LE.1) THEN COM(1)=0 COM(2)=0 ENDIF and computed Cmax/TMAX as $DES IF(CONC.GT.COM(1)) THEN COM(1)=CONC COM(2)=T ENDIF $ERROR CMAX=COM(1) TMAX=COM(2) Recently I applied the same procedure to compute Cmax following 1 hr IV infusion. Unexpectedly, Tmax was estimated at times > 1 hr, and Cmax was higher than 1-hr concentration (true Cmax is at 1 hr). After some experiments, the explanation was that Nonmem computes concentration-time course (with infusion ON) for longer than 1 hr, and resulting Cmax/Tmax are at the end of the "computation window" rather than at 1 hr. Turns out that the results also depend on ADVAN routine. The largest deviation (still small, 1-3 percents) was for ADVAN8, ADVAN9, and ADVAN13. ADVAN15 was better but still off. ADVAN14 was almost perfect but still slightly (0.01%) off. ADVAN6 provided correct answer (up to the precision of the output). So, the discrepancy is small but if 1-2% difference is important, one has to be careful when using DES block computations. Thanks Leonid