Re: treatment of BQL

Date: Tue, 05 Oct 1999 10:09:29 -0700 From: LSheiner <lewis@c255.ucsf.edu> Subject: Re: treatment of BQL Leonid, I can see the problems with extensive data sets. As I think I mentioned before, the reason for deleting all but the key BQLs (i.e., those where the observations first dip below detection, or those just before the observations first rise above detection) was pointed out to me by Dennis Fisher I think: Imagine fitting a biexponential to single dose PK observations that are only slightly off from being mono-exponential, and that observations dip below detection at say 12 hours, followed by BQLs at 18, 24, 30, 36, 42, and 48 hours. If you set all the BQLs to QL/2 and leave them in, the second exponential will not be the one you seek, but will be estimated as very long, since the set of BQLs are operating to "tell" the fit that that there is a long flat tail. But if you delete all but the value at 12 hours, then the BQL there simply helps the fit understand that the levels are low at 12 hours and beyond ... LBS. -- Lewis B Sheiner, MD Professor: Lab. Med., Biopharm. Sci., Med. Box 0626 voice: 415 476 1965 UCSF, SF, CA fax: 415 476 2796 94143-0626 email: lewis@c255.ucsf.edu