I remember an abstract on this topic that was presented at a Society of
Forensic Toxicologists meeting in the U.S. about 4-5 years ago. Authors
were M. Huestis and E. Cone from the U.S. National Institute for Drug Abuse.
They collected urines from "volunteers" who smoked a single marijuana
cigarette per week in a controlled setting. The 11-nor-9-carboxy-delta
9-THC (THCCOOH) concentration in urine as obtained by GC-MS was "normalized"
by dividing by the urine creatinine concentration.
A ratio was taken of THCCOOH/creatinine from 2 sequential urine samples as
follows:
Ratio = [(THCCOOH/creatinine) sample #2] / [(THCCOOH/creatinine) sample #1]
where the sample #2 was collected a minimum of 24 hours after sample #1.
Using ROC analysis, the best overall accuracy for predicting new marijuana
use was found at a ratio of >= 0.5 (overall accuracy = 85.4%, 5.6% FP, 8.6%
FN). Using a ratio of >=1.5 decreased the FP rate to 0.1% but increased
the FN rate to 27.0%.
Hope this helps. Sorry I don't have a complete literature citation - just
some notes in my file - but you should be able to find this on a PubMed
search. /Sal
Salvador F. Sena, Ph.D., DABCC
Associate Medical Director, Clinical Chemistry
Danbury Hospital
Danbury, CT 06810 USA
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> -----Original Message-----
> From: Peter Stromberg [SMTP:[log in to unmask]]
> Sent: Monday, March 11, 2002 5:45 AM
> Subject: CANNABIS
>
> My understanding is that cannabis can remain detectable in urine ( >50
> ug/l) for
> around 4 weeks after the last drug usage.Furthermore at the approach to no
> drug detection the urines can vary from positive to negative depending on
> dilution and other factors.This makes establishing whether the client is
> continuing to abuse cannabis quite difficult.How do list-users out there
> handle this situation?
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