Published: Journal of Analytical Toxicology, Volume 23, Number 5, September 1999, pp.384
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Letter to the Editor
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Interference of Gemfibrozil with Roche TesTcup
To the Editor:
As part of a workplace drug-testing policy, a coal-mining company implemented
random on-site preliminary urine testing using the OnTrak TesTcup (Roche Diagnostic
Systems, Inc.). Presumptive positives were sent to our laboratory for confirmation.
A potentially embarrassing situation arose when the mining manager underwent
a random test and produced a positive reaction to cannabinoids. He denied using
any illicit drugs and claimed to be taking Gemfibrozil (Lopid) for treatment
of hypertriglyceridemia. Upon receipt at the laboratory, the TesTcup still showed
a positive reaction to cannabinoids. The urine specimen was analyzed for cannabinoids
using EMIT™ (Behring Diagnostics) at the 50-µg/L cutoff and was found
to be negative. The urine specimen was then analyzed for the presence of carboxy-THC
using pentafluoropropyl derivatization and gas chromatography–mass spectrometry
(GC–MS). No carboxy-THC was detected. The same sample was tested for cannabinoids
using the Syva RapidTest (Behring Diagnostics). The test was negative.
Gemfibrozil was added to drug-free urine and subjected to the same tests. No
false positives were obtained. Urine was later obtained from a healthy subject
taking only Gemfibrozil medication. The urine was subjected to the TesTcup (positive),
EMIT (negative), and Syva RapidTest (negative). The urine was subjected to a
comprehensive drug analysis including EMIT, high-performance thin-layer chromatography,
and GC–MS. No drugs other than gemfibrozil were detected.
Gemfibrozil is metabolized to hydroxymethyl and carboxyl products. It is suggested that one of these metabolites is responsible for the false-positive result of the Roche TesTcup test for cannabinoids in urine.
J.H. Lewis
Toxicology Unit
Pacific Laboratory Medicine Services
North Ryde NSW 2113
Australia
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