Efavirenz interference in urine screening immunoassays for tetrahydrocannabinol.

BACKGROUND It has been known for some time that the antiretroviral drug, efavirenz (EFV), cross-reacts in urine immunoassays for tetrahydrocannabinol (THC). Because published studies investigating this phenomenon are limited, cross-reactivity information for several immunoassays is lacking. Reports of possible false-positive THC results from clinicians conducting workplace testing prompted us to investigate cross-reactivity for assays frequently employed in our own setting. In light of the potentially deleterious consequences of misclassification, information about EFV cross-reactivity should be included in product information to facilitate interpretation of results and assay selection. METHODS Random urine samples from 30 patients on EFV therapy were analysed for THC metabolites by two near-testing devices (THC One Step Marijuana and Rapid Response(®) Drugs of Abuse Test Strips) and two automated immunoassays (Roche Diagnostics Cannabinoids II and Beckman Coulter SYNCHRON(®) Systems THC2). THC confirmatory testing was performed by gas chromatography-mass spectrometry (GC-MS). RESULTS GC-MS failed to detect THC metabolites in any of the samples, as did three of the four immunoassays. However, the Rapid Response(®) test strips yielded positive results in 28 out of 30 samples, which could be reversed on re-testing after sample pretreatment with glucuronidase. CONCLUSIONS Our study supports previous findings that interference is attributable to a glucuronidated EFV metabolite. We postulate that cross-reactivity is influenced by the composition of immunogens used to elicit anti-THC antibodies. Since access to such information is restricted, contributions from scientists in the antibody industry may be enlightening.