Efavirenz interference in urine screening immunoassays for tetrahydrocannabinol

Abstract

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 w Drugs of Abuse Test Strips) and two automated immunoassays (Roche Diagnostics Cannabinoids II and Beckman Coulter SYNCHRON w 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 w 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.

Guilty or not guilty? — False positive results of common medicines in drug tests: review and practical guide

“…This was despite the urine samples being verified as THC-free using a separate method. This cross-reactivity of efavirenz with the tests may be due to the glucuronidated metabolite of the drug [59]. The same issue was found with three other tests: Triage® TOX Drug Screen by BioSite Incorporated, Cedia® Dau MultiLevel THC by Microgenics Corporation, and Cannabinoids ELISA Kit by Immunalysis Corporation.…”

“…For example, in the ENRICO study, all 24 participants on efavirenz tested positive for THC [63]. However, the inconsistency and lack of reports in recent years suggest a need for more current research in this area [58,59]. Interestingly, another drug, raltegravir, has also been found to give false-positive results in cannabinoid screenings [15].…”

Southern African HIV Clinicians Society guidelines for harm reduction

“…False-positive urine tests for cannabis in patients using EFV (used in > 90% of HIV-positive South Africans at the time of writing this guideline) have been described. 78 EFV, efavirenz.…”