Prescription Drug Efavirenz Yields False Positive Test Results For THC, Study Says

Pretoria, South Africa: Prescription doses of the antiretroviral prescription drug efavirenz (EFV) cross-reacts in urine immunoassay tests for the carboxy THC metabolite, according to clinical data published online in the journal Annals of Clinical Biochemistry.

Investigators at the University of Pretoria in South Africa analyzed random urine samples from 30 patients on EFV therapy 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).

Authors reported: “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.”

Separate studies have previously documented that efavirenz may yield so-called ‘false positive’ test results for the carboxy THC metabolite on various types of presumptive immunoassay urine tests.

For more information, please contact Paul Armentano, NORML Deputy Director, at: Full text of the study, “Efavirenz interference in urine screening immunoassays for tetrahydrocannabinol,” appears in Annals of Clinical Biochemistry.