Characterization of Interference with 6 Commercial Δ9-Tetrahydrocannabinol Immunoassays by Efavirenz (Glucuronide) in Urine

We observed that antiretroviral therapy with efavirenz (EFV) produces urine samples that screen positive for Δ9-tetrahydrocannabinol (THC) exposure, despite the absence of 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (THC metabolite). These observations were made when we were using immunoassay-based reagents to screen for drugs of abuse in patients enrolled in a study on the effects of inhaled marijuana on HIV-related neuropathy. Extensive anecdotal literature exists regarding the interference of EFV with antibody-based assays for THC metabolites in urine. In product literature, the manufacturer of EFV (Sustiva®; BMS Virology) states that EFV may interfere with THC metabolite immunoassays (1). An earlier letter to Clinical Chemistry discusses the interference of EFV with an ELISA for estradiol(2). We therefore characterized the occurrence of EFV cross-reactivity with several commercial reagents used to screen for THC exposure. We hypothesized that cross-reactivity with THC immunoassays was the result of the interaction of EFV metabolite (and not a parent drug) with the antibody complexes used in …