WOS: 000378754600011Cross-reaction is an important problem with immunoassays that may lead to interferences and potentially yield wrong results. This case study describes a false positive tetrahydrocannabinol (THC) screening with CEDIA immunoassay reagents after the use of efavirenz for anti-retroviral therapy. A 31-year-old HIV (+) probationer transsexual was admitted to Ege University Institute of Drug Abuse, Toxicology and Pharmaceutical Sciences Toxicology Laboratory due to marijuana abuse. He had been using 600 mg/day Efavirenz to overcome his disease. As the routine application in the laboratory, the urine specimens of the probationer were first screened for drugs of abuse using CEDIA reagents. The analysis results were all found to be positive. Subsequently, the urine specimens were also analyzed for their THC content with another urine screening immunoassay kit using DRI (Diagnostic Reagent Inc.) reagents. The results were found to be negative. In order to ensure a correct result, the confirmation of the analysis was performed with the gas chromatography-mass spectrometer for the urine samples and the results were found to be below the detection limit (LOD < 1.97 ng/mL), Additionally a hair sample belonging to probationer was obtained, analyzed with a fully validated method and the result was also found to be negative. Therefore, it was concluded that the result obtained with CEDIA reagents was false positive for THC which is, and was caused by Efavirenz use by the probationer. The results demonstrated that there is a strong need for the careful interpretation of the analysis results by an experienced forensic toxicologist to decrease the chances of obtaining wrong results
