Background: Individuals who have tested positive for cocaine have claimed that lidocaine, or its primary metabolite, norlidocaine (monoethylglycinexylidide (MEGX)), have caused false positive results for the cocaine metabolite benzoylecgonine (BE) on urinary immunoassay testing.
Objective: The goal of the study was to determine if lidocaine exposure from routine medical procedures can result in false positives on a commercially available cocaine immunoassay urine drug screen (UDS).
Methods: We performed a cross-sectional observational study of patients receiving lidocaine as part of their regular care. Standard immunoassay drug screens and confirmatory liquid chromatography-mass spectrometry (LC-MS) were performed on all urine samples to assess for MEGX and BE.
Results: In total, 168 subjects were enrolled; 121 samples positive for lidocaine were ultimately included for analysis. One hundred fourteen of the 121 were also positive for MEGX. None of the 121 were positive for cocaine/BE on the UDS (95% CI), 0-3.7% for the full sample and 0-3.9% for the 114 who tested positive for MEGX.
Conclusion: The present study found no evidence that lidocaine or norlidocaine are capable of producing false positive results on standard cocaine urine immunoassays.
Keywords: Cocaine; Cross-reactivity; False positive; Lidocaine; Norlidocaine; Urine drug screen.