False-Positive Amphetamines in Urine Drug Screens: A Six-Year Review.

Immunoassays are routinely used to provide rapid urine drug screening results in the clinical setting. These screening tests are prone to false positive results and ideally require confirmation by mass spectrometry. In this study, we have examined a large number of urine specimens where drugs other than the amphetamines may have caused a false positive amphetamine immunoassay screening result. 12,250 urine drug screens in a clinical laboratory that used the CEDIA amphetamine/ecstasy method were reviewed for false positive results over a six-year period (2015-2020). An additional 3,486 referred samples, where confirmatory mass spectrometry was requested were also reviewed. 86 in-house samples and 175 referral samples that were CEDIA false positives screens were further analyzed by an LC-QTOF general unknown screen. Potential cross-reacting drugs were identified and their molecular similarities to the CEDIA targets were determined. Commercial standards were also analyzed for cross-reactivity in the amphetamine/ecstasy CEDIA screen. Positive amphetamine results in 3.9% of in-house samples and 9.9% of referred tests for confirmatory analysis were false positive for amphetamines. Of these false positive specimens, on average, 6.8 drugs were detected by the LC-QTOF screen. Several drugs were identified as possible cross-reacting drugs to the CEDIA amphetamine/ecstasy assay. Maximum common substructure scores for 70 potential cross-reacting compounds were calculated. This was not helpful in identifying cross reacting drugs. False positive amphetamine screens make up to 3.9 - 9.9% of positive amphetamine screens in the clinical laboratory. Knowledge of cross-reacting drugs may be helpful when mass spectrometry testing is unavailable.