![]() When confirmatory testing is performed, results are generally unavailable for several days. Regrettably, MS testing is limited or nonexistent in many hospital laboratories. The best practice following a positive UDS involves confirmation with the mass spectrometry (MS) technique such as gas chromatography–mass spectrometry (GC-MS) or liquid chromatography–tandem mass spectrometry. False negatives are not covered in this review but present opportunities for significant patient mismanagement if not understood. False-negative results can be caused by a variety of factors including the cross-reactivity of the antibody used by the assay, the cutoff concentration for a positive result and length of time between drug ingestion and specimen acquisition. Immunoassays for selected drug classes, e.g., opiates and benzodiazepines, are also subject to clinically important false negatives ( 2). Further complicating the issue are the many available platforms with differing cross-reactivities. Unfortunately, they are subject to cross-reactivity with structurally related and unrelated compounds potentially yielding false-positive results. Immunoassays dominate urine drug screens (UDSs) because they are simple to use, easy to automate and provide rapid results ( 1). These false-positive results support the generally accepted practice that immunoassay positive results are considered presumptive until confirmed by a second independent chemical technique. False-positive results were described for amphetamines, opiates, benzodiazepines, cannabinoids, tricyclic antidepressants, phencyclidine, lysergic acid diethylamide and barbiturates. The discussion is separated into six sections by drug class with a corresponding table of cross-reacting compounds for quick reference. These articles were then carefully analyzed and condensed to 62 that included data on causes of false-positive results. English language articles were searched via the SciFinder platform with the strings ‘false positive urine’ yielding 173 articles. We present an inclusive review of analytes causing false-positive interferences with drugs-of-abuse UDS immunoassays, which covers the literature from the year 2000 to present. ![]() ![]() Knowledge of these potential interferents is important in determining a course of action for patient care. Many cross-reactivities exist with other analytes, potentially causing a false-positive result in an initial drug screen. Urine drug screen (UDS) immunoassays are a quick and inexpensive method for determining the presence of drugs of abuse. ![]()
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