Immunoassays for drug screening in urine : Chances, challenges, and pitfalls.

Immunoassays are presently used worldwide for the rapid screening of drugs. Despite the fact that they are a highly valuable tool for the testing of legal and illicit drugs, there is a real risk of false-positive and false-negative findings and many pitfalls must be taken into account when these tests are used in an uncritical manner and without valid confirmation procedures. In a long-standing successful cooperation with a large psychiatric hospital, we checked doubtful and nonplausible immunochemical findings in urine with gas chromatography-mass spectrometry (GC-MS) confirmation methods. The reported case histories demonstrate typical pitfalls, for example, influence of changing nutritional habits, use of sweeteners (cyclamate), uncritical creatinine correction, impaired elimination, or cross-reactivities of simultaneous therapeutic medication. In accordance with national and international associations, immunoassays are recommended as a useful tool for screening. However, confirmation analysis with conclusive methods (GC-MS or liquid chromatography-MS) is unavoidable for valid substance identification, discrimination between active and inactive metabolites, detection of congeners, and accurate determination of concentrations in body fluids.