The manuscript purpose is to provide a resource for clinicians on the functionality and pitfalls of the rapid urine drug screen for clinical decision making. Many providers remain under-informed about the inherent inaccuracies. The rapid urine drug screen is the first, and often only, step of drug testing. In the majority of emergency departments the urine drug screen is a collection of immunoassays reliant on an interaction between the structure of a particular drug or metabolite and an antibody. Drugs in separate pharmacologic classes often have enough structural similarity to cause false positives. Conversely, drugs within the same pharmacologic class often have different enough structures that they may result in inappropriate negatives. This lack of sensitivity and specificity significantly reduces the test utility, and may cause decision-making confusion. The timing of the drug screen relative to the drug exposure also limits accuracy, as does detection threshold. Confirmatory steps following the initial immunoassay include chromatography and/or mass spectrometry. These are unavailable at many institutions and results rarely return while the patient is in the emergency department. In addition, institutional capabilities vary, even with confirmatory testing. Confirmation accuracy depends on a number of factors, including the extent of the catalog of drugs/metabolites that the facility is calibrated to detect and report. In summary, the standard emergency department urine drug screen is a test with extremely limited clinical utility with multiple properties contributing to poor sensitivity, specificity, and accuracy. The test should be used rarely, if ever, for clinical decision making.