Written by: Ben Kiesel , MD (NUEM ‘23) Edited by: Jason Chodakowski, MD (NUEM ‘19) Expert Commentary by: Joe Kennedy, MD
Expert Commentary
The urine drug screen is one of the most frequently ordered and more frequently cursed tests in all of medicine. It is one of the few tests that generates more argument than the banal discussions between surgeons and emergency physicians regarding the utility of a white blood cell count. Nevertheless, in skilled hands, this cheap and simple immunoassay can answer a few quick questions. The key is in knowing your Achilles heel:
Opioids and opiates produce similar toxidromes, but there are virtually limitless combinations of street and prescription drugs, lab assays, and confirmatory tests to tell these apart. If the test is negative, who cares! Treat the patient in front of you. Fentanyl and its analogues are perhaps most often missed. Wake the patient up, discuss their substance use, and move on.
Hopefully you noticed that *many* things other than phencyclidine can lead to a positive urine drug screen for PCP. Ever have a conversation with the mother of a 12 year old about their PCP use? Best go into that conversation knowing that many other substances cross-react. Perhaps the kid is actually on lamotrigine for seizures, and that is why they were combative. Or perhaps like most humans, they took a few ibuprofen at some point. Either way—when child protective services is consulted for a positive result, either call the poison center or your local toxicologist for help.
Perhaps the most important point is that this is a screen and absolutely by no means a confirmatory test. When it matters (placement, custody, fired/hired, transplant eligibility, excluding other diagnoses), get an actual level or result. Not sure how to do that? Call your lab! Or call me. Toxicologists love solving these problems and teaching and helping others do the same!
Joe Kennedy, MD
Attending Physician, Emergency Medicine
University of Illinois Hospital
Senior Toxicology Fellow
Toxikon Consortium
How To Cite This Post:
[Peer-Reviewed, Web Publication] Kiesel, B. Chodakowski, J. (2020, Oct 19). U Tox: Clinical Utility and False Negatives. [NUEM Blog. Expert Commentary by Kennedy, J]. Retrieved from http://www.nuemblog.com/blog/utox-clinical-utility-and-false-negatives.
