UMEM Educational Pearls

Category: Toxicology

Title: Tox Screen - The False Positives

Keywords: urine toxicology screen (PubMed Search)

Posted: 3/18/2010 by Fermin Barrueto, MD (Updated: 3/27/2010)
Click here to contact Fermin Barrueto, MD

When you draw a urine toxicology screen it can mislead more often than help you. Here is a quick list of the test followed by some medications that cause false positives - when in doubt, call your lab to find out specifics since results will vary lab to lab:

TCA - diphenhydramine, carbamazepine, cyclobenzaprine (side note: TCA screen should never be used to determine TCA toxicity, your ECG and physical exam should be enough to determine if the patient is toxic from TCA

Cocaine - the most accurate test on the screen, positive for up to 5 days

PCP - dextromethorphan and ketamine can turn it positive

Amphetamines - pseudoephedrine, ephedrine, phenylephrine and many other OTC cough decongestants can as well, the worst screening test with the largest number of false positives