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May 2, 2011

Unnecessary testing

I compiled a bigger table with all of the false positives I could find here.

In January, Emergency Medical Abstracts reviewed "Commonly prescribed medications and potential false-positive urine drug screens" by Brahm in the American Journal of Health-System Pharmacy.

It's not news that urine tox screens are useless; that's what I've been taught since medical school (admittedly, not all that long ago).

Emergency textbooks such as Tintinalli's (I checked) and PEER VII, the major board review for EM residents, agree. To me, the issue seems pretty much settled.

However, the people we admit our patients to -- internists and psychiatrists -- (anecdotally) always ask for the tox screen. The seemingly compelling argument from psychiatrists is that knowing what drugs were abused now helps with long-term care, i.e. they can catch their patients lying about drug abuse.* Internists seem to be genuinely interested in finding the cause of the patient's current illness, so that other causes can be lowered in the differential.

Although these goals are laudable, unfortunately, as the key table below from the Brahm paper highlights, tox screens are terrible tests. Not surprisingly, most labs put disclaimers in the their u-tox results, such as:
This assay provides a preliminary qualitative analytical test result. A more specific alternate chemical method must be used to obtain a confirmed analytical result, and should be correlated with clinical findings.
Maybe the patient was smoking some marijuana laced with PCP while popping phenobarbital; or, maybe he had a headache and took some legal, over-the-counter ibuprofen.
My general practice has been to order the urine tox to placate the inpatient teams as I don't want to start fights all the time, but I clarify with the nurses that obtaining a urine sample is their lowest priority.

The point here isn't that tox screens or internists are useless, merely that we should know how good our tests are when we decide whether or not to use them. 

Routine "screening"** labs are no different.

Unfortunately, my department recently came to an agreement with our Internal Medicine department about standard labs resulting before patients are listed for admission. The general idea is that there are a few levels of care -- a nurse practitioner/hospitalist floor service, a teaching/resident floor service, a handful of stepdown beds, and the ICU. Some patients are too sick for a the NP/hospitalist service and routine labs may identify a subset of those. 

But a large group of patients are clearly sick enough for the teaching service but obviously not sick enough to need to be upgraded to a stepdown or unit bed -- and routine labs are not going to identify any patients that might be. An INR of 7 or a surprising new renal failure does not mandate a stepdown bed. Yet these patients now, by interdepartmental policy, cannot be listed for admission until their routine labs result, regardless of the fact that those results will not alter the patients' disposition. This means that patients wait for generally at least an extra hour before they can be admitted, leading to further ED boarding, which leads to, among other things, higher mortality. This is another unfortunate example of misapplied testing leading to worse patient care.


*This argument was presented by a psychiatry resident at a joint EM-Psych resident conference last year.
**The term "screening" for routine labs is broadly misapplied, occasionally by myself. A screening test should be sensitive but not specific to catch all possible instances of disease; routine labs (e.g. chemistry and complete blood count) are incredibly insensitive for disease processes.

Brahm NC, Yeager LL, Fox MD, Farmer KC, Palmer TA. Commonly prescribed medications and potential false-positive urine drug screens. 
Am J Health Syst Pharm. 2010 Aug 15;67(16):1344-50.

most benzos are missed

summary, aka why urine tox is fairly useless


  1. there was some miscommunication. minimal testing isn't an agreed-upon policy, after all.

  2. it's great that it's not a policy, but there are still delays to assigning teams & beds due to admitting residents waiting for routine labs.

  3. outstanding comprehensive lecture on urine tox tests by Dr. Jon Cole at Hennepin http://hqmeded.com/video/39433095 , approx 30 min

  4. add: Tramadol causes PCP false positive:

  5. great new post on UDS by Rob Orman at ERCAST: http://blog.ercast.org/2012/09/mysteries-of-the-urine-tox/ and http://www.royalmedical.com.au/pdfs/10-Mayo-Clinic-Clinicians.pdf

  6. amoxicillin & NSAIDS can cause cocaine FPs: http://www.jfponline.com/Pages.asp?AID=4319

  7. great PV card by Michelle Lin: http://academiclifeinem.blogspot.com/2010/07/paucis-verbis-card-urine-toxicology.html

  8. more! may cause propoxyphene false positives: methadone, cyclobenzaprine, doxylamine, diphenhydramine, imipramine