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Peeing into the wind? Urine drug screens, part 1 (benzodiazepines)

Posted Aug 20 2012 12:00am

Let’s be honest.

When was the last time results from the urine drug screen (UDS) changed your management plan?

Many times it takes hours for the patient to give the urine sample anyway. And, with all of the false positives out there, how do we know what the heck the result is actually telling us?

OBJECTIVE 
Today’s post will help you properly interpret the UDS, particularly focusing benzodiazepines.

Introduction:
The original UDS was termed the NIDA-5 (amphetamines, cannabinoids, cocaine, opiates, and phencyclidine) because they are the five drugs that were recommended by the National Institute on Drug Abuse (NIDA) for drug screening of federal employees back in the late 1980s. Drug-screening immunoassays are also frequently done for barbiturates and benzodiazepines and less frequently for methadone.

That has to raise some caution flags right off the bat! This test was not designed for Emergency Departments or hospitals for that matter. It was developed to screen federal employees. The fact that we have made it part of our standard practice affords various limitations.

In general, the qualitative UDS for each drug is looking for a particular structure. If the immunoassay identifies that structure (or one similar), it will trigger the test positive. Let's take a closer look at the test for benzos.

Benzodiazepines:
Benzodiazepines are pretty popular. In the U.S., alprazolam, clonazepam, lorazepam, and diazepam are among the most commonly prescribed medications in the outpatient setting. Here are the important points regarding this test:
  1. Most benzo screens look for oxazepam. If you're wondering how this could possibly be helpful since few patients are on oxazepam, you're not alone. However, diazepam and chlordiazepoxide both are metabolized to oxazepam. So, by looking for oxazepam, you actually pick up three benzos in one.
  2. The tricky part is that benzos vary in reactivity and potency and can trigger a positive result due to cross-reactivity.
BOTTOM LINE
A negative result doesn't rule out benzodiazepine ingestion and a positive result only guarantees that oxazepam, diazepam, or chlordiazepoxide is present. Not very helpful, in my humble opinion.

Next time we'll fully explore the opiate screen. Stay tuned...
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