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Drug Tests Not Immune from False Positives

If your child insists his positive drug-test results are a mistake, there's a chance he could be telling the truth.

Drug tests generally produce false-positive results in 5 percent to 10 percent of cases and false negatives in 10 percent to 15 percent of cases, new research shows.

Eating as little as a teaspoon of poppy seeds - less than the amount on a poppy seed bagel - can produce false-positive results on tests for opioid abuse, according to Dr. Dwight Smith, of the Veterans Affairs Medical Center in Black Hills, S.D.

The poppy seeds can lead to false-positive results for two or three days, he said, yet one recent study showed only about half of doctors were aware of the problem.

Knowing the test's limitations is crucial given that about 150 million drug tests were conducted in the United States last year, he said.

"We drug test everyone, our students, our athletes," Smith said.

Also, many private and federal employers require regular testing, he said.

False-Positive Results in 5 Percent to 10 Percent of Cases

To get a better picture of the test's flaws, Smith and colleagues at Boston Medical Center reviewed scientific articles on drug screening published between January 1980 and September 2009.

The results were presented at the American Psychiatric Association's annual meeting.

Cold medications, the antidepressant Wellbutrin and tricyclic antidepressants can trigger false-positive results on tests for amphetamines, according to the review, and the antidepressant Zoloft and the painkiller Daypro can show up as a benzodiazepine problem.

The quinolone antibiotic drugs can trigger false positives for opioids, and the HIV medication Sustiva can show up as marijuana use, Smith said.

On the other hand, just being in the room with someone who is smoking marijuana is not going to trigger false positive results, no matter what your child claims, he said.

"Unless [they] were in the van with Cheech and Chong, that's not what happened," Smith said.

And "cocaine is cocaine - you don't get many false positives or false negatives," said Dr. Petros Levounis, director of the Addiction Institute of New York at St. Luke's and Roosevelt Hospital in New York City.

Standard Drug Tests Sometimes Miss Oxycodone

Another problem is that most standard drug tests have a substantial false-negative rate for oxycodone (OxyContin, Percolone, Roxicodone), an opioid drug that's been associated with high levels of abuse. In some cases, a person could be taking oxycodone, and the routine drug screen might miss it and report as negative. Oxycodone is also found in the medications Percocet, Roxicet and Tylox.

One study in the review showed that 88 percent of doctors didn't know that they may need to order a special test to accurately screen for oxycodone.

Other opioids missed by standard tests include methadone, fentanyl, Ultram, Subutex and Suboxone, Smith said.

Likewise, standard tests may miss some sedative and hypnotic drugs.

Another problem is that there are no federal guidelines setting threshold levels for positive results, Smith said.

Over the years, people have come up with all sorts of ways to try to beat the tests, mainly by diluting urine samples, he said.

"We really have no good numbers on how prevalent drug-test cheating is," Smith said, so further research is needed.

This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.
By Charlene Laino
Reviewed by Laura Martin
©2005-2010 WebMD, LLC. All rights reserved

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