How False Positives Work (and What They Could Mean for Your Health)


False Positives in Drug Testing
Martial Saugy, head of the Swiss Laboratory for Doping Analyses (LAD) of the Centre Hospitalier Universitaire Vaudois poses with an anti-doping sample of urine in his laboratory.
Martial Saugy, head of the Swiss Laboratory for Doping Analyses (LAD) of the Centre Hospitalier Universitaire Vaudois poses with an anti-doping sample of urine in his laboratory.
© LAURENT GILLIERON/Keystone/Corbis

There's no question it would be terrifying to be erroneously diagnosed with cancer, thanks to a false positive test. But cancer screenings aren't the only types of tests that can turn up false positives. Drug tests can as well. And falsely testing positive for drug use, especially illicit drug use, can be just as devastating as a false cancer diagnosis.

As with cancer screenings, false positive drug tests certainly aren't the norm; preliminary results from one study, released in 2010, showed 5 to 10 percent of drug tests resulted in false positive results [source: Laino]. Yet with some 150 million drug tests given in 2009 in the U.S. alone — to prospective employees, athletes, students and more — that equates to 7.5 to 15 million people winding up with false positives.

To protect yourself or a loved one, here are some things to know. Kids sometimes claim they falsely tested positive for marijuana because they were in a room where other kids were toking, not them. But standing in a cloud of marijuana smoke won't cause a false positive test. And cocaine is a drug that, for whatever reason, rarely shows up as a false positive or false negative [source: Laino]. The issue of false positives enters into the picture more if you ingest a legal medication or food that contains a trace amount of a drug, which then causes a false positive drug test. Here are a few of the more problematic sources [source: Borreli].

  • Poppy seeds. Eating a poppy seed bagel can result in a positive test for opiates and morphine, as the seeds contain minute traces of opiates.
  • Ibuprofen. Pop a few for a sore back, and you might test positive for marijuana, barbiturates or benzodiazepines. The pain reliever is thought to interfere with an enzyme found in some drug tests
  • Cold and allergy meds. If you're sick and taking some of these over-the-counter (OTC) drugs, beware. All may result in a positive test for amphetamines due to pseudoephedrine, a synthetic amphetamine used in many OTC medications.
  • Tonic water. If you need to take a drug test, hopefully a gin-and-tonic isn't your go-to cocktail. Tonic water, aka quinine water, contains a small amount of quinine, a drug used for centuries. Drink too much of it, and you can test positive for opiates.

Author's Note: How False Positives Work (and What They Could Mean for Your Health)

I've never had a false positive test per se, although once a mammogram showed "suspicious" results, so I had to return for a more accurate test. Luckily, it was a noninvasive procedure. And the second test showed I was fine. But a year ago, a friend's mother was given a Cologuard test, and the result came back positive. My friend and her family went through several days of hell before a colonoscopy revealed she was actually fine. Her mother was very upset that her physician hadn't done more research on the various screening tests and their false positive rates, and was so traumatized by the experience that she quit seeing him.

I hope I never receive a false positive on a health screening test. But I'm going to continue to be tested. And if a test comes back positive, I'll remind myself not to panic unless a second test does, too.

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