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

By: Melanie Radzicki McManus  | 

rapid COVID-19 test
A medical worker performs a rapid COVID-19 test at the COVID-19 Rapid Test Center in New York City. Noam Galai/Getty Images

You can't believe this is happening. In a proactive health move, you undergo a CA-125 test, which screens for ovarian cancer. The test result comes back positive. Shocked and reeling, you spend an agonizing week waiting for the results of a subsequent surgical procedure.

When the results are finally in, they're negative — you don't have ovarian cancer after all. That first result was a false positive. You're elated, yes, but also angry. What's the use of having a screening test if it might not be accurate?

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When it comes to your health, the more informed you are, the better. Medical tests can produce one of four results:

  1. True positive. The test says you have condition "X" and you really do.
  2. True negative. The test says you do not have condition "X" and you really don't.
  3. False positive. The test say you have condition "X," but you really do not.
  4. False negative. The test says you do not have condition "X," but you really do.

While many medical tests yield very accurate results, others do not. So if you're facing a medical screening, it's important to discuss with your physician how reliable the results will be. It's also good to be aware that if your test yields a positive result, you will likely undergo a second, more accurate, test. This isn't a big problem if the second test involves, say, a noninvasive blood draw. But sometimes the subsequent test is a surgical procedure and/or is risky to perform. You don't want to undergo something like that if it isn't necessary.

Keep in mind, too, that it can be days or even weeks before the results of a second test come in, potentially leaving a healthy you and your loved ones stressed, terrified and sleep-deprived for no reason.

So how common are false positives? A study published in 2009 in the Annals of Family Medicine found a high risk for false-positive results in screening tests for prostate, lung, colorectal and ovarian cancers. And the risk went up with the number of tests administered. Results were compared after participants received four tests for various cancers versus 14:

  • The cumulative risk of a false-positive result after four tests was 36.7 percent for men and 26.2 percent for women.
  • The risk of a false-positive result for those who had 14 tests nearly doubled, to 60.4 percent for men and 48.8 percent for women [source: Cancer Connect].

Perhaps more alarmingly, a study published in a 2017 edition of American Family Physician found nearly 60 percent of current or former smokers who underwent a lung cancer screening via low-dose CT were given some type of positive result – but nearly 98 percent of those ended up being false positives [source: American Family Physician].

In 2020, with the rapid development of various COVID-19 tests to tackle the global coronavirus pandemic, the issue of false-positive test results became a concern. Some of the molecular tests, done via nasal or throat swabs, or by testing saliva and other bodily fluids, carry a false-negative rate of 20 percent. That may sound high, but that rate was for testing performed five days after people noticed COVID-19 symptoms. For those tested earlier, the false-negative rate skyrocketed to as much as 100 percent [source: Shmerling].

False-positive rates for COVID tests are generally rare; a March 2021 review of four rapid COVID-19 tests found that the tests correctly gave a positive COVID-19 result in 99.6 percent of patients who took it. Another review of 16 different rapid tests found that the antigen tests correctly ruled out infection in 99.5 percent of people who had symptoms and in 98.9 percent of people without symptoms.

So we can see that false negatives are a bigger problem than false positives but both exist. Part of the issue with COVID-19 test accuracy is that the U.S. Food and Drug Administration (FDA) gave the green light to the use of hundreds of diagnostic tests under its emergency use power. So there are numerous tests available, all with varying degrees of accuracy. In fact, in October 2021, an Australian maker of an at-home COVID-19 test had to recall the kits because of an unusually high rate of false positives.

Recently, the FDA began warning health care providers and others about false-positive results for COVID-19 antigen tests, which many people take because the results are quickly available. The FDA believes that false-positive results may be due to health care providers not following the test instructions, the processing of multiple specimens in one batch, insufficient cleaning of instruments or workspace and other reasons [source: FDA].

Of course, all these test inaccuracies we've discussed don't mean screening tests should be avoided, or that you shouldn't have more than a certain number. Clearly, the tests do catch a lot of diseases, which translates to lives saved. And in the case of COVID-19, you have to keep in mind that tests are being rapidly developed due to the extreme seriousness of a pandemic, and one in which the virus keeps morphing. Still, it's important to know the accuracy rate of any test you're considering, and the positives and negatives associated with any necessary follow-up tests so you can make an informed choice.

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How False Positives Occur

Unfortunately, it's difficult to eliminate false-positive test results. For example, a mammogram may accurately show abnormal cells in the breast ducts that could become cancerous, when in actuality they are not life-threatening and won't cause symptoms [source: National Cancer Institute]. Patients screening for ovarian cancer may be given a cancer antigen 125 blood test (CA-125). If a woman's CA-125 level is elevated, cancer is suspected and additional testing may result. The problem is, a lot of other conditions can cause an elevated CA-125 level, such as diverticulitis, pregnancy and even normal menstruation, so the test isn't recommended for women with only an average risk of ovarian cancer [source: Laughlin-Tommaso].

False-positive test results aren't just annoying. They're responsible for driving up health care costs, as they necessitate additional testing. A 2015 study put the collective cost for false-positive mammograms and breast cancer overdiagnoses alone at a whopping $4 billion a year [source: HealthAffairs].

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Many would argue a far greater cost is in personal angst. A Danish study, published in a 2013 issue of the Annals of Family Medicine, found women who receive a false-positive mammogram suffer from anxiety and depression up to three years after the faulty diagnosis — long after they know they do not have breast cancer. Many women also have problems sleeping and with their sex lives [source: Moisse]. And a study by Penn State researchers found women with false-positive mammograms are more likely to start taking anti-anxiety and depression meds.

Still, no one is suggesting scrapping these tests, which save many lives. The takeaway is to do your homework. Work closely with your medical professional to determine which tests are in your best interest and how frequently you should have them done. And try not to become too alarmed if you receive a positive test result. It very well may be inaccurate.

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False Positives in Mammograms and Colon Screenings

Dr. Rim Villard, mammogram
Dr. Rim Villard looks at the results of a mammography, on Oct. 9, 2017, at the Paoli-Calmette Institute, in Marseille, France. ANNE-CHRISTINE POUJOULAT/AFP via Getty Images

As you've seen, false positives can show up in a variety of medical tests. Two of the more notable are mammograms, which screen for breast cancer, and colorectal cancer tests.

Mammograms were long recommended for women starting at age 40, and earlier if there was a family history of the disease as a way to detect breast cancer. But in 2009, the U.S. Preventive Services Task Force recommended women ages 50 to 74 should have mammograms every other year rather than yearly. And women under 50 and over 74 should only have mammograms after careful consultation with their physician. Others experts disagreed. The American Medical Association and American Cancer Society (ACS), for example, both reasserted their positions that women should have annual mammograms starting at age 40 [source: Walker]. In 2015, the ACS changed that recommendation to 45 (see sidebar).

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One of the reasons behind this kerfuffle is the prevalence of false positives in mammography. Although the test correctly identifies breast cancer 87 percent of the time, women under 50 and those with dense breasts are more likely to have a false-positive mammography result. And somewhat paradoxically, once you've had 10 annual mammograms, your chance of receiving a false positive soars to 50 to 60 percent. This, of course, means additional testing, expense, inconvenience and worry [source: Susan G. Komen].

Besides your age, breast density and mammogram frequency raising your risk of a false-positive result, you're also more at risk if you've had breast biopsies, have a family history of breast cancer or take estrogen [source: National Cancer Institute]. Thus, the time that you begin mammogram testing, and the frequency with which you repeat it, should be carefully discussed with your family physician.

Colorectal cancer is the third-leading cancer-related death in the U.S. The American Cancer Society estimates there will be nearly 150,000 new colorectal cancer diagnoses in 2021. The traditional means of screening for these cancers is through colonoscopy, which a 2014 study by University of Utah researchers showed catches 94 percent of colorectal cancers [source: The Scope].

But many people find the procedure and its preparation unpleasant, with only about 50 percent of the people recommended for a colonoscopy actually undergoing the procedure [source: Lazarus]. This has led to the development of noninvasive screening tests that examine people's stools for cancer, not the colon itself.

While many people who skipped colonoscopies are more likely to take a poop test, they're not as accurate. Cologuard, a leader in stool tests, has a 92 percent accuracy rate, for example, but a 13 percent false-positive rate. And the fecal immunochemical test, or FIT, is only about 79 percent accurate, with a 25 percent false-positive rate [sources: Paturel, Cologuard, Polymedco]. These tests also need to be repeated every few years and not once a decade, as with most colonoscopies

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False Positives in Drug Testing

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 aren't the norm. Preliminary results from one study, released in 2010, showed only 5 to 10 percent of drug tests resulted in false-positive results [source: Laino]. However, an award-winning 2019 study by Vanderbilt University Medical Center researchers showed as many as 43 percent of its urine drug-screening tests produced false positives, often due to other drugs patients were taking for legitimate reasons [source: Vanderbilt University Medical Center].

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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.

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Originally Published: Nov 18, 2015

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More Great Links

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