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

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].

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.

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).

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

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].

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].