Like HowStuffWorks on Facebook!

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

False Positives in Mammograms and Colon Screenings

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 have been in the news for some time now. A reputable way to uncover breast cancer, mammograms were long recommended for women starting at age 40, and earlier if there was a family history of the disease. But in 2009, the U.S. Preventive Services Task Force recommended women ages 50 to 74 should have mammograms every other year. 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 84 percent of the time, younger women, and those with dense breasts, are more likely to have a false positive mammography result. In fact, women who have regular mammograms between the ages of 40 and 50 have a 50 to 60 percent chance of receiving a false positive during this decade, which will result in additional testing, expense, inconvenience and worry [source: Susan G. Komen].

Women more at risk for a false positive test result previously have had breast biopsies, have a family history of breast cancer and take estrogen [source: National Cancer Institute]. Thus, when you begin mammogram testing, and the frequency with which you repeat it, should be carefully discussed with your family physician.

Colorectal cancer is the second-leading cancer-related death in the U.S. The American Cancer Society estimates there will be 132,700 new colorectal cancer diagnoses in 2015. 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 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. The problem is, these tests are less accurate.

One German study of six different stool tests (which look for trace amounts of blood) showed accuracy rates of 25 to 72 percent, and false positive rates of 3 to 30 percent. Cologuard, one of the newer colorectal cancer tests on the market, inspects the DNA in stool to check for colorectal cancer. While studies show the test finds 92 percent of the cancers that colonoscopies uncovered, and 42 percent of the polyps, it has a false positive rate of 13 percent. And the new Epi proColon test, which checks your poop for chemical changes to one gene, carries a false positive rate of 19 percent [source: Pollack].