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


The implements for a screening test for colon cancer are displayed. Many cancer screening tests have high rates of false positives.
The implements for a screening test for colon cancer are displayed. Many cancer screening tests have high rates of false positives.
Media for Medical/UIG via 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 in positive. Shocked and reeling, you spend an agonizing week waiting for the results of a subsequent surgical procedure. When the results are in, they're negative. You don't have ovarian cancer after all. The 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?

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

  • True positive. The test says you have condition "X," and you really do.
  • True negative. The test says you do not have condition "X," and you really don't.
  • False positive. The test say you have condition "X," but you really do not.
  • 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. When undergoing such a screening, it's important to discuss with your physician how reliable the results will be. If your test yields a positive result for a certain condition, 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 tests are surgical procedures, and/or are risky to perform. You don't want to undergo that if it isn't necessary.

Plus most, if not all, people are upset to learn they have a particular medical issue. It can be days or even weeks before the results of the second test come in, potentially leaving healthy people and their 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 for those who had all 14 tests nearly doubled, to 60.4 percent for men and 48.8 percent for women [source: Cancer Connect].

These results don't mean screening tests are unreliable, 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. But 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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