Can a cancer screening test be wrong?

Yes, a screening test can be wrong. All screening tests have risks, and two of them are false results.

• A false-positive result shows there is cancer when there really is not. It can cause anxiety and is usually followed by more tests and procedures, which also have risks.
• A false-negative result shows there is no cancer when there really is. A person who gets a false-negative result may delay seeking medical care even if they have symptoms.

Being wrong sometimes is built into screening, not a defect in it. A screening test examines large numbers of people who feel perfectly well and tries to spot the few with a hidden problem. Tune the test to catch nearly every cancer and it will also flag healthy people. Tune it to stop alarming healthy people and it will miss cancers. No setting avoids both. So the real question is not whether a test is ever wrong, but how often, and in which direction.

Mammography shows the scale of it. The US Preventive Services Task Force estimates 1,376 false-positive results for every 1,000 women screened across a lifetime of screening. That total runs higher than 1,000 because one woman can be called back more than once over decades. Most callbacks end with extra pictures and nothing more. Some go further. With screening every two years, the 10-year chance of having a biopsy that turns out to be benign is about 6.6%.

There is a third way a screening test can mislead, and it is the least intuitive one. Sometimes screening finds cancers that don't matter because they would have gone away on their own or never caused any symptoms. That is called overdiagnosis. The test was not technically wrong. Cancer cells really were there. But finding them did not help, and treating them can cause real harm. USPSTF models estimate about 14 overdiagnosed breast cancers per 1,000 people screened over a lifetime, with estimates across models running from 4 to 37.

Some screening procedures can also cause other problems, such as bleeding. Colon cancer screening with sigmoidoscopy or colonoscopy can cause tears in the lining of the colon.

None of this is an argument against screening. USPSTF still recommends mammography every two years for women aged 40 to 74, a grade B recommendation, meaning the benefits are judged to outweigh the harms. The strongest evidence about screening comes from randomized clinical trials, which is how these tradeoffs were measured in the first place.

Three habits follow from all this. First, treat a callback as a question, not a verdict. Most people called back after a screening test do not have cancer. Second, never let a normal screening result cancel out a symptom. A lump, unusual bleeding, a lasting cough, or weight loss you cannot explain needs its own evaluation, no matter what your last screen showed. That is exactly the trap a false negative sets. Third, ask before you screen what happens if the result is abnormal: what the next test is, how long it takes, and what it involves. Knowing the path in advance removes most of the fear from a callback letter.

This is why it helps to understand a test's benefits and harms and to talk them over with your healthcare provider before deciding whether it is right for you.