False-Positive Test Results: What You Should Know

Medically Reviewed By: David E. Kozono, MD, PhD

Testing is a key part of a medical diagnosis, including cancer. Different kinds of tests and screenings are used to diagnose cancer and monitor it as a patient is treated.

Test results can have two different outcomes: positive or negative. A positive test result indicates the condition being looked for is or may be present, or that something abnormal was discovered. A test is negative when the condition is not found.

Tests are not 100 percent accurate and can sometimes report incorrect results. An incorrect result is called a false positive test if it incorrectly reports the presence of a condition or abnormality, or a false negative if it incorrectly reports the absence of a condition.

It is important to remember that a false positive result is not necessarily a bad thing: It can simply indicate that something abnormal was found, which can be further investigated through other tests.

Here, David Kozono, MD, PhD, a radiation oncologist at Dana-Farber, answers some of the most common questions about false positive tests.

Are false positive results common?

In short, it depends on the type of test. Screening tests and confirmatory tests — tests that confirm a finding or diagnosis — have different parameters, so in turn, have different false positive statistics.

Screening tests are intended to look for something. For example, mammograms are used to screen for breast cancer. They are usually fairly sensitive tests because they are looking for anything that is unusual. In the case of a mammogram, when the scans are studied, anything that looks out the norm is flagged. The mammogram may show that a person has some dense breast tissue in an area, or a cyst, but without further testing, the physician may only see something abnormal — causing them to order further tests. Those additional tests may conclude that the abnormality is only some dense breast tissue and not something to worry about.

While that initial false positive can cause inconvenience because of the extra tests, it is also better to confirm that the issue is harmless, rather than worry that it might be something more.

Can a biopsy give false positive results?

The likelihood that a biopsy test will result in a false positive is slim. Confirmatory tests, like biopsies, are used after screenings in order to confirm the identity of what an initial screening might show. These tests are highly specific, and are therefore less likely to result in false positive results. A physician wants a clear answer as to what the biopsy says.

How will I know if I get a false positive result?

You will know because further testing into the issue initially discovered will reveal that it was not what was being tested for. Let’s say in a hypothetical scenario that a mammogram shows something abnormal in the left breast: In actuality, that abnormal area is a benign lump, but the physician may not be entirely sure of that without further investigation. A biopsy reveals that the lump is not cancer, confirming that the mammogram gave a false positive.

If that mammogram were the only test that was used in a diagnosis, the patient could have been given unnecessary treatment for cancer that they didn’t have.

False positive test results do happen because tests cannot be 100 percent accurate, but they are not something to be overly worried about, Kozono stresses. Your doctors will know when to follow up and when to let things be. Their job is to keep you as healthy as possible, so they will do everything they can to get you the proper care and treatment.

About the Medical Reviewer

David E. Kozono, MD, PhD

David Kozono, MD, PhD, is a board certified radiation oncologist who specializes in the treatment of thoracic malignancies including lung cancer. His career goal is to improve treatment for patients with these cancers through basic, translational and clinical investigation. He graduated in 2005 from the Johns Hopkins University School of Medicine with an MD and a PhD in Biochemistry, Cellular and Molecular Biology, which he obtained studying under Nobel Laureate Dr. Peter Agre. In 2006, he completed an internship in internal medicine at Brigham and Women’s Hospital, and in 2010, he graduated from the Harvard Radiation Oncology Program. Upon graduation, he joined the Department of Radiation Oncology at the Dana-Farber/Brigham and Women’s Cancer Center, where he is a full-time clinician and researcher.