What Are False-Positive Test Results, and What Causes Them?

Medically reviewed by Robert J. Mayer, MD

Tests for the presence of cancer are far from infallible. Sometimes they fail to detect an existing cancer – a false-negative result – but a far more common problem is a false-positive result. These are findings that suggest cancer is present when, in reality, it isn’t.

A false-positive test suggests cancer is present when, in reality, it isn’t.
A false-positive test suggests cancer is present when, in reality, it isn’t.

False-positive test results cause patients needless anxiety and worry. They also lead to additional testing, which may include invasive procedures such as biopsies that carry their own risks, and unnecessary costs – all to reveal there is no cancer present after all.

Two kinds of tests can generate false positives —diagnostic tests and screening tests. Diagnostic tests, such as blood studies, X-rays, and CT scans, are used when symptoms suggest the possibility of a specific cancer. Causes of erroneous results include taking antibiotics or certain drugs, infections, rare antibodies in the patient’s blood, or mishandling of laboratory specimens.

An unusual false positive case in 2012 involved a woman who was told following a PET-CT scan that her cervical cancer had spread to lymph nodes in her thighs and buttocks. Surgeons, however, found that the “cancer cells” spotted by the scan were actually bits of pigment deposited in her lymph nodes from extensive tattoos that covered most of her legs and thighs.

Screening tests aimed at early detection in people not suspected of having cancer include mammograms, Pap smears, PSA (prostate-specific antigen) testing for prostate cancer, and chest X-rays of people with a history of smoking. Suspicious shadows on mammograms may be harmless cysts. An abnormal Pap smear may be due to an infection, and an enlarged prostate can raise levels of PSA.

Imaging scans may spot abnormalities that when examined further turn out to be tissue scarring in the lungs, calcifications in lymph nodes, or harmless cysts in the liver and kidneys.

A 2009 study found that the more cancer screening tests people experienced, the more likely they were to have one or more false-positive results. The researchers analyzed data from the Prostate, Lung, Colorectal, and Ovarian Cancer Screening trial, in which 68,436 participants ages 55 to 74 years underwent as many as 14 tests over a three-year period. They included cancer antigen 125 blood tests and transvaginal ultrasound exams to screen for ovarian cancer in women; PSA tests and digital rectal exams for prostate cancer in men; and sigmoidoscopies and chest X-rays in both groups.

The risk of a false positive result after four screening tests was about 37 percent for men and 26 percent for women; by the 14th test, the risk was 60 percent for men and 49 percent for women. These numbers don’t mean the tests aren’t beneficial, but they highlight the need to consider their risks versus benefits for individual patients, the researchers said.

Learn more about cancer screening from the Center for Cancer Genetics and Prevention at Dana-Farber.