What You Should Know About False Positive Test Results

False positive refers to a test result that tells you a disease or condition is present, when in reality, there is no disease. A false positive result is an error, which means the result is not giving you the correct information. As an example of a false positive, suppose a blood test is designed to detect colon cancer. The test results come back saying a person has colon cancer when he actually does not have this disease. This is a false positive.

Also Known As: type I error, alpha error

Doctor talking to patient in office
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Why a Test Might Give a False Positive Result

There are a variety of reasons why a test will have a false positive result. Some are due to the limitations of the test itself; others are due to mishandling or medical error.

  • Non-specific Results: An example of a non-specific result is a positive guaiac fecal occult blood (FOB) test. It is positive when there is blood in the stool. That can be a sign of colon cancer. But it may also mean that you have hemorrhoids, inflammatory bowel disease, or bleeding ulcers. Your healthcare provider may order further tests to rule out colon cancer and diagnose these other conditions. If there is no colon cancer, the fecal occult blood test might be said to be a false positive for that condition. Because of the many different reasons the FOB can be positive, it is used as a screening test rather than a diagnostic test.
  • Cross-reactions: In this case, a test is positive because it is reacting to something other than what it is designed to detect. In the case of the guaiac FOB test, it can show a positive result when there isn't blood in the stool, but you have recently consumed a large amount of red meat, broccoli, horseradish, oranges, and a few other foods. It also may react with colchicine, iodine or boric acid. When the healthcare provider suspects there is a false positive for this reason, a repeat test or a different test may be performed.
  • Problems in specimen collection, handling, and processing: Lab tests have specific handling requirements. If something goes wrong anywhere in the process from blood being drawn, transported to the lab, processed, sampled, and analyzed, it can produce a false positive or a false negative result. For example, a urine sample collected to look for a urinary tract infection may show a false positive if it isn't collected cleanly and kept refrigerated. Those conditions allow it to be contaminated with skin or vaginal bacteria and allow them to multiply, making it appear that the bladder is infected. Your healthcare provider and the lab may suspect mishandling if these results don't match other tests or your symptoms.
  • Mix-ups in specimen identification and reporting: A human error has a large role in medical errors. Your healthcare provider should suspect that there has been a mix-up if your test results don't match your other symptoms. They could be from someone else entirely. Imaging results can be misidentified as well as lab results. The lab and your healthcare provider will usually compare your current results to previous tests and if there is a discrepancy they may suspect a mix-up.

Questioning Your Lab Results

Your healthcare provider will normally make a diagnosis based not only on lab results but also on your physical exam, history, symptoms, imaging and biopsy analysis. If a test doesn't match up with what those factors are signaling, it should be repeated or further tests done.

As a patient, you should ask questions to clarify what your test results mean and whether there are other interpretations. Getting a second opinion or asking whether a test should be repeated or further diagnostic tests performed is within your rights as a patient.

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  • Fecal Occult Blood Test, LabTestsOnline.org, American Association for Clinical Chemistry, October 30, 2015.

  • False-Positive, American Association for Clinical Chemistry.

By Suzanne Dixon, MPH, RD
Suzanne Dixon, MPH, MS, RDN, is an award-winning registered dietitian and epidemiologist, as well as an expert in cancer prevention and management.