What Are False Positives and False Negatives?

While many of today's medical tests are accurate, false negative or positives do occur. What causes these erroneous results?

A false negative is a test result that indicates a person does not have a disease or condition when the person actually does have it, according to the National Institute of Health (NIH). False negative test results can occur in many different medical tests, from tests for pregnancy , tuberculosis or Lyme disease to tests for the presence of drugs or alcohol in the body.

Correspondingly, a false-positive test result indicates that a person has a specific disease or condition when the person actually does not have it. An example of a false positive is when a particular test designed to detect melanoma, a type of skin cancer , tests positive for the disease, even though the person does not have cancer.

Double-checking

Because tests differ, the reason behind an inaccurate result and the rate at which they happen depend on the test and on the follow-up protocol used to double-check test results.

An example of how testing protocols are designed to catch false readings and double-check test results can be seen in HIV testing. HIV testing is done using two different types of tests: screening and confirmatory, according to the Centers for Disease Control and Prevention (CDC).

The first test is a screening test called the Enzyme-linked immunosorbent assay (ELISA) that determines a person's status based on the presence of HIV antibodies in their blood. If the initial ELISA test is positive, the lab usually repeats the test using the same sample, according to the CDC.

If the both ELISA test results are positive, a confirmatory test (using different laboratory techniques, such as a western blot or an immunofluorescence assay) is conducted. Both initial and confirmatory tests must have reactive, or positive, results in order for a person to be given a positive result.

What causes false positives

Depending on what a person is being tested for, false positives can occur for several reasons. For example, with tests used to diagnose syphilis (such as the Rapid Plasma Reagin or VORL antigen tests), common causes of false positives include acute viral and bacterial illness, pregnancy and drug addition, according to the State of Alaska Health Care Services.

Some vaccinations (such as flu shots) can occasionally cause a person to test positive for the flu when they do not actually have it, but when the test is repeated, the result is negative, according to the CDC.

For example, a study conducted at a Denver emergency department and published in the July 21 issue of the Journal of the American Medical Association (JAMA) showed that 41.7 percent of HIV-negative people who participated in clinical trials for HIV vaccines tested positive on routine HIV tests - even though they were not actually infected. Those rates differed depending on the type of vaccine administered, ranging from 6.3 percent to 86.7 percent.

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