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.

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