False-Positive / False-Negative

False-positive means the test result came out “positive,” indicating you have that condition, but it was wrong.  Vice versa with false-negative: it looks like you don’t have the disease, but the test was wrong, so you really do.  It’s not that the lab or radiologist made a mistake with the blood test or x-ray.  Rather, many tests are simply not 100% accurate, through nobody’s fault.  So when a test is “positive” or “negative,” we clinicians to use our judgment, & consider all information.

We also use the terms “Sensitivity” and “Specificity” in this regard. A test that’s very sensitive (high Sensitivity) has very few false-negative results. So if your result is indeed negative, you can feel comfortable knowing you don’t have that disease. Analogously, a test that’s very specific (high Specificity) has very few false-positive results. So if yours come out positive, it’s highly likely that you do, indeed, have that condition.

Sometimes it’s better not to do tests!ย  For example, a person has joint pains for a few weeks. The most likely cause is one of many viruses, which will go away on its own within 1-2 months. But say we seek a diagnosis like Rheumatoid Arthritis, an order a blood test for it (Rheumatoid Factor antibody), which lo-and-behold comes back “positive.” Well, many people have Rheumatoid Factor in their blood and will never get the disease (even up to 5-10% of the elderly). But now we’ll label our patient with a chronic illness they don’t have, which may eventually result in prescriptions for unnecessary medications, some of which may be dangerous.

There’s also a very interesting statistical fatal flaw to the idea, “Let’s do the test ‘just in case’.” See topic Why Not to Test People for Unlikely Conditions.

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