Tests for viral infections are right most of the time, but they are not perfect. When you are tested for COVID-19, there are four possible outcomes: two where the results are correct (“true” results) and two where they are incorrect (“false” results).
- True positive result. You are infected, and the test says correctly that you are infected.
- True negative result. You are not infected, and the test says correctly that you are not infected.
- False positive result. You are not infected, but the test says incorrectly that you are infected.
- False negative result. You are infected, but the test says incorrectly that you are not infected.
What can go wrong?
- There’s not enough of the thing the test is supposed to detect. For a viral test, that means there’s not enough virus in your body. Or not enough virus made it onto the swab. For a serological (antibody) test, that means there’s not enough antibodies in your blood.
- The timing isn’t right. Some tests work better if they’re done right away. Other tests work better if more time has passed since the infection.
- Mistakes. Sometimes the samples get contaminated. And sometimes people make errors when running the test. Fortunately, there are lots of procedures in place to keep this kind of thing to a minimum.
Tests that work the best have high numbers of true positives and true negatives. So scientists do their best to develop tests that reduce the number of incorrect results, but no test will be perfect.
Last update: April 24, 2020, 9:53 AM
Science review: ERS, JAB