What is specificity and sensitivity in COVID testing?
Whenever a new test is evaluated for accuracy it is compared to a reference test that is considered to be the “gold standard”. In the case of evaluating COVID antigen tests, the reference test is a laboratory PCR test. PCR tests (the tests that require either a deep nasal or throat swab or a saliva sample), are considered to be the most accurate because they look for and identify genetic material that comes only from the virus.
To evaluate a new antigen test, a number of tests are performed with the new antigen test as well as with a PCR test. The results of the PCR test are considered to be “true” results. The evaluation compares how well the results of a new antigen test agree with the PCR results in one of four categories:
- True Positive: antigen result is positive and the PCR test is positive
- True Negative: antigen result is negative and the PCR test is negative
- False Positive: antigen result is positive and the PCR test is negative
- False Negative: antigen result is negative and the PCR test is positive
Sensitivity is a measure of how well the comparison test can detect true positive COVID results. The higher the number of “true positive” results, the higher the sensitivity. Current studies seem to show that the antigen tests are approximately 80% sensitive, meaning that they get “true positives” about 80% of the time.
Specificity is a measure of how well the comparison test can detect true negative results. Current data shows that antigen tests are almost as specific as PCR tests, which means they produce few false positives.
Overall “accuracy” is a combination of the measures of sensitivity and specificity. The most accurate tests have very few false negatives (meaning they are highly sensitive) and very few false positives (meaning they are highly specific).