Accuracy, Sensitivity and Specificity of COVID Testing
In previous blog articles we have discussed how the accuracy of drug test kits is measured. We’ve discussed the concepts of specificity and sensitivity and how they relate to overall accuracy. Given the current intense interest in COVID testing, including the newer antigen tests, we thought it would be a good time to review these concepts as they apply to COVID tests.
Evaluating COVID Tests
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 and Specificity
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.
Accuracy Defined
Overall “accuracy” is a combination of the measures of sensitivity and specificity. The most accurate tests have very few false negatives (they are highly sensitive) and very few false positives (they are highly specific). PCR tests are a good example of a test that is highly sensitive and highly specific. Antigen tests score high on specificity (few false positives), but not as high on sensitivity (more false negatives than PCR tests), so they are not quite as accurate as PCR tests. But knowing that the antigen tests may miss some positive tests, repeated antigen testing can help overcome this shortcoming.
Assaf Shouval says
How taking multiple tests can increase the accuracy? Does it have to be taken with different conditions (time, other saliva samples etc..) or is it by pure chance?
“But knowing that the antigen tests may miss some positive tests, repeated antigen testing can help overcome this shortcoming.”
Jack Singleton says
You raise a good question about how repeated antigen testing can increase the accuracy of the tests. Because the Covid viral loads in an individual change over time, the CDC has suggested repeating the tests at least twice a week if an initial test is negative. An individual with a low viral load may be negative on the first test, but as the infection grows and the viral load rises a subsequent test is likely to be positive. Some researchers also suggest that there is a small variability in how the Point of Care antigen tests perform in detecting low levels of the virus. They suggest that performing more than one test at the same time can give a better chance that one of the tests will detect a positive result at low levels.