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Defining COVID-19 Terms: Test Sensitivity

September 25, 2020

Author

Elizabeth (Izzy) Montgomery, MPA
Policy Analyst
501-526-2244
efmontgomery@achi.net

 

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In this installment in our series explaining key terms and phrases used by public health officials in discussions of the COVID-19 pandemic, we look at the term “test sensitivity.”

For medical diagnostic tools, such as COVID-19 tests, it is important to gauge their ability to accurately identify whether a person is infected with the virus. One key aspect of diagnostic testing is the test’s sensitivity.

In medical testing, sensitivity measures how often a test generates a positive result for people who actually have a certain condition. For example, if 100 people are tested and 50 are infected with COVID-19, a test that is 100% sensitive would identify all 50 of the infected people. However, if a test only identifies 25 of the 50 infected people, it is only 50% sensitive and misses half of the infected people. Those who are infected but nonetheless have a negative test result are known as false negatives, since they are infected but the test failed to identify them as positive.

In COVID-19 testing, PCR tests (a type of molecular test) are almost always able to identify a person who has been infected for more than a few days, and therefore the tests are considered to be very sensitive. Antigen tests, another type of diagnostic test that yields faster results, are not as reliable as PCR tests at identifying everyone who is infected, making them less sensitive. Importantly, though, if a positive result is obtained with either a PCR or antigen test, there is high degree of certainty that the individual is indeed positive for COVID-19 infection.