False Negative Rates of Coronavirus Tests - Info from Harvard Medical School

What are the chances that a coronavirus test will tell me I am not infected when I actually am?

The chances that a coronavirus test will give you a false negative (indicating that you are not infected when you actually are infected) depend upon the type of test you have and when in the course of your infection the test is performed. There are two main types of tests:

• nasal/throat swab tests and saliva tests, both of which detect the virus itself
• blood tests that detect antibodies that your immune system produces in response to the infection.

If you get the nasal/throat swab or saliva test, you will get a false negative test result:

• 100% of the time on the day you are exposed to the virus. (There are so few viral particles in your nose or saliva so soon after infection that the test cannot detect them.)
• About 40% of the time if you are tested four days after exposure to the virus.
• About 20% of the time if you develop symptoms and are tested three days after those symptoms started.

This possibility of a false negative test result is why anyone who has symptoms that could be due to COVID-19, or has been exposed to someone known to be infected, must isolate even if they test negative for coronavirus.

The blood antibody test does not become positive (or might never be positive in some people) until many days after exposure, and is therefore not the primary test used for diagnosis. It is very useful for research and public health decision making.

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