Testing: Infection versus Immunity


Gina C Pervall, MD

There are two categories of lab tests for COVID-19: Molecular and Serology. Molecular testing is used to test for active infection and is performed by obtaining respiratory specimens through a nasal swab. Serology testing is used to test for immunity through a blood test that detects antibodies to COVID-19.

Not everyone needs to undergo molecular testing for active COVID-19 infection. The Centers for Disease Control and Prevention (CDC) has guidance that prioritizes testing of individuals suspected of COVID-19 infection (link below). In addition, the states, local health departments, and individual clinicians are able to exercise discretion regarding testing for active COVID-19 infection. If an individual has symptoms including fever, cough, and shortness of breath, they should call their medical provider. They may also visit their local health department’s website for information on local testing.

Serology testing is used to detect antibodies that are specific proteins made in response to infections. Testing positive for COVID-19 antibodies indicates that an individual, whether symptomatic or asymptomatic, was previously infected with COVID-19 and developed an immune response. However, it remains unclear if individuals with COVID-19 antibodies have protective immunity to prevent reinfection.

Currently, the Food and Drug Administration (FDA) has given authorization to only one antibody test.  However, the agency has cleared more than 70 companies to sell tests that are not yet FDA approved. There are a number of variables that affect specificity/reliability of available antibody tests. These variables along with timing of testing could result in false test results.

If the antibody test used does not only target antibodies produced COVID-19, a false positive result may occur. This result means than an individual tested positive because of antibodies to a coronavirus other than COVID-19. A false-negative test result can occur if antibody testing is performed too soon after an individual has been infected with COVID-19. Testing too early before the body has time to develop antibodies to the virus could result in a false-negative test result.

Currently, serology/immunity testing is recommended to detect antibodies to COVID-19 in order to identify people who may have been exposed to the virus or have recovered from the infection. Serology testing should not be used as sole basis to diagnose COVID-19. Serology testing for antibodies and immunity will play a key role in reopening businesses and returning to everyday life. However, the role for using COVID-19 antibody testing to determine that time has not yet been determined.

Read More Testing Patients with Suspected Infection