There are 3 kinds of tests for SARS-CoV-2 under the FDA’s Emergency Use Authorization (EUA): Molecular, Antibody, and Antigen testing.
Molecular test — RT-PCR (Reverse transcription polymerase chain reaction): This is a molecular in vitro diagnostic test that identifies the presence of virus in the body through collection of nasopharyngeal, oropharyngeal, or other respiratory specimens by swabbing.
A false-negative result may occur if:
- a specimen is improperly collected, transported or handled
- inhibitors are present in the specimen
- there are too few virus particles present in the specimen.
Detecting the presence of the virus doesn’t indicate whether or not someone is infectious.
A single negative result does not mean you can skip social distancing or stop wearing a mask.
A negative test result only means that you didn’t have COVID-19 at the time of testing. It’s possible your sample was collected too early in your infection.
You could also be exposed to COVID-19 after the test and then get infected and spread the virus to others.
If you develop symptoms later, you may need another test to determine if you are infected with the virus that causes COVID-19.
Antibody Test — Serology: Blood test with a turn-around time of typically same day, but sometimes can take a couple days. Serology is especially important because it may detect previous infections in people who had few or no symptoms.
Antibodies are proteins made by the immune system in response to a foreign substance (called an antigen). Each antibody recognizes a small portion of a single antigen (antibody specificity) so an antibody that recognizes the virus that causes chickenpox will not recognize the virus that causes measles.
There are 5 different types, or classes, of antibody: IgG, IgM, IgA, IgE, and IgD. Most rapid novel coronavirus tests look for IgG and IgM. After an infection, your body starts to produce IgM antibodies first, which can be detected after about a week and hang around for a couple weeks after your symptoms start. IgG antibodies are part of your immune system’s long-term arsenal, detected after 2 weeks from the start of symptoms, peaking around 30 days, and hang out for months or even years after you recover.
IgM antibodies: Produced first, detected ~ 1 week after start of symptoms. Because it is only detectable for a couple weeks, it is used to determine recent infection.
IgG antibodies: Produced later, detected ~ 2 weeks after start of symptoms. Because it can be detected for months or years after infection recovery , it is used to determine past infection.
In general, there are two kinds of antibody test methods: binding antibody or neutralizing antibody tests.
Binding antibody tests: These tests use purified proteins of SARS-CoV-2, not live virus, and can be performed in lower biosafety level laboratories (e.g., BSL-2).
Point-of-care (POC) tests usually use lateral flow to detect IgG, IgG and IgM, or total antibody in whole blood, serum, plasma, and/or saliva. A common example of a lateral flow device is a home pregnancy test. Some point-of-care tests can be performed on blood collected from a fingerstick rather than a blood draw.
Laboratory tests typically use ELISA (enzyme-linked immunosorbent assay) or CIA (chemiluminescent immunoassay) to detect IgG, IgM, and IgA antibodies, either individually or as total antibody.
Neutralizing antibody tests: As of July 2020, the FDA has not authorized the use of neutralization tests for SARS-CoV-2 yet. In this test, a serum or plasma sample is infected with live virus in a test tube to determine if the antibodies are functionally capable of preventing infection. Because this method involves the live virus, it requires a higher biosafety level (BSL-3 or BSL-2 laboratory).
Virus neutralization tests (VNT): Here, the SARS-CoV-2 virus comes from a clinical isolate or is modified for the assay. This testing must be done in a BSL-3 laboratory and can take up to 5 days.
Pseudovirus neutralization tests (pVNT): This test uses a non-SARS-CoV-2 modified virus (e.g. vesicular stomatitis virus, VSV) that has a piece of protein from SARS-CoV-2 attached to it. This testing can be performed in a BSL-2 laboratory, depending on the particular strain.
Unfortunately, just because we can detect them doesn’t automatically mean that they can provide immunity/protection from reinfection.
Antibody testing results shouldn’t be used to decide if someone can return to work. It also shouldn’t be used to group people together in settings such as schools, dormitories, and correctional facilities that increase the risk of transmission.
Antigen test — Rapid: Rapid diagnostic test with a turn-around time of less than one hour. Antigen tests look for specific proteins on the surface of the virus and can diagnose an active infection.
Unlike antibody tests that detect your immune system’s response to the virus, antigen tests detect viral proteins directly. The two major antigenic targets of of the SARS-CoV-2 virus are the spike glycoprotein (S) and the nucleocapsid phosphoprotein (N).
Depending on the complexity of the method, these tests can be performed in <30 minutes or may take a couple of hours.
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Interim Guidelines for COVID-19 Antibody Testing (CDC, updated 8/1/2020)
Great Video Explanation of Antibodies & Serology Tests (Beckman Coulter, 5/22/2020)