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Serological Testing and COVID-19

We have already solidified tests to detect COVID-19 through viral tests (which test if you currently have an infection). However, there's another type of test, a serological test, that can assess individuals who have the virus, or who had the virus in the past.

What is Serological Testing?

Serological testing is a blood test that searches the blood for antibodies, focusing on proteins made by the immune system to fight pathogens; yet, to reiterate, this test is not a diagnostic test.

How it works

The serum component of the blood is analyzed, which contains antibodies to specific components of pathogens, called antigens. These antigens are recognized by the immune system as foreign and are targeted by the body's immune response.

Types of Serological Testing

Neutralization Tests

Neutralization tests determine if the patient has active and functional antibodies to the pathogen. It measures how much the patient's antibodies can inhibit viral growth in the lab.

Step by step process demonstrating a neutralization test (Johns Hopkins)

Chemiluminescent Immunoassay (CLIA)

CLIA shows if the patient has antibodies by displaying a fluorescent signal whenever patient antibodies interact with virus proteins.

Step by step process demonstrating the CLIA test (Johns Hopkins)

Enzyme-linked Immunosorbent Assay (ELISA)

This test places the patient's antibodies between reporter antigens and the viral protein in question to detect the active antibodies.

Step by step process demonstrating the ELISA test (Johns Hopkins)

Lateral Flow Assay (LFA)

Also called a Rapid Diagnostic Test (RTD), LFA is a relatively rapid test which takes a sample from the patient and flows the blood across a strip with a target antigen anchored. If the sample contains the specific antibody relevant to that antigen, they form a complex reaction that results in a coloured band on the strip.

Step by step process demonstrating the LFA test (Johns Hopkins)

The Uses of Serological Testing

COVID-19 serology tests are used to detect various antibodies:

  • Immunoglobulin M (IgM)- This antibody is generally a marker of early infection and is usually detected in the first week of infection.

  • Immunoglobulin A (IgA) - Like IgM, this antibody is also a marker of an early infection but it is typically produced by the mucosal surfaces and can be used to diagnose infections that affect that respiratory mucosa

  • Immunoglobulin G (IgG) - Develops later in the infection compared to IgM and IgA and continues to circulate even after IgM and IgA levels drop in the blood

Serological testing uses antigens to detect antibodies specific to SARS-CoV-2. Most of the time, spike (S) and nucleocapsid (N) proteins are used:

  • Spike (S) proteins - This protein is a viral surface protein that binds to host cells during infection

  • Nucleocapsid (N) proteins - This protein is a structural protein that binds and packages viral RNA and is the most abundant protein of SARS-CoV-2

Diagram depicting the S and N protein together (Sigma Aldrich)

For more information, consider checking out this video from The New York Times.


Johns Hopkins. (2020, June 23). Serology testing for COVID-19 [PDF]. Baltimore: Johns

Hopkins Center for Health Security.

Ontario Agency for Health Protection and Promotion (Public health Ontario). Serology testing

and COVID-19 – What We Know So Far. Toronto, ON: Queens’s Printer for Ontario; 2020

O'Connell, K. (2012, June 08). What Is Serology? Retrieved August 20, 2020, from


Featured image courtesy of The New York Times

Article Contributors: Sara Gehlaut, Olivia Ye