A Closer Look at COVID-19 Serology Tests

You probably know someone who was exposed to COVID-19 and maybe even had a cough, fever and a bad sore throat for a couple of days, but just did not test positive on an Antigen Rapid Test (ART).

You'd think it puzzling, remarkable, in fact that someone could possibly be immune to the virus despite having prolonged and close contact with someone infected with it.

So was this person infected with COVID-19, or were the ART test results a series of a false negatives?

Here's one way to put this debate to rest: a COVID-19 serology test. Unlike the ART or Polymerase Chain Reaction (PCR) test, a serology test does not give an indication of whether you are currently infected with COVID-19. Rather, it helps detect whether you've had a past COVID-19 infection by detecting virus antibodies in your bloodstream.

Dr Serene Wee, general practitioner at Parkway Shenton Medical Clinic at Robinson Road shares more about COVID-19 serology tests, how they work, and the various types of tests you can take based on your needs.

What is a COVID-19 serology test?

A COVID-19 serology test involves a blood test, taken to detect a past COVID-19 infection.

But how does that work? We detail the process below.

The COVID-19 serology test detects antibodies

The COVID-19 serology test detects antibodies in the bloodstream to determine the presence of a past COVID-19 infection.

Antibodies fight viruses, like COVID-19

Antibodies are Y-shaped proteins in our bloodstream that are produced by the immune system in response to an antigen or foreign substance, such as bacteria. Antibodies can also be introduced into the body through vaccination.

To fight and eliminate antigens, antibodies attach to them.

Similarly, after being infected by the SARS-CoV-2 virus (COVID-19) and exposed to the antigens from this virus, your body produces antibodies to help fight and eliminate the virus from your body.

Antibodies produced during a past infection remain in the bloodstream, thus detectable during a serology test

Antibodies continue to be present in the bloodstream after recovery from the virus. This helps ensure that if exposed to the same antigen again, like those from COVID-19, the body is prepared to tackle it more efficiently.

Uses of the COVID-19 Serology Test

Undergoing a COVID-19 serology test can help you find out if you had COVID-19 in the past. Here's why that can be useful:

• If a past COVID-19 infection has been detected through the serology test, it is advisable to hold off undergoing a procedure 7 weeks post-infection to prevent complications after surgery.
• A serology test can help diagnose multisystem inflammatory syndrome (MIS-C) in children, since many children with MIS-C have COVID-19 antibodies.
• All individuals* vaccinated overseas will need to verify their vaccination records. A serology test can help you do so, unless you present a digitally verifiable vaccination certificate exempting you from taking it.
  • Including Singapore Citizens, Singapore Permanent Residents, Long-Term Pass Holders, and Short-Term Pass Holders staying in Singapore for more than 30 days.

Understanding a COVID-19 serology test result

What does a positive COVID-19 serology test mean?

A positive test would mean there are COVID-19 antibodies detected in the blood sample. This will point to a past COVID-19 infection. It could also be a false-positive test, which could be a result of an inaccurate test or because the antibodies detected are from a closely related virus.

At present, more is still being understood about COVID-19 antibodies and the extent to which they can protect individuals from getting infected with COVID-19.

Hence, a serology test should not be used as an indicator of immunity to being infected or re-infected by COVID-19.

What does a negative COVID-19 serology test mean?

This would mean an absence of COVID-19 antibodies in the blood sample collected, so you may not have been infected by COVID-19 in the past. It could also be a false-negative result, due to a faulty test, or because the test was taken before sufficient antibodies (usually 2 – 3 weeks is required to achieve this) were produced by the body to be picked up during the test.

What are the different types of COVID-19 serology tests?

Here are the 4 types of COVID-19 serology tests available in Singapore

1. SARS-CoV-2 total antibody test

This is a non-specific antibody test which detects IgM (early phase) and IgG (late phase) antibodies, which develop in response to the SARS-CoV-2 virus. IgM and IgG are immunoglobulins or antibodies in our immune system which help eliminate the virus from the body. IgM is produced immediately after an antigen enters the body and dissolves after a few weeks, while IgG is produced some time after IgM, and remains in the bloodstream longer than IgM.

*Mean Titer refers to the concentration of antibodies present in the bloodstream.

2. Anti-SARS-CoV-2 S antibody test

This test detects quantitative antibodies against SARS-CoV-2 spike (S) receptor binding domain. Its main use is to detect and quantify vaccination-induced immune response by providing you with a numerical value of the level of antibodies present in your bloodstream.

3. Anti-SARS-CoV-2 N antibody test

This test detects qualitative antibodies to the nucleocapsid (N) protein of SARS-CoV-2 virus. It detects the presence of antibodies in the bloodstream, rather than the level of antibodies present. Its main use is to detect the body's immune response following a natural infection.

4. cPass SARS-CoV-2 test

This test is intended for the qualitative detection of neutralising antibodies against SARS-CoV-2 virus. Its main use is to aid in identifying individuals with immune response to SARS-CoV-2 post infection or vaccination. A positive result (30% and more) indicates the presence of the SARS-CoV-2 neutralising antibody.

Serology testing can help you understand whether you were previously infected with COVID-19, which can be beneficial for various reasons. That said, it must be noted that this form of testing also has its limitations. Besides the possibility of giving you false test results, a COVID-19 serology test should not serve as an indicator that you are immune from getting re-infected with COVID-19.

It is thus important to continue adopting practices to reduce your risk of infection and prevent infecting others as we continue to live with COVID-19.

To find out more about the serology tests available at our clinics, locate a Parkway Shenton clinic near you and drop us a call to schedule a serology test.