What are the types of COVID-19 tests?
Nasal or oral swab

This is the most common type of test available to determine positive cases of COVID-19. It uses polymerase chain reaction (PCR) testing. Sample is collected by a medical professional using a swab, similar to a Q-tip, to collect mucus from your nasopharyngeal or oropharyngeal passages— located deep inside your nasal and oral cavities in the back of your throat. PCR testing works by directly detecting the presence of the virus, instead of the presence of the body’s immune response or antibodies. However, preliminary data suggest that PCR testing may have a high false-negative reporting rate (up to 30%), which means those who actually have the virus may wrongly receive a negative test result. However, most of the false negative results are due to improper nasal swab application.

Blood test
This type of test looks for antibodies, which are specific blood proteins created by your immune system when it comes into contact with a foreign invader. Antibody testing is the only way to detect if you have previously been exposed to the virus.

What are antibodies?
Antibodies are infection-fighting proteins created by our immune system in response to the characteristics of the infectious agent. Over the course of an infection, our immune system produces different types of antibodies including IgM, which develops early in the immune response, and IgG, which develops later. Because antibodies to a virus are only produced once someone has been exposed to the virus (or to a vaccine, which doesn’t yet exist for COVID-19), the presence of antibodies to a virus is a solid indicator that an individual has been infected. 

What is an antibody test? 
The antibody test is a serology test which measures the amounts of antibodies or proteins present in the blood when the body is responding to a specific infection. However, unlike molecular tests for SARS-CoV-2, antibody tests are more useful as tools for gauzing proportions of the population exposed to the virus. Currently, antibody testing is mistake-prone and clinically unverified and should not be used as sole test for diagnosis. 

What is the difference between IgM antibodies and IgG antibodies? 
IgM antibodies are produced by the body during an active infection, while IgG antibodies come later and typically signal the infection has passed. 

IgG antibodies may also indicate immunity, or resistance to reinfection with the same virus, which is the case for certain other viral diseases such as measles, hepatitis A and polio. It is not yet known whether IgG antibodies are indicative of immunity to COVID-19, but research is ongoing.

How does COVID-19 antibody testing work?
For COVID-19 antibody testing, there are three different subclasses of antibodies that can be used including immunoglobulin A (IgA), immunoglobulin (IgG), and immunoglobulin M (IgM). 

The COVID-19 antibody test works by detecting whether the specific types of antibodies in your blood match those that correlate with the SARS-CoV-2 antigen, the virus responsible for COVID-19. If both IgM and IgG antibodies are still present in your system, it means that you have recently been exposed. If you only have IgG antibodies, it means it’s likely been over two weeks since you were infected. If you don’t exhibit any symptoms for more than 72 hours, it means (likely) you have an inactive infection which is no longer contagious.

A negative antibody result does not rule out SARS-CoV-2 infection, especially in individuals who have been exposed to COVID-19 positive patients. Further testing with a molecular test should be considered to rule out infection. 

Positive results may be due to past or present infection with non-SARS-CoV-2 coronavirus strains, such as coronavirus (eg. HKU1, NL63, OC43, or 229E). 

While the presence of antibodies doesn’t necessarily prevent future infections, they can reduce the severity of future infections of the current COVID-19 strain.

What are “point-of-care” antibody tests? 
Handheld “point-of-care” test kits only require a sample of blood from a pricked finger and results can be produced within minutes. There are over 100 versions of this kind of test being offered globally, and the accuracy of these kits is highly variable. This led the World Health Organization to recommend against using them except for research purposes.

How is antibody testing different from PCR tests?
PCR testing for COVID-19 involves detecting presence of viral nucleic acid or protein and determine whether an active virus is present. On the other hand, antibody testing (also called serology testing) is done with blood samples, looking for evidence of the body’s immune response to the virus. 

An individual’s exposure to a foreign pathogen results in the white blood cells ability to ‘learn’ about it and make antibodies to neutralize it. So, an antibody positive to SARS-CoV-2 result means 1) exposure to SARS-CoV-2 at some point in the past and 2) presence of an immune system robust enough to launch an antibody-forming immune response.

The blood tests check for specific iterations of Immunoglobulin-M (IgM) and Immunoglobulin-G, (IgG), that are created by the immune system in response to Covid-19.

How long does it take to generate these antibodies?
Sufficient data related to antibodies against SARS-CoV2 is currently lacking. While IgM is created quickly as part of the body’s response to infection and dissipates soon, IgG stays in the body and represents memory of past exposure to the virus. Some reports suggest most healthy people start making antibodies 11 to 14 days after symptoms first appear. 

A recent report from China, published in Nature Medicine, suggests that most patients developed SARS-CoV-2 specific antibodies within two to three weeks of their first symptoms. In this study patients developed IgM antibodies starting from day 7 and IgG antibodies after 2 – 3 weeks.

What are the potential benefits of antibody testing?
It can help in detection of PCR-negative cases in patients having other symptoms and also help in identification of people who can be convalescent plasma donors. Antibody testing can guide epidemiologic studies to determine disease prevalence in a community and can also assist in verification of vaccine response in vaccination programs.

What are the potential drawbacks of antibody testing?
Antibody testing’s false negative results can lead to missing carriers of the virus, due to either the fact that individuals have not developed antibodies yet or that the test is inaccurate.

False positivity also risks giving the wrong impression of immune protection, due to either cross-reactivity of antibody with common cold coronaviruses (e.g. HKU1, NL63, OC43, 229E) or a non-specific antibody.

Why should individuals know their antibody status?
The test will let individuals know their antibody status which can tell them if they have been exposed to the virus or if their bodies have cleared the virus. It is critical to understand their own health and slowing spread of the virus. 

The more we get community testing out into the world, the faster we can help slow the spread of the disease. Knowing who and who has not mounted an antibody response is critical to our understanding of disease progression.

By Dr. Zaheed Husain, Ph.D.

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