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Antibody testing for COVID-19 explained

by Dr. Ann Lima, Clearwater Valley Hospital and Clinics

Antibodies are proteins that our immune system develops when it fights off invaders.

When we get exposed to a new infection, such as a virus that we haven't seen before, it takes a little bit of time for our immune system to develop these antibodies. In the meantime, we may get sick from the infection and show signs of illness like fever, cough, diarrhea, or other symptoms. However, after the antibodies get developed, our bodies now have a template for fighting the infection. So, the next time the body gets exposed to this infection, our immune system quickly identifies it, finds the memory cells that we made the first time around, and can knock it out before showing symptoms or spreading it to others.

This is the concept that vaccines are based off of. Vaccines place a piece of the virus or bacteria into our bodies so that we can develop immunity via antibodies without ever getting sick. For most vaccines, this piece of the virus or bacteria is not the actual infectious particle, but enough of the picture that when we get exposed to the real infectious particle it will recognize it.

The antibody test for COVID-19 does not mean you are actively infected with the virus. Antibodies are an indicator of exposure, but not necessarily active disease or active infection. For people that are vaccinated against disease, they will show up positive for antibodies but have never actually been infected by the disease. There are also different types of antibodies, those that show up early in the infection and then go away, and those that stick around to fight off future infections. The IgG test is the antibody to see if you have been exposed to COVID-19, which seems to have shown up in most people after two weeks of having the disease. We have this test available at Clearwater Valley Hospital and Clinics and St. Maries Hospital, and the turnaround time is 1-2 days.

The antibody testing is not 100 percent accurate, and we have identified issues that make interpreting the results hard. There will be some people that test positive that have not actually been exposed. Locally, since we have had so few people with the disease, there may be more false positives than true positives at this point in time. There are also people who have been exposed, and may even have been sick, but fail to develop antibodies. As we test more people for antibodies that we are sure had the virus (those that tested positive with the nasal swab when they were sick), then we can find out how well these tests work. We are not certain whether people that have been infected with other coronaviruses (the more common types that cause a regular cold) will come up positive for this antibody test, and if they do this test is not so helpful.

Historically, if you have been exposed to viruses like this and develop an immune response, you should be immune. But we have not yet proven this in COVID-19. We won't know for sure, until we have a large amount of people that have known antibodies - from exposure, infection or vaccine - and then are re-exposed to the virus to see if the immunity holds up.

If you believe you are actively infected by COVID-19, then the nasal swab is the right test. If you believe you had COVID-19 at least two weeks ago, due to past symptoms, or exposure to an actively infected person, then the antibody test could be done, although there are limitations to interpretation of the test.

Photo: Dr. Ann Lima

Window on the Clearwater
P.O. Box 2444
Orofino, ID 83544
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