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Can we trust antibody tests to tell who has been infected and who hasn't?


by Health DeskPublished on June 7, 2020Updated on November 8, 2021

What our experts say

An antibody test is a tool that can provide information about whether a person currently has antibodies against the COVID-19 virus in their blood. Antibodies can be a result of two things:

  1. A previous infection with SARS-CoV-2 the virus that causes COVID-19
  2. Vaccination against COVID-19

According to the United States Center for Disease Control and Prevention, antibody tests should not be generally used for diagnosing a COVID-19 infection, because the body can take up to three weeks to produce antibodies in response to a new infection. For this reason, the presence of antibodies is a good measure of protection and is generally not reliable for diagnosis.

Individuals who have antibodies in their blood from vaccination or a previous infection are more protected against developing severe disease, hospitalization and death compared to individuals who do not have these antibodies. However, this is not an absolute truth. We still need more research to answer questions like “which type of antibody provides reliable immunity against COVID-19 infection?” and “At what level should these antibodies be present in a person to provide immunity?”. Not knowing the answers to these questions makes information provided by antibody testing less actionable. More research is needed in order to establish a concrete relationship between antibodies and protection against new COVID-19 infections.

Additionally, the U.S. Food and Drug Administration does not currently recommend that healthcare providers use antibody testing to assess the level of immunity against COVID-19 after an individual has been vaccinated. This is because the available antibody tests have not yet been evaluated for this purpose. Available tests are also not standardized, which means two people getting an antibody test may not get the same test. In other words, their blood may be examined in different ways. Standardization will need to be done in order to have more reliable testing on a national level, as have been done previously for other infections such as Measles.

Currently, viral tests are used to diagnose whether someone has a current COVID-19 infection. There are two types of tests available, that work by either detecting the genetic material of the virus (NAATS, an example of which is the Polymerase Chain Reaction test, or PCR) or by detecting a specific part of the virus that is threatening to the body and is referred to as “antigen” (antigen tests)

Any person who has symptoms of COVID-19 or a suspected recent exposure will need a viral test to examine whether they have a current infection or not. This testing is recommended regardless of whether they have antibodies or not. Until they get the results they should maintain social distancing to minimize the risk of transmission to others.

Context and background

While COVID-19 diagnostic tests determine whether or not a person is currently infected with the virus, antibody tests are used to determine whether or not a person was infected with COVID-19 in the past. This occurs through a process in the immune system whereby the body develops a type of protein called 'antibodies' in response to the presence of a foreign object threatening the body, also called an "antigen", which in this case is the virus. The antibodies latch onto antigens and then work to remove them from the body. Antibodies are detected in the blood of people who are tested after infection; they show the body’s efforts to fight off a specific infection. This is why antibody tests can determine whether or not a person has had a specific virus.

In comparison, tests for active cases of the virus (such as a polymerase chain reaction (PCR) test) looks for the actual genes of the virus itself in a person. This means that COVID-19 has to be active and have a large enough amount infecting in a person in order for the test to be accurate. One difference between these tests is the fact that antibody tests look for prior infection from a virus using blood, whereas PCR-based tests are run on saliva or mucus samples to look for the virus's genetic material and diagnose people who are currently infected with the novel coronavirus.

Antibody tests are now available in many parts of the world, and they can play an important role in understanding COVID-19 trends and identifying groups at higher risk for infection.  Diagnostic tests are never 100% accurate. Sometimes, a test will produce false positives (when you test positive for the virus, but don't have it) and false negatives (when you test negative for the virus, but you really have been infected). The antibody tests for COVID-19 are similar. To be able to trust the results of an antibody test, we must look at its ability to detect the presence of COVID-19 antibodies when they are present, and its ability to confirm the absence COVID-19 antibodies when they are truly absent.

In the context of the current pandemic, it is important for us to reduce the amount of false positive test results that can occur by choosing a test that is highly specific. This means we must choose a test whose ability to correctly identify those without the disease is as accurate as possible, so that a negative test result truly means that person has not been infected with COVID-19. This is because an individual with a false positive test result may be mistakenly reassured that they are safe, and may pay less attention to basic prevention (e.g., social distancing). In this context, the U.S. CDC recommends choosing a test with very high specificity, perhaps 99.5% or greater.

Irrespective of where people reside, they should choose the test available on the market with the highest specificity. However, it is important to note that we still don't know exactly what a positive antibody test means in practice. We are not sure yet if a positive antibody test means that the antibodies will give the person immunity (i.e. prevent them from getting sick again with COVID-19); and if they do, how long this protection can last (weeks? months?). Several studies are currently underway.

Used with Permission from Health Desk, a public health hub that explains emerging COVID-19 science.

This article was written and edited by the Tayo editorial desk and has been reviewed by an independent panel of subject matter experts.

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