Q: When discussing the quick antibody tests, can you explain the difference between IgM and IgG antibodies?

A: Immunoglobulin M (IgM) is the first antibody the body produces when it encounters a threat of infection—they stick to the virus and help to neutralize it. As part of the adaptive immune response, the body adapts to the threat and starts to induce little mutations in the areas that give antibodies their binding specificity. When this happens they begin to bind more tightly to the threat, and switch their classes from IgM antibodies to IgG antibodies.

IgG antibodies have very good specificity, but it takes much longer for the body to begin producing IgG antibodies than IgM antibodies. When we detect IgM antibodies on the test, it tells us that the patient has recently been exposed. But as time goes on, the patient will start to develop an IgG response.

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These tests have very limited clinical use.  The need for antibody testing would be to determine if a patient has recovered from COVID-19, and whether we could take a sample of the patient’s plasma to be used as convalescent plasma, or donor plasma, to treat acutely ill patients.

Before we can do that, however, the tests have to be better validated because there’s a potential for cross reactivity from other seasonal respiratory viruses, including other coronaviruses. This seems to be more of an issue with IgM antibodies, where the cross reactivity seems to be higher.

The time component is so important here, because if you test somebody that’s on day 5 of a COVID-19 infection, they are unlikely to be mounting a significant IgG or IgM response. It’s not until about the 20th day, or at least after the second week, that one see these numbers go up to reliably expect positive test results.

And it’s worth mentioning that we don’t know how protective these antibodies will be on re-exposure. While we would hope that the immunity would be long-term, looking at other human coronavirus infections, our immunity may wane over time.

A: If a NP/PA has a patient who has a negative RT-PCR test, but whom you suspect of having COVID-19, what do you recommend?

Q: In most cases, we will repeat the RT-PCR test on the assumption that the original test was conducted too soon or got contaminated. Adding the PCR antibody testing could tell us a little more about where in the course of the virus the patient is. But in terms of acute infection, the antibody testing is not going to give us the information we need to respond in a clinically appropriate amount of time.

Remember that the RT-PCR test is just one part of how to diagnose and treat a patient with suspected COVID-19. It is important to look at the rest of the patient’s laboratory results and their images.  No test is 100%, so we always have to keep in mind our pre-test probability that the patient had COVID— before I ordered this test how likely did I think this patient has COVID?

If I am convinced that my patient has COVID and the test comes back negative, we will leave that patient in isolation until we can repeat that RT-PCR test, just to make sure we’re not exposing our staff and other patients to this infection. As mentioned, the test is most accurate when given within the first 10 days of symptoms. Typically, I would recommend to re-test after a minimum of 24 hours but not delay more than 72 hours.

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A: What test would you recommend for community screening purposes?

Q: The antibody test is actually well suited for this because it can help determine how prevalent the virus is in our community.  We know that there are people that have been infected with COVID-19 and had mild symptoms, or no symptoms at all. They have completely recovered and so we could start to see, for example, that 20%, 40%, or 60% of our community already had COVID-19. The antibody test is much better suited to answer those types of questions.

Q: Is it realistic to be able to test all students and employees for COVID-19?

A: Perhaps, but there is a big manufacturing bottle-neck in an environment that is not necessarily coordinated for major screening programs. If we could get a sufficient number of tests, I think that wide-spread testing is going to be part of the answer on how we safely open back up.

Jonathan Ives, is a pulmonary/critical care PA-C with Texas Pulmonary and Critical Care, which provides critical care coverage at a large community hospital in Fort Worth, Texas. Mr Ives is also an adjunct faculty member at University of North Texas, Health Science Center, Physician Assistant Studies, where he lectures on a wide variety of topics.