COVID-19 Antibody testing information (as of 05/01/2020)
For weeks it seems, we have eagerly been awaiting the availability of antibody tests for
COVID-19. It was thought that with widespread antibody tests, we would have a better sense of how many have been exposed or infected with the disease, and more importantly how many are now immune. We, as providers, have heard numerous times, “I was so sick in January, I know it was COVID-19 and that I’m immune.”
Unfortunately, the reality is that antibody testing may not be very useful.
A few facts about the tests
Only 4 tests have been approved by the FDA. None of those are rapid, in-office tests. Many of the tests are actually very poor. For example, the advertised sensitivity of one test is pretty good, but the validation of the test was only conducted on <70 patients. Not great.
Typically, after a person is exposed to a disease, they will form antibodies in their blood. The earliest (and less durable) antibody formed is the IgM. IgG will then form. IgG is the more durable antibody response. For COVID-19, it appears that IgM forms about 7 days after infection, and IgG appears 14 days after infection.
Even if exposed, a negative test can occur if a patient cannot develop a strong enough immune response to create antibodies.
A false positive test can occur if the test cross-reacts with another similar molecule, for example, a different coronavirus.
Many people are assuming that if their IgG antibody test is positive, they are now immune to COVID-19.
We do not know if people with COVID-19 IgG are actually immune.
1. Even if having IgG means immunity, how much?
2. What level of titer (concentration of antibody) is necessary for immunity?
Even if we knew the answers to these questions, the test would not help as it is a simple “yes or no” test.
3. We also do not know how long any immunity would last. Immunity to SARS-CoV and MERS-CoV declines after about a year. However, people can get infected with the common coronaviruses (ie the common cold) repeatedly through a winter.
We don’t know where COVID-19 is on this spectrum.
The PCR (swab) virus test is used to test for current infection with COVID-19. The antibody test (blood test) can tell us whether or not a person has been exposed to COVID-19. It cannot guarantee immunity. The very real risk is that people test positive and assume they are immune and thus safe. They may then place themselves and others at risk by stopping necessary precautions and safe distancing.