What is Coronavirus “Immunity Testing”? Should I Get Tested?
Immunity tests, also called “serology” or “antibody tests,” detect novel coronavirus antibodies (IgM, IgG) that the body makes in response to COVID-19 infection. Antibody tests are designed to identify whether you have been exposed to the virus by testing a blood sample from a vein (ELISA) or from a finger prick (“lateral flow”) test. Both operate on the same principle, detecting the presence of IgM or IgG antibodies in a patient’s blood by binding them to reactive molecules that show up as a visual positive or negative result. While the ELISA-based tests require going to a lab to have your blood drawn and some processing time, results tell you the quantity of IgG and IgM antibodies made in response to infection. Lateral flow tests could potentially offer a simpler “yes or no” test at home or in the doctor’s office, showing a positive or negative result with the presence or absence of a band (think of a home pregnancy test).
The problem with both kinds of tests is the rate of false-positives. Lateral flow tests have wildly variable false-positive and false-negative results, and are not recommended. Even the more accurate ELISA tests have surprisingly variable manufacturer-reported rates of false-positives, from zero to 9.4%, but few have been independently verified. In an insightful article for The Atlantic, Ed Yong uses one of these tests as an insightful example: “it claims to correctly identify people with those antibodies 93.8 percent of the time. By contrast, it identifies phantom antibodies in 4.4 percent of people who don’t have them. That false-positive rate sounds acceptably low. It’s not. Let’s assume 5 percent of the U.S. has been infected so far. Among 1,000 people, the test would correctly identify antibodies in 47 of the 50 people who had them. But it would also wrongly spot antibodies in 42 of the 950 people without them. The number of true positives and false positives would be almost equal. In this scenario, if you were told you had coronavirus antibodies, your odds of actually having them would be little better than a coin toss.”
We worry that patients who receive a positive antibody test could feel more comfortable being less careful with social distancing, compromising protection of at-risk loved ones or themselves.
With all of this in mind, if you are interested in knowing whether you have positive antibodies to COVID-19, these options seem best right now:
Stanford serology test: You need an order from your doctor for this test and it is drawn at a Stanford lab draw center. Give us a call or email us if you are a patient of ours and interested in serology testing and we can discuss.
Quest Diagnostics serology test: You can order this yourself and schedule online for an appointment at a Quest Diagnostics lab draw site.
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(Sky Pittson, MD, August 12, 2020)