May 05, 2020
There are a lot of mixed messages about Covid-19 antibody testing circulating out there. We hear from various sources in the press that antibody tests are inaccurate, or that they are meaningless. Even official government health departments are sometimes unenthusiastic about antibody testing, for various reasons - or even for no given reason at all!
Considering all these different opinions, it’s worthwhile reviewing again why these tests can be important and extremely worthwhile for some of us.
1. Accuracy. There are many Covid antibody tests out there now, with varying degrees of accuracy. News reports and some health departments focus their criticism on lower quality tests, unfortunately giving the impression that ALL the antibody tests are suspect. This is not the case. The lab we use for our tests is Quest Diagnostics, a nationwide reference lab with state of the art facilities and equipment. There documented accuracy rate is 99.5-100% for positive antibody results (meaning the patient has been infected with Covid-19 in the past); this accuracy has been confirmed by outside researchers as well (google it if you like). Their accuracy for negative results is about 90%, which would mean you had not been infected. The accuracy is lower for a variety of reasons, the main one being that some people simply don’t produce a lot of antibodies in response to the infection, therefore the test can’t detect them. This rate of accuracy for antibody tests is considered to be quite good.
2. What does a positive result mean? Am I immune? A positive result means you have been infected with the Covid-19 virus sometime within the past few months, and your body’s immune system has produced antibodies against that virus. Despite what officials from the World Health Organization and even CDC are cautioning, you are probably immune from re-infection for some period of time. One of the reasons these big health agencies are beating around the bush about immunity with antibody levels is that we don’t know exactly what percentage of people will be fully immune, partly immune, or not immune at all, and we don’t know how long the immunity lasts. Typically for younger healthy people with good immune systems, antibody responses to most viruses give immunity for at least several months. but we just don’t know for certain with Covid-19.
3. What does a negative result mean? It means it’s very unlikely you were infected with Covid-19. And if you were, your antibody levels are so low the test couldn’t detect them. If you had an illness very recently (symptoms beginning 2 weeks earlier) and you are still suspicious that it was Covid-19, you may consider repeating the antibody test at the 4-6 week mark, when the antibodies would be at their peak levels. But the most important point here is that you’re still fully susceptible to becoming infected with COVID-19. If you have a negative test result, you have no immunity to it.
4. Perhaps the most important aspect of antibody testing is the information it gives us about a community. You have probably read about testing in New York City, New York state, and Los Angeles county. Those antibody testing programs have revealed that many, many more people have been infected with COVID-19 than previously known. This seems to be because most cases of Covid-19 have mild symptoms or no symptoms at all. For our community in northern Michigan, we have performed nearly 500 tests now, And have results on almost 400. Out of those 461, we’ve had 26 positive antibody results, which is less than 6%. Because these tests aren’t conducted randomly, the results aren’t necessarily representative of the entire community. But certainly they show that there are many, many more Covid-19 cases than the 21 reported in Emmet County, the 13 in Charlevoix County, and the 19 in Grand Traverse County. This of course means the fatality rate of the disease is much lower also, and that is crucial information for decision makers (or is there only one decision maker?) in Michigan who are determining when and to what degree our region is able to reopen.