A blood test for COVID-19 is as different from a Swab test for the virus as a photograph of a person is from seeing them face to face.
The blood test detects the body’s memory of having seen the virus. The Swab test picks up molecules of the virus itself from your nose and throat.
The blood test tells us that, at some time, your body had the virus. The Swab test shows the virus is still in and on you, and you are contagious to others around you. If your blood test is positive, you may or may not still have the virus. The blood test alone cannot tell you this important detail. Only the Swab test can tell you if you are contagious.
As of right now, the blood test cannot tell you for certain if you are immune to (protected from) COVID-19. We hope that a positive blood test means you are immune; but studies are just being done on this and it is too soon to say. If you are immune, the blood test does not tell us how long you will be immune. Again, studies are just getting underway that will answer this question. It will take months or perhaps a year for an answer. So, don’t let your guard down. A positive blood test does NOT mean you can afford to stop social distancing, hand washing, wearing masks and all the other important measures that have helped control this pandemic.
A negative Swab test does give some assurance that you are free of the virus. But, not 100% assurance since false negatives can occur in a small percentage of people. So, if you have a negative test and you do not have COVID symptoms but later develop them, or have a negative test but then your symptoms persist or get worse, you need to be retested.
On the other hand, a positive Swab test is highly reliable and shows you are contagious and that your symptoms (if any) are due to COVID. If your test is positive and you have no symptoms, you are still contagious. You should isolate yourself (see our video on isolation and quarantine on the Health Department website) and get advice from your healthcare practitioner.
Some people who are infected with COVID-19 are interested in donating their blood to the Red Cross or a medical center for the purpose of it being used in the treatment of severely ill, hospitalized COVID patients. In that case, the COVID blood test is used to determine if you have enough antibodies (the body’s chemical way of remembering infection) to do any good. This use of the blood test is almost the only one that is useful currently.
So, to summarize: You are much more likely to need a COVID-19 Swab test than a blood test. A Swab test for COVID virus allows you to “see” the virus in and on your body “face to face.” The blood tests are like looking at its photograph – you do not know where it is right now or if it is even alive in and on your body.
We need a lot more studies and the good science that will come from them to answer many of the questions that we now have about blood testing and the finer details regarding how COVID-19 behaves.
William A. Klepack M.D.
Tompkins County Health Department