When we do any test, we need to ask ourselves how reliable is the test in answering the question we are asking, and what are we going to do with the information. Tests are not magical correct answers to questions. All tests can be wrong: either a false positive where the test says you have the disease but you don’t. Or a false negative, where the test says you are fine but you really do have the disease. Even the best tests in the world are wrong between 1-5% of the time. That is for every 20-100 tests done, 1 of them is wrong. Then, when you get what is may be the right answer from a test, how is that going to change what needs to be done?
There are 2 types of tests for COVID-19. We can do swabs of the nose, throat or saliva that measure whether COVID-19 RNA (the genetic material that is unique to COVID-19) is present, and we can do blood testing for antibodies which measure your body’s immune response to COVID-19.
Rapid testing for COVID-19 RNA is only available in the hospitals for patients who are sick enough to be admitted. It is important for the hospital to know who does or doesn’t have COVID-19 so they can provide appropriate protection to other patients and staff. Here the test does make a difference in how people are treated.
RNA testing can be done from doctors, offices, and sometimes you can order the test directly from some labs. These results take anywhere from 2-7 days to come back. So by the time you get the results, you may be back to normal. We have very limited ability to test for COVID-19. These tests must be sent to the lab in special containers which we must get from the lab. We do not have very many of them. Collecting the sample must be done outside the office since the collection process contaminates the exam room. And the person who is doing the collection must be wearing full Personal Protective Equipment. Like you may have heard on the news, PPE is in short supply and we only have a limited amount.
A positive RNA test is very reliable in telling us if someone has COVID-19. There are rarely any false positives. A negative test is less accurate but still pretty good. It can miss COVID-19 very early or very late in the illness because there just isn’t enough virus present to detect.
Antibody tests done on blood are very unreliable and often give misleading information. If this is all you need to know, you can skip the next few paragraphs. If you want more details, keep reading.
Your body makes 2 types of antibodies to fight infection. IgM antibodies are made right away, and are the first line defense. A positive IgM test means you are fighting off the infection. IgG antibodies show up later. They are the body’s memory of the infection. These are what give you immunity to a second infection with the same germ.
If your IgM test is positive, it can mean any of the following:
You have an early COVID-19 infection and are fighting it off.
There are at least 3 other types of coronavirus besides COVID-19. These are viruses that cause the common cold. They are not new like COVID-19. They cause minor coughs, colds and fevers. The IgM test may be positive because you have an infection with one of these other coronaviruses.
If you IgM test is negative, it can mean any of the following:
You don’t have an infection now with COVID-19.
You really do have an infection but it is too early for the test to pick it up. It may take 7-10 days for the test to turn positive.
If your IgG test is positive, it can mean any of the following:
You had COVID-19 and are immune.
You had another infection with another coronavirus that has nothing to do with COVID-19.
No one knows how long these antibodies last in your body. You may have antibodies and be immune now but this will wear off in the future. Or you may have the antibodies that we can measure with the test, but they aren’t the antibodies the body actually needs to make immunity. So you think you are immune but you aren’t.
If your IgG test is negative, you probably have not had an infection with COVID-19.
We strongly recommend against doing antibody testing. The tests are unreliable, may give false information, and are not approved by the FDA.
RNA testing for COVID-19 is available and accurate. However, it rarely, if ever is going to change what we need to do to treat you. It may be important for you to know if you are infected because it may change how you isolate yourself from others who may be at higher risk for complications. Given how common COVID-19 is in our community, and given how long it takes to get the test results, we would assume that if you have fever and cough that you have a COVID-19 infection and you should stay isolated until you are better. This is the safest way to protect other people.