Many people are wondering about testing. There’s more to it than just knowing if a test is accurate or not. It’s a complicated topic — I’ve tried to break it down below with an explanation of the two main tests and major factors to consider.
Nucleic Acid Amplification Test (NAAT) or PCR (Polymerase Chain Reaction) by Nasal Swab
When we talk about “testing for coronavirus” generally we are talking about this test—checking to see if you have an infection right now.
How does it work?
You get a swab from deep inside your nose into the very back of your throat. If you’ve had a flu swab, it is similar. It often will make you gag, if it is done properly. This is meant to collect mucus that would contain replicating viral particles. This sample is then sent to the lab where any genetic material (RNA) matching that of SARS-CoV-2 can be amplified to then identify the presence of the virus. This is a fairly reliable test if done properly and there is enough mucus to test.
Where do I get this test?
You can get these tests at a regular doctor office or urgent care, a “drive through” center, and now you can test yourself at home with a kit. It is likely the kit will be less accurate due to variability in how people do the test.
Why would I get a False Negative?
It can be falsely negative if the swab isn’t done well
If it’s too early to detect the infection (the virus hasn’t replicated enough yet)
If the amount of viral particles aren’t enough to be detected
Can I get a False Positive?
No test is perfect, but because this tests for a genetic sequence specific to SARS-CoV-2, it is highly unlikely.
Antibody Testing by Blood Sample (Serology)
How does it work?
COVID antibody testing looks at your body’s response to infection by checking for IgG and IgM antibodies.
[Hey, wait a minute, what are antibodies anyway? These are special Y-shaped proteins that immune cells produce in response to a new virus or pathogen. These antibodies are specific to some parts or proteins found in the virus. When you are infected with something for the first time, IgM antibodies specific to that invader are produced. They then will stick to and help immune cells destroy the intruder. Meanwhile, IgG antibodies will be produced—they “remember” the infection, and float around your blood ready to prevent or lessen future infections.
IgM antibodies (Immunoglobulin M) are produced about 5-20 days after an initial infection and they appear first. They peak around 80 days, and will subside by around 6 months after initial infection. IgG (Immunoglobulin G) antibodies are also produced, but slightly later— about 7-20 days after the first day of getting infected. IgG antibodies continue for months to years, and in some cases last a lifetime (ie., give you lasting immunity to a type of infection.]
The main reason people are getting these tests is they want to check themselves to see if they are “immune” to COVID, or if they “had it already.” It may give some people a sense of safety — feeling like they can’t get it again or they can’t spread it around. However, there are a few things to think about if you had/are having the test.
Where do I get this test?
You can get this test done at certain doctor offices and urgent cares. It is not widely being done at the state level like the nasal swab. It will be a blood draw that is sent out and you get the results in a couple days. There is also a pin prick which results within 15 minutes. It is not clear if the pin prick is as accurate as the send out.
Why would I get a false positive?
All major manufacturers of SARS-CoV-2 place a disclaimer that reads something like: “we cannot guarantee that there is no cross-reactivity from other similar coronavirus.” Meaning, you may get a positive result because you’ve had other common colds caused by any one of the coronaviruses, that have similar proteins to SARS-CoV-2.
The rate of infection is low and the number of people being tested is high. This is somewhat of a complicated concept that’s related to the accuracy of the test and the prevalence of the disease. It’s called positive predictive value (PPV) in health statistics. Simplistically, for those who get a positive result, the PPV gives us a sense of how likely it is that you really had the disease. For the COVID antibody tests it can range from approximately 35%-75% (for an infection rate 2.5%-10%, respectively). That’s a huge range that’s dependent on both how the test is made and how many people have the infection right now. Which means, even if you do have a positive antibody test, it does NOT necessarily mean you are “safe.” There’s a great Scientific American article describing the statistics that drive the info we can get from antibody tests
Why would I get a false negative?
It’s too early to detect the antibodies. Unlike the PCR testing, which becomes positive closer to the onset of symptoms, your body will produce antibodies at varying rates about a week out from initial infection. If you are in the process of developing them, you may test negative.
You had the infection but your body did not produce enough antibodies to be detectable.
You had the infection but the antibodies you had faded away over some time.
Other considerations
Even if antibody testing accurately shows a person possess antibodies for a SARS-CoV-2, it does not necessarily mean it will confer immunity to future infections. We need more time, and more data on the cases we have to get an idea of how humans respond to this virus in the short and long term.
What should I do?
Right now in Essex county New Jersey, any symptomatic or asymptomatic person can be tested for COVID by nasal swab. Check in your local area to see where they are doing this type of “screening”. It gives the health department a sense of how widespread the disease is, and that really helps clarify the concepts we discussed above. You can and should get this test at some point if it is available where you live.
If you choose to get the antibody test, and you test positive OR negative, you should still follow all the same distancing and isolation guidelines set forth by the CDC and your state. As you read above, limitations with the testing and statistics show that results ultimately do not free you from following guidelines as it may or may not be a true result.
Of note, NONE of the tests are approved by the FDA, though many of them have “EUA”- Emergency Use Authorization.
Here are some articles on testing:
FDA List of EUA (Emergency Use Authorization) Tests by Manufacturer
https://www.medicinenet.com/how_do_the_covid-19_coronavirus_tests_work/article.htm