What You Need to Know About Testing for COVID-19
Due to the holidays, many more people than usual have been getting tested for SARS-CoV-2, the coronavirus that causes COVID-19. However, our family practice doctors are seeing and hearing a great deal of confusion and misinformation regarding the accuracy of COVID-19 testing.
So we thought we’d provide a kind of primer on the different tests, and explain the limits of testing.
Why COVID testing can be inaccurate
The first thing you need to know is that a negative COVID-19 test is not a get-out-of-jail-free card. There are simply too many variables involved. This is true of many medical tests, as well.
Results can vary depending on the type of test given, who administers it, and where you are in the infection.
For example, if you’ve just been exposed, the virus will not have had time to replicate enough throughout your body to show up in a test. Or the person giving the test may not have inserted the swab far enough into the nasal cavity to obtain a sufficient sample.
Or, if you’re one of the millions who are asymptomatic (i.e., never develop symptoms), it’s difficult to know where you are in the course of the infection, so you may be tested too early or too late.
And the estimated 40 percent (or higher, depending on the study) who remain asymptomatic throughout their infection are responsible for the majority of transmissions, according to the Centers for Disease Control and Prevention (CDC). So it’s crucial—although nearly impossible—to pinpoint the best time to test them.
Experts estimate it takes about four to five days after exposure for the viral load to achieve its highest concentration in the body. It then remains at that level for the next five days, so that is the optimal time to test.
Different tests, varying results
Then, of course, there’s the problem of accuracy, which varies from test to test.
PCR tests
This is the gold standard in testing. Polymerase chain reaction (PCR) tests are highly accurate. If done correctly, they can detect the signature pieces of RNA from the virus in the sample. Even so, some studies have suggested that as many as 30 percent of PCR test results are inaccurate. This is about the same percentage as the standard flu test.
These types of tests require highly trained personnel to administer them, because if the sample is not collected correctly, that increases the chances of an inaccurate result.
Antigen tests
These are the majority of the rapid tests available on the market. They also require a nasal swab to search for the distinctive protein cover on the coronavirus’ cell. These are even more dependent on timing. That is, when in the course of infection the person is tested. If it’s too early, the virus may still be replicating itself in the body, and has not yet begun to shed detectable pieces of protein.
The results of antigen tests are available sooner than the PCR test, but are also up to 50 percent less accurate. The CDC advises those who test negative on an antigen test to have a PCR test to confirm the results.
Saliva tests
These tests are relatively new to the scene, having been approved in August of this year. They are more popular with the general public because they are less invasive. In addition, they don’t require the costly reagents and may be done at home.
While they are not yet widely available, studies show their accuracy is comparable to the PCR test.
Antibody blood tests
These tests are not used to detect an active infection. Rather, they use blood samples to look for antibodies that the body already activated to combat an infection of COVID-19. That means, at one time, the person was likely infected. But there’s no way to tell if the virus is still active.
In fact, the CDC says it can take one-three weeks for your body to produce antibodies. Therefore, about the only thing they’re good for is to learn whether you would be a good candidate to donate blood plasma to help fight the infection in those who currently have it.
Final thoughts
Testing can still be a useful tool, as long as you understand there are no guarantees. It is one more weapon in the arsenal we can bring to bear against this deadly enemy. And the testing numbers themselves are less important in detecting community spread than the percent positivity rate. That’s because the percent positive shows how active the virus is in a particular community, rather than how many people are being tested.
You should also understand that, even if a test result is accurate, the moment you come into contact with an infected person, the result is irrelevant, because you can then become infected.
We also want to caution you about any scams involving home testing you might find on the Internet. Please be tested by a reputable entity.
Until the vaccine becomes more widely available (probably not until late spring or early summer), your best bet is to assume anyone you meet outside the home is infected. We advise these necessary precautions:
- wearing a face mask
- avoiding indoor spaces as much as possible
- washing your hands
These steps have been shown to reduce the spread of the virus.
