As the United States continues to grapple with the COVID-19 pandemic, the testing initiative remains contentious and uneven. There have been concerns about availability, accuracy, objectives, and delays in results. Could the wider introduction of a rapid COVID test help?
There are pros and cons to this form of COVID testing, which delivers results in minutes but has less reliable results than other COVID tests. Availability of rapid testing varies by location, so you may or may not have access to one. Be prepared with what’s known so far about rapid COVID testing.
COVID tests fall into two main categories:
- Diagnostic tests tell you whether you have an active COVID infection. Currently, there are rapid antigen tests and more precise molecular tests. The main molecular test in use is based on PCR (polymerase chain reaction), but there are other types of diagnostic molecular tests. Another name for the diagnostic test is a virus test.
- Antibody tests tell you whether you had a prior COVID infection. Again, there are rapid tests as well as conventional laboratory serology tests.
How a Rapid COVID Test Works
A rapid COVID test is a type of diagnostic test. Rapid tests usually measure viral antigens, which are substances that tell your body to produce an immune response to an infection. Antigens are not the same as antibodies, which your immune system produces in response to signals from antigens. Trained personnel in a variety of settings can administer a COVID rapid antigen test.
The other type of diagnostic test is the PCR test, which detects molecules of viral genetic material (RNA). It is labor intensive and performed in an accredited laboratory. The molecular test is the standard diagnostic test for COVID, but you may have to wait several days or more to get the results.
There are other, non-PCR types of molecular tests to detect coronavirus RNA, oftentimes with a quicker turnaround time. One of the tests in use for diagnosing active coronavirus infection involves LAMP (loop-mediated isothermal amplification). It is a rapid molecular test, typically for use in a clinical setting like a hospital or nursing home. An at-home COVID-19 test using LAMP was recently issues an emergency use authorization (EUA) by the FDA (Food and Drug Administration), available in spring of 2021.
Rapid antigen, antibody and molecular tests are so-called point-of-care tests, where results are returned at the time the sample is taken, usually within 15 minutes.
COVID Rapid Test Reliability
Molecular, or PCR tests are more reliable than antigen tests for diagnosing an active infection. Comparing the sensitivity of a PCR test to a rapid antigen test, the PCR test can detect a single molecule of coronavirus RNA in a patient sample. A rapid antigen test would likely need tens of thousands of molecules in the same size sample to get a positive result. Rapid molecular tests seem to be similar in sensitivity to the standard PCR molecular test, meaning if test results are negative you very likely are not infected. However, sensitivity varies between test devices—there are more than 200 different test devices (antigen, molecular and antibody) listed on the FDA website.
Rapid antigen tests are most reliable when people are in the early stages of COVID-19, when the viral load is highest in your throat and nasal passages. You may not experience symptoms at that stage, but you are still contagious and can spread the virus to others. Repeated rapid antigen testing can help because the amount of virus in your nose and throat can increase enough within hours so that you can test positive after one or more negative tests. A positive rapid antigen test result means you most likely have COVID-19. A molecular test can confirm the diagnosis.
If you take a rapid COVID antigen test 5 to 7 days after you develop symptoms, you may not have enough antigens for the test to detect them and you may get a false-negative result. Likewise, if you test too early, perhaps just after you are exposed to the virus, your test will come back negative because there are not enough viral antigens to be detected. If actually infected, you will likely test positive in another day or two.
Rapid antigen tests are a diagnostic tool, but not meant to confirm an active infection or prevent transmission among people who don’t have symptoms. They are intended for people with symptoms of COVID-19, people who are a close contact of a confirmed COVID case, and for tracking people who are part of an outbreak. Today, rapid antigen tests are more likely to be used to screen large numbers of people to identify potentially infected individuals as well as to track spread of the infection within the community. However, the FDA recently granted an EUA to two rapid antigen tests for home use. One requires a doctor’s order; another does not. These tests may be available for purchase starting January 2021.
Experts believe a person with COVID may be contagious for up to three days before they develop symptoms, and most contagious in the two days before they start to feel sick. The duration during which an individual is contagious varies. Guidance about how long to isolate or quarantine may depend on your circumstances. Current guidelines are between 10 and 14 days, but it depends on how sick you are—patients who are hospitalized should remain isolated, if possible, for 20 days or more after symptoms first appear.
Rapid COVID Test Status and Availability
The government has shipped millions of rapid COVID tests to nursing homes, schools and other facilities. Rapid COVID tests are also available at other point-of-care facilities, such as pharmacies. Check with your provider or public health authority for availability in your area. Most rapid tests use a swab to collect mucus but some newer [molecular] tests use saliva. It is unclear if saliva tests are as reliable as those that use mucus. Private and commercial labs and companies are continually developing and distributing new tests and treatments for COVID, and will continue to collect and analyze data about test efficacy and reliability.
Keep in mind that no COVID tests, rapid or otherwise, are officially approved by the FDA, but the agency issued an EUA. That means the tests have not been subjected to the usual strict FDA standards and their accuracy is less well established. Still, evidence points to a high level of accuracy for the PCR and other molecular tests. As developers continue to work on rapid COVID tests, they may become more reliable as well as available.
For the time being, COVID rapid tests can be useful in certain scenarios and contribute to the control of the pandemic. If you have had COVID and want to know if you are no longer contagious, one way to verify that is to see if you have two negative tests 24 hours apart.