COVID-19 testing has been a hot topic since the outbreak of the viral pandemic. Phrases such as “nasal swabs” and “viral genetic material” have moved from clinical vocabulary into the mainstream consciousness, and new tests arrive on the market seemingly weekly. But what do you need to consider if your doctor, employer, or school asks you to get a COVID-19 test? How do you know whether it’s the right type of test? Or, if you can trust the result?
Lab-based PCR Tests
The most common type of COVID-19 test, and the one described as the “gold standard” for its ability to detect even tiny amounts of virus accurately, is called a PCR (or polymerase chain reaction) test. With this test, a swab is taken from the nasal passage or throat by a medical professional. Some versions of the test are approved for use in asymptomatic individuals, while others are intended only for those displaying symptoms.
The swab, which mops up trace amounts of virus present in an infected person, is put in a tube with a special liquid that keeps the part of the virus that will be detected, its RNA (ribonucleic acid), from degrading. Swabs are typically transported to a lab where many samples can be processed at once on sophisticated robotic machines that amplify and detect only the RNA from SARS-CoV-2, the virus that causes COVID-19. This specificity is vital since other coronaviruses that can cause common cold infections might be present. Though this process only takes a few hours, there have often been much longer wait times for results since many lab facilities are overburdened.
The sensitivity of lab-based PCR tests is excellent for detecting infections early. However, a downside is that after a person’s body has fought off the virus and they’re no longer making new viruses that could infect others, some remaining viral RNA in their nose or throat may result in a positive test. Nonetheless, we believe that testing a group of people regularly with lab-based PCR tests is the optimal way to detect all infected individuals, regardless of whether they’re at the beginning or end of their infection. Some tests can even be self-administered at home, which is advantageous when trying to stop the virus from spreading.
On-site PCR Tests
Another way of conducting the PCR test is on specialized machines installed at some doctors’ offices and hospitals, which have a modified amplification and detection process that can run in as little as 45 minutes. These tests require trained personnel to perform them and are designed to confirm suspected infections in symptomatic individuals to help providers make quick treatment decisions in clinical settings. They are not intended for screening of asymptomatic individuals and may be less sensitive than a lab-based PCR test when used for samples with low viral concentrations.
Rapid Antigen Tests
Newly developed “rapid antigen” tests are also now becoming available. An “antigen” is anything that the body mounts an immune response against; for SARS-CoV-2 antigen tests, that is a viral protein. To detect the protein, a nasal swab is taken similarly to that used for PCR but is mixed with a different chemical reagent. The sample is then loaded into a machine that processes it to give a digital result or onto a small cartridge that provides a color change result similar to a pregnancy test. The significant advantage of these tests is the speed, with results delivered within 15 minutes. The drawbacks, however, are lower sensitivity and, at the moment, limited supply.
Whereas PCR and antigen tests identify individuals with an active infection, antibody blood tests, when used two or more weeks after the onset of symptoms, can reliably identify individuals who have developed antibodies against SARS-CoV-2. According to the FDA, testing for these antibodies “can help identify who has been infected and developed antibodies that may protect from future infection as well as identify those still at risk.” However, it’s currently unclear how long you might be protected from further infection if you have a positive antibody test.
COVID-19 antibody testing provides information on the estimated prevalence of the disease in a group or population, such as a school’s students, a company’s employees, or a city’s residents. This information can then be used to inform the testing frequency for active infections.
So, which test should your doctor, school, or employer be ordering for you? The answer depends on how fast they need a result, how often they’re testing, and whether they have medical staff available to administer the test. Supplies of antigen tests are being prioritized to nursing homes in COVID-19 hotspot counties right now so that symptomatic individuals can be quickly isolated if they are infectious. The NBA, on the other hand, has taken the PCR test route. The stakes are high if an infection spreads among athletes, so the NBA is using PCR tests every other day to ensure they catch any infection as soon as possible.
Some blockbuster movie shoots have also taken a daily testing approach. Regular testing would be the most effective way to monitor school or college students, too, with the testing frequency determined by local infection rates. Some school districts have already implemented this approach, but getting the results quickly enough to stop an outbreak is essential and currently a limitation in some areas.
The good news is that with the approval of new types of tests and companies growing their testing capacity, the likelihood is increased that we will soon all have access to the appropriate tests to protect our health.
At Sema4, our COVID-19 testing capacity is 15,000 viral tests a day, and we have optimized the gold-standard PCR test process to deliver results within 24-48 hours. We also have the capacity for 4,000 antibody tests per day, with results delivered within 48-72 hours. And, our customizable digital end-to-end testing solution, which includes web portals and the Sema4 Works symptom monitoring app, can help schools and businesses take steps to safeguard students’ and employees’ health as they return to learning and work.
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