The first test created to detect COVID—and still the most widely used—is a molecular test called PCR (polymerase chain reaction), Dr. Campbell says. “PCR and similar tests look for the COVID virus’s RNA,” meaning genetic material that comes only from the virus, he explains. “They tend to be quite sensitive, but even among these, they are on a continuum of sensitivity and vary a whole lot.”“Sensitivity” measures how often a test correctly delivers a positive result for people with the condition that’s being tested. A test that’s highly sensitive will catch almost anyone who has the disease and not generate a lot of false-negative results.
How does the test work? A molecular test looks for genetic material from the virus. The test uses sophisticated chemicals and equipment to reproduce millions to billions of copies of viral-related DNA from even the smallest sample. Because of that, the test is considered highly sensitive, leading to very few false negatives.
See more: What is the accurate test for covid 19
How is a sample obtained? Usually with a swab inserted into your nose. There are three different methods for nasal collection:
- Nasopharyngeal: A health care professional inserts a long swab deep into your nostril to collect fluid from the back of your nose.
- Mid-turbinate: This method, which someone can be coached to do themselves or is done by a professional, involves placing a soft swab straight back into the nostril (less than one inch) to collect a sample.
- Anterior nasal swab: This test, which can either be self-administered and possibly supervised by a trained health care provider, or done by a health care professional, involves putting a swab three-quarters of an inch into the nostril and twirling it around at least four times to get a sample.
In general, the deeper you go for a specimen, the greater the sensitivity, says Richard Martinello, MD, a Yale Medicine infectious disease expert. “But, we’ve found it’s much more comfortable to do mid-turbinate or anterior nasal swabs, and they provide a reasonable degree of sensitivity,” he adds. “It’s a compromise of sorts, but it does allow us to simplify the collection process.”
Other collection methods include:
- Oropharangeal (throat) swab: A trained health care provider collects a sample using a swab to the back of the throat.
- Saliva: You drool into a sterile, leak-proof cap container. For now, this type of testing is only offered at select locations.
How is the test processed? Most specimens are sent to laboratories.
Where can you get one? Molecular tests are offered at pharmacies, doctors’ offices, and designated testing locations, such as health clinics, as well as locations set up by private or state and local public health systems.
How quickly can you get results? Because the tests are sent to a lab, it depends on lab capacity. Results are typically available within a range of one to seven days, depending on your location.
“Currently, Yale New Haven Health System is turning around about 76% of community test results within 24 hours and just over 95% within 48 hours,” says Scott Roberts, MD, a Yale Medicine infectious disease specialist.
How accurate are they? According to the Centers for Disease Control and Prevention (CDC), laboratory-based tests, such as PCR, have a “generally high” test sensitivity.
“PCR tests are considered the most accurate available,” Dr. Martinello says. “But because these tests are highly sensitive and specific, there is still a risk for a false positive.”
But limiting false negatives might be extremely important, especially with the rise of more transmissible variants. “It’s actually true for those who have—and who don’t have—symptoms, but if you do have symptoms, a PCR test is more likely than an antigen test to pick up an infection accurately,” says Dr. Campbell.