An Expert’s Guide to COVID-19 Testing
An epidemiologist discusses the differences between the molecular and rapid tests, their accuracy, and when people should get tested for coronavirus.
As the highly contagious Omicron variant spreads in the United States, one of the critical tools to help control the spread of COVID-19 is testing. “Testing is one of several important components of the global effort to control the pandemic,” says Dr. David Calfee, chief hospital epidemiologist at NewYork-Presbyterian/Weill Cornell Medical Center. “It allows those with symptomatic COVID-19 to receive appropriate care and treatment and helps to reduce transmission of the SARS-CoV-2 virus from those with symptomatic and asymptomatic or presymptomatic infection.”
But as more and more people seek out tests, questions remain, including the differences between the types of tests, how accurate COVID test results are, and when people need to be tested.
Dr. Calfee, who is also a professor of medicine at Weill Cornell Medicine and deputy medical director of the Department of Infection Prevention and Control at NewYork-Presbyterian, spoke with Health Matters to answer these questions.
What is a PCR test?
The polymerase chain reaction (PCR) and nucleic acid amplification tests (NAATs) are molecular tests. Molecular tests detect the presence of the virus’s genetic material, known as RNA. In general, these tests are considered to be the “gold standard” for diagnosis of COVID-19. They have greater sensitivity than antigen tests, meaning they can detect more people who have COVID-19 infection. It typically takes about 24 hours to get PCR test results, but results can take even longer if labs are backed up.
What is an antigen test?
Antigen tests, also referred to as rapid tests, detect the presence of one or more proteins that are part of the virus. Antigen test results can be acquired quickly because they require less complex technology. They frequently have a faster turnaround than molecular tests, and you can get results from a doctor’s office or urgent care very quickly. Home antigen tests can also be purchased at drug stores and online — several have been cleared by the Food and Drug Administration under an Emergency Use Authorization. The rapid home tests are a good tool to have on hand, especially when it’s difficult to find a PCR test and you develop symptoms.
False positive results on antigen tests are rare, but because antigen tests have lower sensitivity, a false negative result is possible. This is important because a negative antigen test does not necessarily mean you don’t have the virus. If you have symptoms of COVID-19 or have reason to suspect you were exposed, such as having close contact with someone who subsequently tested positive for COVID-19, it’s recommended you confirm a negative antigen test result with a PCR test. It is also important to note that a negative test (either antigen or PCR test) immediately following an exposure to someone with COVID-19 does not prove you were not infected, because it can take several days for the virus to become detectable.
When should people get a test?
While specific recommendations vary among local and state health departments and other agencies and employers, groups prioritized for testing are people with symptoms of COVID-19 and those with known exposure to COVID-19. Other situations in which testing has been recommended include before and after traveling and attending large gatherings, prior to invasive medical procedures and surgery, and for people who work or live in a long-term care facility or another congregate residential setting.
This chart breaks down the different type of COVID-19 tests and how accurate the COVID tests are. A molecular test is also known as a PCR or NAAT test. It detects the presence of the virus’s genetic material (RNA) through a swab of nasopharynx, nose, or throat; or saliva. This test accurately diagnoses or rules out active coronavirus infection and is considered the “gold standard” test that usually doesn’t need to be repeated. An antigen test is also known as a rapid test and detects the presence of one or more proteins that are part of the virus through a swab of nasopharynx, nose, or throat; or saliva. This test rapidly diagnoses active coronavirus infection, with results in as soon as two hours. Positive results are generally accurate. A molecular test may be recommended to confirm a negative result. If an infection is thought to be very unlikely (such as no symptoms and no known exposure), a molecular test may also be recommended to confirm a positive. An antibody test is also known as a serological test. It detects the presence of antibodies produced in response to an infection through a blood test. Antibody tests are used to see if you’ve had coronavirus infection in the past. However, it may be negative in the early phases of infection, and the sensitivity and specificity of antibody tests vary.
Should children who are attending in-person school get tested regularly?
This can vary depending on the district. But regardless of testing requirements in schools, it is important to adhere to preventive and mitigation strategies, such as vaccination if they are eligible, and use of face coverings, social distancing, and hand hygiene.
According to the newest guidance from the Centers for Disease Control and Prevention, a person who tested positive for COVID-19 can stop isolating after 5 days, if asymptomatic, followed by 5 days of wearing a mask when around others. Why is that?
This recommendation is based on data that suggests that infectivity decreases fairly quickly in most people with COVID-19. They found that the majority of SARS-CoV-2 transmission occurs early in the course of illness, 1 to 2 days prior to onset of symptoms and 2 to 3 days after. Molecular tests may continue to be positive even after that person is no longer infectious due to the ability of these tests to detect very small amounts of viral genetic material. However, longer periods of isolation are recommended for people who have more severe COVID-19 and for individuals who have severely compromised immune systems.
David P. Calfee, M.D., M.S., is the chief hospital epidemiologist at NewYork-Presbyterian/Weill Cornell Medical Center and the deputy medical director of infection prevention and control for NewYork-Presbyterian Hospital. He is also a professor of medicine (infectious diseases) and population health sciences at Weill Cornell Medicine. Dr. Calfee is a Fellow of the Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America, and is the editor of Infection Control & Hospital Epidemiology. His research interests include antimicrobial-resistant pathogens and the epidemiology and prevention of healthcare-associated infections.