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.

Woman administers COVID test. How accurate are COVID test results?

In addition to universal masking, social distancing, and handwashing, testing is another critical tool to fight COVID-19. “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 even as testing capabilities improve, many 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 and more.

Dr. David Calfee

Dr. David Calfee

What is a COVID diagnostic test?
A diagnostic test is used to detect the presence of the virus or parts of the virus in the sample obtained from the person. For the SARS-CoV-2 virus, diagnostic tests are performed on a sample from the respiratory tract, such as the nasopharynx (the upper part of the throat behind the nose), the nose, or throat, or from saliva.

What’s the difference between what’s known as the PCR test and the rapid tests? Is one type more accurate?
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.

Antigen tests, also known 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 some can be done at a doctor’s office or clinic.

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 reason to suspect you were exposed, such as having symptoms of COVID-19 or live in a setting in which there is an ongoing outbreak, it’s recommended you confirm the 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 after traveling, after attending larger indoor 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.

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 use of face coverings, social distancing, and hand hygiene.

According to guidance from the Centers for Disease Control and Prevention, a person who tested positive for COVID-19 can be around others after 10 days, and a subsequent negative test is not necessary. Why is that?
This recommendation is based on data that suggests that infectivity decreases fairly quickly in most people with COVID-19. Several studies of people with mild-to-moderate COVID-19 who have recovered from their infection (no longer having fever and other symptoms have improved) have found that infectious virus is no longer present in samples collected beyond this period. Additionally, 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.

What is an antibody test and can it be used to diagnose COVID-19?
Antibody tests are performed on a blood sample to detect antibodies that the human body’s immune system produces in response to an infection. A SARS-CoV-2 antibody test can provide evidence of previous exposure to the virus, but it cannot distinguish a recent infection from a more remote infection. Thus, antibody tests are not used to diagnose acute COVID-19 infection.

Additionally, antibody tests may be negative in the early phases of infection because it takes time for the body to produce antibodies. A positive SARS-CoV-2 antibody test suggests that you have had a previous infection. For some diseases, such as measles, the detection of antibodies indicates immunity to subsequent reinfection with the organism. The extent to which antibodies to SARS-CoV-2 are protective against reinfection and the duration of any protection that they provide have not yet been clearly defined.

David P. Calfee, M.D., M.S., is the chief hospital epidemiologist at New York-Presbyterian Hospital/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 an associate editor of Infection Control & Hospital Epidemiology. His research interests include antimicrobial-resistant pathogens and the epidemiology and prevention of healthcare-associated infections.