How do COVID-19 tests actually work? Virginia Tech infectious disease expert explains
Virginia Tech public health expert Lisa M. Lee explains how testing can be used to understand the impact of COVID-19 and help reopen the economy.
PCR is an acronym for a laboratory technique called polymerase chain reaction.
“PCR tests have been used since the start of the epidemic,” said Lee, a public health expert specializing in infectious disease epidemiology and public health ethics, and the associate vice president for research and innovation at Virginia Tech, where she leads the division of Scholarly Integrity and Research Compliance.
“This test uses a swab from inside a person’s nose and throat and looks for the genetic material of the virus itself. If results show the virus’s genetic material, it means that the person has the virus and can spread the infection. The drawback to the PCR test is that it requires special equipment, is expensive, and can take days to get results,” Lee said.
An antigen test can be done on the spot and provides results in minutes.
“This test looks for a little part of the outside coating of the virus and tells us if the virus is there or not,” Lee said. “It provides results much faster, which is important for contact tracing to help identify individuals who may have been exposed and to stop spreading the disease.”
Antibody tests shows if a person has had past infection with the virus that causes COVID-19.
“When we are infected with a bacteria or virus, our bodies respond by sending out antibodies to attack the invader,” Lee said. “Depending on the bacteria or virus, antibodies stick around in our blood for a period after the infection and can indicate that at one time the bacteria or virus was present.”
The bigger issue is that it is very hard to tell what a positive antibody test means.
“Some bacteria and viruses can cause disease more than once—think about the common cold virus or strep throat. Many of us have gotten more than one cold or strep infection in our lives. We are just not sure yet if having had the virus that causes COVID-19 means one is immune from being infected again, and if so for how long,” Lee said.
“In some cases, having a disease once can be enough to make us immune forever. For example, chicken pox: most people only get it once in their lives because once they have an antibody response it protects them forever. Unfortunately we do not know yet whether the COVID-19 infection will be one of those infections that we get once or again and again.”
“If it turns out that COVID-19 is an infectious disease that we only get once, antibody tests would be very helpful in determining what proportion of the population is immune to the infection. If that proportion is high, it could help guide reopening of communities. If, on the other hand, we can be infected more than once, having an antibody test will be less useful for controlling the disease.”
Lee says that all tests can result in false positives or false negatives. Usually the false positive and false negative rates are low, but even at low rates they can provide misleading information.