The PCR test is often considered the gold standard in detecting coronavirus infection. Some employers ask their employees to do just that test after getting over covid-19, and they are also used for free travel and ending isolation.
However, experts say that PCR tests are not the best for every situation. PCR tests can detect small parts of the genetic material of the coronavirus in a sample taken from a person. The test works by making copies of that genetic material if it is present in a person's sample, the US Centers for Disease Prevention and Control (CDC) explains.
It is this possibility of amplification that allows the test to detect very small parts of the virus in the body, which makes them highly sensitive in diagnosing covid-19, according to the CDC.
Although the sensitivity of the test can be positive for the detection of coronavirus after recent exposure to the virus, this also means that the PCR result can be positive even after you are no longer contagious, writes CNN.
"A PCR test can show you're positive for the coronavirus three or four weeks after you've recovered because it detects past infection and bits of the virus that are still multiplying," CNN medical analyst Dr. Leana Wen said.
"Seven to ten days after infection, PCR is not a suitable test," she claims, as reported by N1.
When should you get tested with a PCR test?
"The right time for PCR testing is when you know that you have had contact with a person suspected of having the coronavirus who has symptoms and when you want to find out if you have also been infected," says Dr. Albert Ko, a professor at Yale University's School of Public Health.
"Knowing if you have been infected with the coronavirus at an early stage can help you find out if you are contagious to other people and you can share this with your family doctor so that you can discuss your symptoms and so that he can prescribe treatment if you need it," says epidemiologist Emily Somers.
When should you be tested with a rapid antigen test?
Contrary to what some think is best, a rapid antigen test is what you need after you have been sick with covid-19 for several days and want to confirm that you are no longer contagious to other people. "Infected" means that I have the virus in me, and "infectious" means that I secrete enough virus to be able to infect someone else," explains Dr. Ko.
Current CDC guidelines for isolation say that for people who have access to testing and want to do it, the best approach would be to do a rapid antigen test at the end of their five-day isolation. Those who are positive even at that point should remain in isolation until the tenth day since the symptoms started. People who are negative can stop the isolation, but they should wear a mask until the tenth day after the onset of symptoms.
"Although the rapid antigen test has a lower sensitivity and is inferior to the PCR test in detecting infection due to its lower sensitivity, it only detects viruses when they are present in larger numbers and probably reveals that you are more contagious," explains Ko.
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