PCR vs. antigen test: Which to take after taking Covid-19

PCR - or reverse transcription-polymerase chain reaction - tests can detect small amounts of coronavirus genetic material in a sample collected from a human. The test then works by amplifying or making copies of the genetic material if something is present in a person's sample, according to the U.S. Centers for Disease Control and Prevention.

The amplifying property of the test allows it to detect very small amounts of coronavirus in a sample, "making these tests very sensitive to diagnose COVID-19," according to the CDC. While this sensitivity may be beneficial for detecting coronavirus after a recent exposure, it also means that PCR results can be positive even after you are no longer contagious.

A PCR test can say you are positive for coronavirus for three or four weeks after recovering because it still "picks up previous infection and the small fragments (of the virus) are still amplified," CNN Medical said. Analyst Dr. Leana Wen, emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, "this PCR test is not an appropriate test."

Whatever your situation, here's what you need to know about the differences between PCR and antigen testing and when to use them.

When a PCR test is the key

The best time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health.

Knowing your Covid-19 status as early as possible in an infection can help you find out if you are contagious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk to you about your symptoms and prescribe any medical treatment if necessary, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences and obstetrics and gynecology at the University of Michigan Schools of Medicine. and public health.

This is important because "up to two days before symptoms, or one day or two days after symptoms (begin), people may have the highest risk of infecting others," Ko said. Once people are exposed, it takes time for the virus to replicate enough for a test to detect it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein at one of the tips of the coronavirus.

The U.S. Food and Drug Administration has approved the use of both PCR testing and rapid antigen testing in laboratory-based or treatment sites (such as a physician's office, emergency room, pharmacy, school health clinic, and temporary sites such as drive-through test sites). Antigen tests are also commonly available as self-tests.

When rapid antigen testing is essential

Contrary to what some still think is best, a quick antigen test is what you should take after you have had Covid-19 for several days and would like to confirm that you are probably no longer contagious to other people. "'Infected' means I have the virus in me," Ko explained. "'Contagious' means I excrete enough virus to infect another, and it only happens during a peak. It follows a curve after someone has been exposed."

The CDC's current isolation guide is that if people have access to a coronavirus test and want to take it, the best approach is to use a rapid test towards the end of their five-day isolation period. Those who continue to test positive at that time should continue to isolate until they reach 10 days after their symptoms began. People who test negative can stop isolating themselves, but should wear a mask around others until day 10.

"Although the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you have been infected, due to its lower sensitivity it only picks up viruses at a higher level and probably higher levels. contagious, "Ko explained.

"At this time, we do not have an FDA-approved test that can tell us exactly" how much virus it takes to be contagious to other people, "Ko added. But new evidence has led public health experts to believe that rapid testing does better than PCR testing to capture when people are leaving the period of maximum infectivity, as PCR testing is too sensitive, he said.

"We have other data to indicate at what point someone is no longer contagious," Wen said. A British pre-emission study published in December 2021 showed that after five days of coronavirus infection, 31% of people were still contagious. On day eight, this percentage dropped to 11% and continued to decline over the next few days.

"Having the presence of two rapid tests - two days in a row - which are both negative, is a very good indicator that a person is no longer contagious," Somers said.

This does not mean that it is completely impossible to be contagious at this time - it just means that after having Covid-19 for several days and then testing negative with a rapid antigen test, the viral RNA is probably at levels low enough to not to be contagious (even if you are still testing positive on a PCR test).

There have been many online reports of employers requiring people to show evidence of a negative PCR test before returning to work, which "probably is not coherent thinking" and have people not returning to work before weeks after their first rapid antigen or PCR test, Ko said.

"I would strongly urge individuals to contact their HR (HR department) - or if there is a medical or Covid manager on their teams - and escalate the issue because it is not fair," Wen said.

In addition to being more practical, the most logical test strategy for returning to work is to use rapid antigen testing, Ko said.

"As with any test for anything, if you use it for the wrong purpose, you will get an inappropriate result," Wen added. However, "if someone has had multiple antigen tests that are negative but they have symptoms of Covid, a PCR test can help confirm whether they have it or not."

Some places, such as Antarctica and Kosovo, require travelers to show evidence of a negative PCR test before entering. "If the requirement is a PCR within 72 hours of travel or the like, a lot can happen in those three days," Somers said. "The negative PCR from a sample taken three days before the trip may not be better than a quick one on the day of the trip.

"For the question of someone who had an infection and is recovering, I think this is where it really is an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error," she added. "If a country tries to be extremely strict in keeping all infections out, then a negative PCR will be a safer way to do it. However, it will potentially extend the time between when someone can enter the country compared to when they were contagious. "

CNN's Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.