People are reporting long queues and wait times for polymerase chain reaction (PCR) tests so is there a disadvantage for opting to take a rapid antigen self-test (RAT) instead?
According to Professor Peter Richmond, Head of Paediatrics at The University of Western Australia’s School of Medicine and Head of the Vaccine Trials Group at the Telethon Kids Institute, the PCR is our most sensitive and specific test.
“It’s sensitive which means that it can detect very low levels of the virus, potentially even before you’re infectious and certainly after you’ve had the infection you continue to be able to detect the RNA for a period of time,” Professor Richmond said.
“It’s not clear if that ongoing detection of the RNA of the virus is actually representing live virus. It may be that your PCR is positive but you are no longer infectious.”
Unlike a RAT, which only takes 15 minutes to show a result, the sample taken from a PCR test can take about 24 hours to be analysed.
“The issue is in that interval time while you’re waiting on the result you can still be spreading the virus without knowing that you’ve got it,” Professor Richmond said.
RATs, as the name suggests, give a very rapid response and work by detecting a protein that is made by the virus.
“The issue is that when you get infected it takes several days to develop into an infectious virus and make the proteins that produce positive RATs,” he said.
So a positive RAT means you are definitely infectious whereas PCR tests can detect the virus before you become infectious but will also detect very low levels of the virus once the infectious period has ended.
“I think we heard during the Olympics, these people who had COVID several weeks before but suddenly come up with a positive PCR test,” Professor Richmond said.
“They probably had a low level PCR test all the way along but there are some indicators that are detecting very low levels of virus.”
His advice is if you have symptoms and think you might have COVID and you test negative to a RAT but still have symptoms the next day, you should take another RAT.
“In that interval, if you do have COVID, you will have increased the amount of virus you have and therefore increased the amount of protein and will then get a positive RAT,” Professor Richmond said.
“But if you’ve got a negative PCR with symptoms, you’re probably okay, you don’t need to necessarily repeat it the following day.”
And there is no point rushing out to get a test the day after you think you may have been exposed to COVID.
“The incubation period for Omicron is between two to four days from point of exposure rather than that three to seven days that it was with Delta and previous variants,” Professor Richmond said.
“You want to wait two to three days before you get your PCR test, and if it’s positive then obviously you have got COVID and need to isolate. If you have got a negative PCR test you might still want to do a RAT test the following day just to make sure.”
