Simple everyday acts, like brushing your teeth, could be producing incorrect rapid antigen test results, the Therapeutic Goods Administration (TGA) has warned.
On Twitter, the TGA said it was important people do not eat, drink, smoke, brush your teeth or chew gum up to 30 minutes before they collect saliva for a rapid antigen test.
Doing so could produce an incorrect result on the at-home saliva Covid tests. The rules do not apply for a nasal swab test.
The message has also been added as an update on the TGA's web page about rapid antigen tests.
In a consumer fact sheet, it warns that people should refer to the instructions on the test kit for the recommended timing.
"Many home use tests are available and each one is different," the TGA said.
"It is important to follow the steps in the instructions provided with the test you use."
There are several different ways to test for Covid with a rapid test, including a nasal swab, an oral fluid sample or saliva.
Of the RATs approved by the TGA, there are only three that use saliva for testing purposes. All were approved late last year.
RAT advice comes 'months' too late
After the advice was posted to Twitter, many claimed it was being shared "too late" and would have been handy when rapid testing was approved by the TGA.
In NSW alone, where 11,807 new cases were reported on Wednesday, more than half of the cases were from positive RAT tests.
"This info could have been helpful.... Months ago," one person tweeted in response to the TGA.
"Bit late with this advice," another said.
"Are you serious? Do you know how many results would have been different if this information was reported before directions for mass use?" an outraged viewer wrote.
However, at least one person found the information to be humorous in a hypothetical situation.
"Imagine isolating for 7 days because you brushed your teeth," they quipped on Twitter.
Rapid saliva test kits should include the instructions in the packet, though it's likely some were sold without the information as some retailers were separating tests in the height of the RAT shortage.
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