If you have Covid-19 symptoms but test negative with a rapid antigen test, you should test again 1-2 days later, or on day four of feeling crook, new research suggests.

A study out of the United States, conducted between January-May 2021 and published in JAMA Internal Medicine on Saturday, compared the PCR and RAT results of 225​ children and adults with the virus.

It found that, over the whole infectious period, RATs picked up an average of 64%​ of the same-day PCR-confirmed Covid-19 cases (just over two-thirds).

But the sensitivity of RATs appeared to peak four days after symptoms began, picking up 77%​ of Covid-19 cases in the test group.

Researchers also found that doing a second test 1-2 days after an initial negative test helped to increase the number of cases detected via RATs (81%-85%)​ – particularly early in the infection.

Overall antigen test sensitivity was 50%​ during the infectious period, the study found.

Six days after symptom onset, antigen test result positivity was back down to 61%.​

By 11 days, when most people were no longer considered infectious, PCR positivity remained high (86%), while antigen test positivity was much lower: 16%.​

Of the 255 cases, 194 (86%)​ had never received a Covid-19 vaccine. The most common variants detected among participants (in order) were: Alpha (56%)​, Epsilon and Gamma.

RATs look for proteins – antigens – found on the surface of the SARS-CoV-2 coronavirus. These proteins are only present if you have an active Covid-19 infection.

PCR technology, meanwhile, amplifies (by copying) even small amounts of viral genetic material, making it very sensitive to picking up Covid-19, even after a person has recovered and is no longer infectious.

Infectious diseases physician and Professor of Biochemistry at the University of Otago, Kurt Krause, said the study underscored that, generally speaking, RATs are less sensitive than PCR tests.

It was important people keep this in mind, and make optimal use of them, saying it was better for people to do multiple tests – at least two, if not more.

The study also demonstrated that sensitivity varied depending on strain.

Krause said when new variants of concern appear it is really important that existing RATs are analysed to ensure they are good at testing for emerging variants.

This would rely on the likes of ESR validating whether the tests in the community suit the circulating variant(s).

The findings reaffirmed that PCR is “still really the gold standard” for Covid-19 testing, he said.