How do Rapid Antigen Tests work, and are we using them all wrong?

Like last season’s clothes sent to the op shop, the PCR test is out of sight, out of mind. Instead, the quick, slick, more expensive cousin – the Rapid Antigen Test (RAT) is the way we’ve been told to check if our sniffles are something more sinister.

But you’ve probably also heard tales of people with all of the symptoms and a repeated negative result. To understand why, it’s important to analyse how RATs work and what that ominous red line actually means – and even better, it could help us use these COVID-checkers more effectively.

What is a RAT?

A Rapid Antigen Test measures the presence of proteins – called antigens – on the surface of the COVID-19 virus. Although we all know about the spike protein, RATs detect a protein further below the surface, called the nucleocapsid (or N) protein.

Similar to a pregnancy test, a little chemistry experiment occurs in the box: the virus’ N protein reacts to the solution, turning the ‘T’ line on the test red.

If you test positive on a RAT, that means there are building blocks for the virus in your nose or throat. The RAT test picking up N protein suggests that the virus is still replicating, and you could be infectious.

This is a completely different process from a PCR test, which tests for tiny pieces of RNA.  We’re aware now that a PCR test can show the existence of COVID fragments weeks or months after first testing positive for COVID-19. That’s not the case for RAT tests.

“RNA will persist longer in the body than the viral protein,” says Dr Kirsty Short, a virologist at the University of Queensland.

“We equate these rapid antigen tests as more indicative of when you’re infectious than a PCR test, because the viral protein is indicative of replicating virus.”

Am I no longer infectious?

Under the current rules in Australia, you’re allowed to leave isolation seven days after a positive COVID test – no negative RAT required. This is even shorter in the US, with only five days needed at home.

But despite this, it might be a good idea to test yourself with a RAT before visiting grandma.

A small study published in the New England Journal of Medicine last week looked at how long it took patients with Omicron and Delta to no longer have something called “culturable virus” – live virus that is able to infect cells.

Five days after the patients’ first positive PCR test, 50% of those with Omicron still had culturable virus. At 7 days, 25% of those with Omicron still had culturable virus. At 15 days, all the patients had finally cleared.

Researchers only looked at a small sample – about 60 people – so it’s clear more research is required. However, it highlights that COVID doesn’t automatically switch off at day 7.

“I personally would want to be RAT negative before I was out and about,” says Short.

“Our internal policy for our group at work is that you don’t return to work without a negative RAT and you have to be symptom free. When we do return to work, we mask.”

What if you’ve never tested positive on a RAT?

RATs can be a wonderful tool, but they’re not perfect. recent studies have estimated that even in the best-case scenarios they are likely only diagnosing somewhere around 73% of the symptomatic cases that a PCR test would pick up, and reports from the Therapeutic Goods Administration suggest that number could be lower again with Omicron.

So, what does it mean if you’ve been exposed to a COVID case, gotten sick, but repeatedly tested negative for COVID? Unfortunately, no, it’s probably not just a cold.

Rapid antigen tests require upwards of thousands of viral proteins per millilitre of solution in your nose or throat to test positive. But just a small amount of the virus can be enough to kick a primed immune system (like one that has been vaccinated or infected before) into gear. This means you can get symptoms days before testing positive on a RAT.

A study on health workers published in Nature last year suggested that you can even get sick and get better without ever having enough virus for a RAT (or even a PCR test) to pick up. Your immune system is actually working as intended. 

But with RATs being less than perfect at picking up the virus, it’s best to stay home if you’re feeling sick, even if your RAT’s second line doesn’t appear.

Do we still need PCR?

In some states around the country, PCR testing is being scaled back and RATs are being used as the preferred testing. With COVID cases climbing once again is there still a place for PCR in our testing regime?

“There is still room for PCR testing, because PCR testing is more sensitive than a RAT and it means you could pick up the infection earlier. That’s really, really important if you’re somebody who’s eligible for anti-viral treatment,” says Short.

“If it’s taking six days to pick up the infection on the RAT – and then you discover you’re infected and you want to get antiviral treatment – it’s too late.”

The other advantage of PCR tests over RATs is that they’re free for almost everyone. Having to pay for multiple RATs every time you’re sick is a cost burden for those with low incomes, potentially allowing COVID to spread undetected.  Other countries around the world have provided free RATs to tackle this problem.

No one method is more perfect, and with more variants and new waves part of our future now, research continues on other more accurate testing. Until then, those little white RATs remain part of our lives as well.