Covid PCR vs rapid antigen tests: Which test you should use, when and why

The two tests work in different ways, and have different strengths and weaknesses. Which one to take is all about context

Nathan Place
New York
Friday 21 January 2022 15:24
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PCR tests scrapped for asymptomatic Covid cases in UK

For any American wondering if they have Covid-19, there are two kinds of tests available: polymerase chain reaction (PCR) and rapid antigen (sometimes just called “rapid tests”).

The two tests work in different ways, and each type has its strengths and weaknesses. Which one is preferable depends on the situation, but official guidelines on testing are long and complicated. Here’s a quick guide on which test to use when, and why.

PCR Tests

When it comes to accuracy, PCR tests are considered the gold standard. The test works by detecting genetic material from the coronavirus – even in tiny amounts – and replicating it several times over.

As the Centers for Disease Control and Prevention (CDC) explains, PCRs belong to a broader category of tests, called Nucleic Acid Amplification Tests.

“Amplifying those nucleic acids enables NAATs to detect very small amounts of SARS-CoV-2 RNA in a specimen, making these tests highly sensitive for diagnosing COVID-19,” the CDC says. “NAATs can reliably detect small amounts of SARS-CoV-2 and are unlikely to return a false-negative result of SARS-CoV-2.”

In other words, if you have Covid, a PCR test is extremely unlikely to miss it. The tests almost never give false negatives.

This can also be a disadvantage. If you’ve recently tested positive for Covid and are trying to determine when you’re no longer contagious, the PCR may not be the test to take. That’s because the test can keep picking up minuscule amounts of the virus, long after you’ve already recovered. According to MIT, a PCR test can keep coming back positive several weeks or even months after your infection has ended.

On the other hand, if you suspect you could be at the beginning of an infection – for example, if you’ve recently been exposed to someone with Covid – the PCR is the test to take. Some workplaces and schools require a negative PCR result if you’re returning after an illness, and some countries require it before you travel there.

Most importantly, if you are at risk for severe illness because of another condition – a compromised immune system, obesity, or simply old age – a PCR test is highly recommended to determine whether you have the virus.

Rapid Antigen Tests

Rapid antigen tests – also known as “rapid tests” or “rapid lateral flow tests” – work by detecting a certain protein found in the coronavirus. They are less sensitive than PCRs, which is both an advantage and a disadvantage.

Because it’s less likely to detect tiny amounts of the virus, a rapid test can more easily yield a false negative result – so if you’re trying to determine for certain whether you’ve caught the virus, a PCR would be a better choice.

On the other hand, if you’ve already been infected for some time and are trying to find a safe endpoint for your isolation, the rapid test would be the one to take. That’s because while a PCR test could keep telling you you’re infected for months, a rapid test will likely only come back positive if you have a significant amount of virus in your system – enough to be contagious to others. As you try to safely leave your self-quarantine, that’s the question that matters.

“Even though the rapid test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Dr Albert Ko, a professor at the Yale School of Public Health, told CNN.

The other advantage to rapid tests is that they’re faster. While the results from a PCR test can take several days to come back from a laboratory, a rapid test typically takes 15 to 30 minutes.

The bottom line is that if you’re at the beginning of an infection – or worry you are – you should take a PCR test. If you’re approaching the end – or hope you are – you should take a rapid test.

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