They’re good enough to get the job done, but the full answer is a bit more complicated than you might think.

With travel restrictions being eased – but oftentimes requiring a negative Covid test to go along with your photo ID – inexpensive self-test kits are gradually becoming as ubiquitous and in-demand as face masks were at the beginning of the coronavirus crisis.

These tests, which are technically ‘rapid antigen’ test kits, are easy, fast, and fairly cheap. So that begs the question: Just how good are they? Can they be trusted to deliver accurate results?

The short answer is that they’re not nearly as sensitive or reliable as the PCR tests – the polymerase chain reaction method that’s considered the gold standard for detecting the presence of the virus. But the complete answer has more nuance.

HOW DO RAPID ANTIGEN TESTS WORK?

The PCR tests actually pick up the presence of the coronavirus’s genetic material – its RNA. They are incredibly sensitive and, consequently, have a very high degree of accuracy. They’re also relatively expensive and take longer to deliver results.

The rapid antigen tests, on the other hand, look for antigens, a protein located on the surface of the virus. They are not as sensitive, and cannot pick up small amounts of the virus the way a PCR test can. The trade-off is that these tests are fast and inexpensive.

Though they could miss small amounts of virus, rapid antigen tests are nevertheless excellent at identifying large amounts. “You have to have quite a bit of virus to turn the antigen test positive,” according to Sheldon Campbell, who is a pathologist, microbiologist, and professor of laboratory medicine at the Yale School of Medicine.

What this generally means is that these tests are not very effective at detecting asymptomatic cases of Covid. A large meta-analysis found that rapid antigen tests detected about 72% of symptomatic cases which were confirmed positive by a PCR test. The rapid tests were even less sensitive with asymptomatic infections, catching on average about 58% of those cases.

Though that seems troubling – knowing that the average rapid antigen test will only pick up a little over half of asymptomatic cases – the reality is, from a public health perspective, these tests are actually quite effective, simply because they’re good at picking up instances where the virus is present in enough quantity to make a person contagious. That’s the main thing, so from that angle, these tests do their job just fine.

Indeed, studies have shown that rapid antigen tests catch nearly all of the cases (93%) that have a solid chance of being transmissible. The rapid antigen tests do this by their ability to identify large viral loads, which indicate a person could be contagious. (People with smaller viral loads are, logically enough, generally believed to be less contagious than those with higher viral loads.)

“Someone who is antigen-negative but PCR-positive is almost certainly less contagious than someone who is antigen-positive and PCR-positive,” said Campbell.

So, if you take the rapid antigen self-test and get a negative result, that means you’re free to board the flight to Langkawi (or wherever may be opened next), but it doesn’t mean with 100% certainty that you are truly Covid-free. However, if you did happen to test negative while having an asymptomatic case of Covid, it nearly always means you’d have so little of the virus in your body that you’d very likely not be contagious. And from a public health standpoint, that’s the most important point.

The Malaysian government has enacted price controls for the wholesale and retail pricing of Covid-19 self-test kits. Currently, the maximum price retailers may charge for any kit is RM19.90.