Covid-19: Rapid antigen vs PCR tests

DIY Covid-19 antigen rapid test kits being unpacked before being put on display for sale in a Singapore pharmacy. Such kits are useful for their quick results, although they are not as sensitive as PCR tests. — Reuters

On June 10 (2021), Singapore’s Health Ministry announced that DIY (do it yourself) Covid-19 antigen testing kits would be available for public to buy off the shelf for self-testing purposes from June 16 (2021).

But what is a Covid-19 antigen test, and how is it different from a Covid-19 PCR (polymerase chain reaction) test?

Imagine that the SARS-CoV-2 virus that causes Covid-19 is a person wearing a coat.

This coat is called the antigen, while the virus’ RNA (ribonucleic acid) is the person’s body.

Antigen tests work by detecting the presence of the coat, while PCR tests work by detecting the presence of the body.

Vaccines and our own immune systems also target the antigen.

Interestingly, just as we don’t wear the same coats every day, viruses might also change their antigens when they become a new strain of the same virus.

Currently, most of the Covid-19 antigen tests offered in clinics and done by health professionals or trained personnel, take a sample from all the way to the back of the inside of the nose.

While it is largely painless, this procedure may cause some discomfort like watering eyes, or a feeling that you need to sneeze.

Conversely, some of the DIY Covid-19 antigen kits that can be used by the public only require the person to take a sample from the front part of the nostrils.

This makes it easier for untrained members of the public to take a sample.

Covid-19 PCR tests also use samples from the back of the nose.

As it detects the “body” or RNA of the virus and only requires a small amount of it to be present, this means that this test is the most accurate and sensitive in detecting whether a person is infected or not.

Less sensitive, but faster

Covid-19 antigen tests are known to be less sensitive than the Covid-19 PCR tests.

This means that they have a higher chance of giving a negative result, even when the person being tested is actually infected.

One reason is that, as mentioned previously, many of these DIY Covid-19 antigen test kits only require samples from the front of the nostrils.

The amount of virus may be less than if the samples were taken from the back of the nose.

It also comes down to the fact that the Covid-19 PCR test only needs a tiny amount of the “body” of the virus to produce an accurate result, as the PCR technology is able to multiply a tiny amount of the “body” of the virus millions of times.

Therefore, this test will be able to detect an infection even if the person only has a very small amount of the virus.

You may wonder then why are we using Covid-19 antigen tests at all if they are less sensitive than the Covid-19 PCR tests.

The main reason is that Covid-19 antigen tests give quick results.

It requires roughly about 15-20 minutes to give an idea of whether someone is infected or not.

In contrast, Covid-19 PCR tests take anywhere from a few hours to 24 hours for a result.

Thus, if the aim is to try to identify and isolate people quickly to reduce the chance of spreading it further, then using the antigen test may be more appropriate.

So, if you think that you may have been exposed to the SARS-CoV-2 virus, you may want to do your own testing first.

If the DIY test kit turns positive, then you should go to a health professional for further advice.

Please make sure that you only buy from reputable sources if you do wish to use these DIY Covid-19 test kits.

You may be tempted to buy these online, but in some cases, these kits are fake, not properly manufactured or poorly stored.

This can result in you purchasing a malfunctioning test kit.

Additionally, please follow the instructions of the kit strictly.

If you don’t, you may cause the result to be falsely negative, even though you might actually be infected.

Lastly, if you do test positive using these DIY test kits, please visit the nearest medical professional as soon as possible to seek further advice and testing.

Please do not brush off a positive result, even if you do not have any symptoms, as you might be endangering the lives of your loved ones at home, as well as your colleagues and friends.

You might not be lucky enough to win the lottery, but you certainly don’t want to take any chances when it comes to a Covid-19 infection!

Dr Julian Ng is the chief medical officer of a primary care clinic group with branches in Singapore, Malaysia and Vietnam. For more information, email [e-mail]. The information provided is for educational purposes only and should not be considered as medical advice. The Star does not give any warranty on accuracy, completeness, functionality, usefulness or other assurances as to the content appearing in this article. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.