Fast Covid-19 tests to be added to Singapore's testing arsenal: How do antigen rapid tests work?

The Health Ministry will be turning to Covid-19 antigen rapid tests to complement existing polymerase chain reaction tests – the gold standard for testing – as Singapore further opens its economy.

Covid-19 antigen rapid tests will complement existing polymerase chain reaction tests.
Covid-19 antigen rapid tests will complement existing polymerase chain reaction tests.PHOTO: AFP

Why are these rapid tests being piloted?

• Successfully conducting rapid tests before certain mass events will give the Government greater confidence to allow for more activities, which have yet to be approved, to resume.

• This will allow more activities to resume.

Antigen rapid tests (ARTs)

What does it look for

Proteins on the surface of the virus called antigens.

How will they be used

• Participants will be tested before certain mass events. These pilot programmes will be carried out till the end of the year.

• However, this does not replace the need for safe management measures as they target two separate things. The tests reduce the probability that an infected person might enter the venue while safe management measures prevent an infected patient in the venue from transmitting the virus, lowering the chances of clusters forming.

Benefits

• Faster (less than 30 minutes) and hence more feasible for pre-event testing.

• Cheaper.

• Easier to administer.

Drawbacks

• Tests have lower sensitivity and specificity, and may carry a higher risk of false positives and false negatives.

• The World Health Organisation recommends at least 80 per cent sensitivity and 97 per cent specificity. This means that at least 80 per cent of those infected are identified, and at most 3 per cent of healthy subjects are tested false positives.

How it is done

Some involve using a nasal swab from the lower part of the nose.

Polymerase chain reaction (PCR) tests

What does it look for

Genetic sequences of Covid-19.

How will they be used

For most other tests, including tests for essential workers, those with symptoms and people entering the country.

Benefits

The most accurate tests available today.

Drawbacks

• Long duration: The process from the point of swab to the release of results takes about one to two days.

• Its lab-based nature requires a logistical chain to convey the samples and specialised systems to monitor/report the results.

How it is done

Swabs are taken from the back of the nostrils, or midway up the nose and from the back of the throat.

What is the difference between sensitivity and specificity?  

Sensitivity

• The test’s ability to identify those infected as positive.

• This means that a test with lower sensitivity will have more false negatives.

• Given the current Covid-19 community prevalence rates, very few individuals are expected to be infected in the community. Among the infected few, around 12 to 24 per cent are expected to test false negative.

Specificity

• The test’s ability to identify those not infected as negative.

• This means that a test with lower specicity has more false positives.