Explainer: Are antigen rapid test kits less effective in detecting Omicron?

SINGAPORE — Three weeks ago, Mr Lee Xing Shun developed a slight cough and sore throat.

Worried that he was exhibiting symptoms of Covid-19, the 37-year-old did an antigen rapid test and polymerase chain reaction (PCR) test under the supervision of a doctor, only for both results to show negative.

Two days later on Jan 20, Mr Lee, who does infrastructure operations and management, tested positive on with an antigen test as part of his weekly Covid-19 test for work.

Another antigen test the same day under the doctor’s supervision showed negative for the coronavirus, but his PCR results came back positive the next day.

Mr Lee, who has since recovered from Covid-19, said that there was a “high chance” he could have missed detecting his coronavirus infection if he had relied on the results of the antigen rapid test alone.

“If I had skipped the second PCR test offered by the doctor, I may not have known that I was positive,” he added.

Mr Lee’s situation echoes the experience of some other people in Singapore who had Covid-19 symptoms but tested negative on their antigen rapid test, only to find out shortly after that that they had been infected.

Mr Yee Jenn Jong, former Workers’ Party Non-Constituency Member of Parliament, for instance, detailed in a Facebook post on Jan 27 about how he had self-tested a “faint positive”, and then a negative the next day, followed by a positive again on the third day.

There have also been social media accounts of some Singaporeans who continued with their Chinese New Year visiting last week because they had registered negative on their antigen rapid test and came to realise later that they had Covid-19.

With the number of new local Covid-19 cases tripling on Friday (Feb 4), some people have questioned whether the antigen test is a reliable way to detect the Omicron coronavirus strain, which has fast become the dominant strain here.

Singapore’s Ministry of Health has maintained that antigen rapid test kits are effective in detecting Omicron cases, but the United States’ Food and Drug Administration has warned that some kits may have reduced sensitivity for the fast-spreading strain.

TODAY spoke to some infectious diseases experts here to find out if Singapore should change the way it detects and tests Covid-19.

The experts said that the current strategy of relying on antigen rapid test kits remains relevant and urged people to regularly test themselves and self-isolate if they suspect they have the virus.

IS RAPID TEST EFFECTIVE IN DETECTING OMICRON?

There is no conclusive evidence to show that an antigen rapid test is less effective in detecting Omicron, the infectious diseases experts said.

Dr Leong Hoe Nam of Rophi Clinic said that rather, the sensitivity rate for an antigen test improves to more than 90 per cent if done frequently and consistently over three to five days daily.

The sensitivity rate is a measure of a test’s ability to detect a positive case.

“It’s like a lucky draw. The more times you do it, the better chance you have of picking up (the infection),” he said.

Professor Dale Fisher, a senior consultant at National University Hospital (NUH), said that there are other variables that could affect the test’s sensitivity, such as the viral load and sampling technique.

Another senior consultant from NUH, Professor Paul Tambyah, said that there is more likely to be false negative antigen rapid test results due to the lower viral load of the Omicron compared to the Delta strain.

However, Dr Leong said that Omicron has a viral load that is comparable to the earlier Alpha and Beta strains, and antigen rapid test kits were initially designed for these variants.

As such, it does not mean that people infected with the Omicron variant are more likely to register a false negative on their tests.

All three experts were of the view that there is no need to change Singapore’s current approach of relying on widespread self-testing, because it continues to offer an easy and accessible way to detect most cases.

SHOULD WE INTRODUCE THROAT SWABS?

In addition to nose swabs, some scientists have suggested using self-test kits to swab throats to improve the chance of detecting the virus.

This is because people can transmit Omicron when the virus first infects their throat and saliva, but before it reaches their noses.

While some countries such as Israel have already recommended throat swabs, infectious diseases experts here suggested sticking to the current guidelines unless advised otherwise.

Prof Tambyah said that there needs to be more data before Singapore changes its approach to swabs.

WHAT TO DO IF YOU HAVE SYMPTOMS BUT TEST NEGATIVE?

Even before Chinese New Year, various political leaders here had reminded people to stay home and avoid visits if they feel unwell, even if they test negative on an antigen rapid test.

In a Facebook post last Sunday ahead of Chinese New Year, Prime Minister Lee Hsien Loong urged residents to isolate themselves if they feel unwell and catch up on visiting after they recover.

The infectious diseases experts also advised people to isolate themselves if they suspect that they are infected.

Dr Leong reiterated that people should repeatedly test themselves to increase their chances of detecting an infection, while Prof Fisher urged those who need to go out to wear their masks, avoid crowds and adopt good hand hygiene.

For those who do not exhibit symptoms and test negative on an antigen rapid test, Associate Professor Alex Cook from the National University of Singapore's Saw Swee Hock School of Public Health said that there is no reason for them to change their behaviour unless they suspect they have been exposed to the virus.

What if those who test negative are harbouring the virus but not infectious yet?

Assoc Prof Cook said that it is not possible for these individuals to know if the virus is in the latent phase, which refers to the time when they are first infected to becoming infectious. 

They should treat the negative test result as an indication that they are not infectious at that point, he added.

For Omicron, the latent phase is typically shorter than the three-day period it takes for symptoms to show.

The authorities here used to treat people with exposure to the virus as potentially infected regardless of their test result and quarantine them, but doing so now given the lower severity of Omicron and high vaccination rates is “an excessive amount of caution”, Assoc Prof Cook said.