PCR tests here to stay despite rise in rapid tests, say scientists

Scientists here say that despite the recent rise in the number of rapid Covid-19 tests available on the market, conventional polymerase chain reaction (PCR) tests are likely to remain in use at least for the near future as they complement each other.

On Oct 20, the Ministry of Health (MOH) announced that participants in some mass events would have to take an antigen rapid test (ART) and obtain a negative result before being admitted, under a new pilot programme.

And on Oct 29, National University of Singapore (NUS) spin-off Breathonix said it was in discussions with MOH to deploy breathalyser-type tests in trials at public locations.

Those taking a conventional PCR test usually have to wait a day or two for their results. In contrast, ARTs return results in about 30 minutes, while the breathalyser-type test does so in under a minute.

The ART and breathalyser-type test are less accurate than a PCR one. The PCR test has been described as the "gold standard" of Covid-19 detection during the pandemic.

But experts say rapid tests certainly have a role to play.

Associate Professor Alex Cook, vice-dean of research at NUS' Saw Swee Hock School of Public Health, said that frequent testing using a less sensitive test, such as a rapid test, will probably pick up more infections than infrequent testing with a more sensitive test like PCR .

"We could potentially reduce the length of quarantine for incoming travellers by testing them frequently with rapid tests, rather than giving them a single exit test after two weeks," he told The Straits Times last week.

Associate Professor Hsu Li Yang, who is also from NUS' Saw Swee Hock School of Public Health, pointed out that more frequent testing creates more opportunities to detect the coronavirus.

Prof Hsu, who is vice-dean of global health and leader of the infectious diseases programme, added: "In a situation where you need results in a hurry, (like) an hour or two before a mass event, you wouldn't be able to get a PCR result at all - hence the need for these rapid tests."

Professor Dale Fisher from the NUS Yong Loo Lin School of Medicine noted that laboratories may be hard to access in some countries, making point-of-care rapid tests a useful alternative, as they do not require a lab.

But Prof Hsu emphasised that more frequent testing with a rapid test cannot entirely replace PCR tests, as the former has its drawbacks.

"More frequent testing increases the likelihood of a false-positive result for tests that are less specific, increases inconvenience for those taking the tests, and also increases the costs as compared to doing the tests less frequently," he said.

The specificity of a test refers to its ability to correctly identify those without the disease, while the sensitivity of a test refers to its ability to correctly identify those with the disease.

Currently, PCR tests remain the most sensitive form of testing for Covid-19, said Prof Hsu. "Therefore in uncertain clinical situations, especially (with) hospitalised patients, these would still remain the preferred tests," he said.

Prof Cook agreed, saying that until rapid tests are able to match the accuracy of PCR tests, the latter will still be the test needed for clinical confirmation of infection.

MOH had said in October that those who test positive through an ART during the pre-event swabbing pilot need to get a PCR test done to confirm the results.

Prof Hsu noted that Singapore has been able to get to phase two of its reopening using PCR tests alone, and can achieve phase three using the same.

But he added that the availability of rapid tests might allow for greater reassurance and safety when used for pre-entry screening for mass events where hundreds mingle together, although this would come at the logistical costs of setting up the screening area, as well as having to deal with the occasional false-positive test result.

Prof Cook said: "It's horses for courses. Fast tests like antigen have a different role than do PCR tests, and both roles are important in our response. PCR tests are going to remain the gold standard for clinical diagnosis, I'm sure, but the rapid tests allow different kinds of testing."

For example, he pointed out that rapid tests are already being used on cruises to nowhere to test passengers prior to embarking.

"This is far more practical than having passengers go to a clinic a few days before for a PCR test, and reduces the risk of interval infections slipping through... these being infections any time from a little while before going for the PCR test to boarding the ship, which will clearly not be identified by a PCR test," said Prof Cook.

Prof Fisher said: "Different tests allow us to tailor efforts to the context. In some situations speed may be more important, (while) in others it will be accuracy."