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Test kits that can determine if a person has Covid-19 in minutes are not being used in national swabbing exercises as they are not sensitive enough to detect all infections.
“They give false negative results, they miss true cases,” said associate professor Raymond Lin, director of the National Public Health Laboratory at the National Center for Infectious Diseases (NCID).
Professor Lin told The Straits Times: “Currently available rapid antigen test kits can miss up to one in six cases, or even more. If we miss cases, we will have more outbreaks.”
NCID will continue to search for test kits that are fast and accurate, he added.
More than 120 Covid-19 diagnostic test kits have received provisional clearance from the Health Sciences Authority (HSA), ranging from polymerase chain reaction (PCR) kits to antigens and serologicals.
All three work differently.
PCR tests look for fragments of the virus’ genetic material in a sample from a patient and are considered the “gold standard” test for Covid-19.
But the whole process, from taking samples from a patient to sending the sample to a laboratory and the PCR process itself, can take up to a day. It can also be expensive and requires trained personnel and specialized equipment to administer.
Antigen test kits, on the other hand, are site-of-care tests that look for proteins on the surface of the virus and can produce quick results, usually within minutes.
Serology tests, which detect the presence of antibodies, are mainly used to find out if a person has been infected in the past and not to diagnose patients.
Last Friday, the World Health Organization presented its general recommendations on the use of these rapid test kits.
The guidelines include specific circumstances, such as treating suspected Covid-19 outbreaks in remote settings where PCR testing is not immediately available, and for monitoring trends in the incidence of the disease in communities.
Professor Teo Yik-Ying, Dean of the Saw Swee Hock School of Public Health at the National University of Singapore, noted that because rapid diagnostic tests are cheaper and less invasive than PCR tests, their prudent implementation and repeated could complement national measures to control Covid-19.
But they should be used in a way that reduces the chance that false readings, whether positive or negative, will hamper the system, he said.
If protocol dictates that rapid test readings be acted upon, for example, false negatives could lead to new outbreaks, while false positives could add strain to the hospital system and the patient’s pocket.
But Professor Teo said there is the possibility of implementing rapid test kits in certain settings, such as in population segments that undergo routine monitoring.
“Migrant workers undergo biweekly tests, and the process of undergoing PCR smears every 14 days can be quite traumatic, as the nasal smear is quite unpleasant,” said Professor Teo.
It might be possible to use rapid tests, perhaps two or three times every fortnight, to perform the same surveillance, he noted.
“Because the cost of some of these rapid tests is lower, a higher frequency of testing can be less expensive than the PCR swab test, and certainly if there is enough demand, the price will drop as well.”
The American medical technology company Becton Dickinson (BD) has developed a rapid test for Covid-19 that has obtained provisional approval from the HSA.
Its BD Veritor Sars-CoV-2 Rapid Antigen test can detect an infection in 15 minutes, its spokesperson said, adding that each test has its pros and cons.
PCR tests are expensive, require trained molecular technicians, and take more time to produce results. Its robustness also depends on the segment of the viral genetic material that is being tagged.
Rapid tests can complement PCR tests, not replace them, as different tests are appropriate for use at different times, the spokesperson said.
“Having rapid antigen testing at the points of care … complements PCR testing, allows hospitals and laboratories to prioritize their PCR testing for urgent cases and not overload the laboratory system,” he added.
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