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SINGAPORE – The use of saliva testing to detect Covid-19 is undergoing field testing in Singapore as health authorities assess whether it can be scaled up for widespread use.
In response to inquiries from The Straits Times, the Ministry of Health (MOH) said on Thursday (September 17) that it is evaluating factors such as the consistency and integrity of sample collection, the efficiency of laboratory processes, and the capacity of serving large volumes of tests through pooled tests.
He added that most samples for Covid-19 testing are collected via nasopharyngeal (back of the nose) or oropharyngeal (back of the throat) swabs, which can be pooled, allowing for efficient processing of large test volumes daily.
Pooled testing involves the use of a Covid-19 test on a group of suspected cases, and if it tests positive, all individuals who were part of the group will be tested individually to determine Covid-19 cases.
The Health Ministry added that its preliminary findings showed that saliva tests may be “a little more comfortable for some people,” although mechanisms are still being developed to expand testing in laboratories, including combined tests.
Associate Professor Raymond Lin, director of the National Public Health Laboratory at the National Center for Infectious Diseases (NCID), said the center is aware of three local studies conducted on saliva testing, all of which reached different conclusions, including one previous study conducted by the center.
“To validate various sampling methods, NCID is conducting further study and analysis using a standardized structured testing approach with approval from the Ministry of Health,” he said.
In a study led by Associate Professor Hsu Li Yang, Infectious Disease Program Leader and Co-Director of Global Health at the National University of Singapore’s Saw Swee Hock School of Public Health, saliva testing through secretions was shown to taken from the back of the throat may be a viable method for testing for Covid-19.
The study, which has not been peer-reviewed, was conducted from June 2 to June 26 and was conducted among 200 migrant workers who were recruited from community care facilities and migrant worker dormitories.
They consisted of confirmed and suspected cases, including those who were asymptomatic close contacts of confirmed cases, as well as patients who had symptoms of acute respiratory infection.
Three tests were performed on the patients at the same time (nasopharyngeal swabs, saliva tests, and self-administered nasal swabs) to compare their efficacy.
The patients were tested up to three times, at intervals of three to four days, to compare the sensitivity of different samples over time, as the viral load of patients varies during the course of infection, explained Professor Hsu.
Patients had a higher viral load in the early stages of their infection in all three types of samples, particularly those with symptoms.
Of the three, the saliva test reflected the highest percentage of positive results, suggesting that it may be more sensitive than nasopharyngeal swabs, while self-administered nasal swabs were the least sensitive.
Since the swabs were self-administered, it is likely that they only reached the front of the nasal passage, which has a lower viral load, so it gives poor results.
To perform the saliva test, patients must clear their throat using a butchery process, so that their sample contains throat secretions with high viral load.
When asked about the possibility of false positives in test results, Professor Hsu noted that false positives and negatives cannot be completely ruled out as they occur in routine tests, but the chances of this occurring and the impact are very low.
However, considering the number of studies published around the world on saliva tests, the sensitivity of saliva in detecting Covid-19 showed variable results, which can be attributed to different test methodologies, such as the shape where saliva is collected and processed, said Prof Hsu.
Noting various trade-offs, such as a longer time required for specimen collection and a more complicated laboratory testing process, he added that study results showed saliva testing was just as viable as nasopharyngeal swabs “and caused less discomfort among patients, which are important points of consideration for local implementation. “
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