PENANG’s mass Covid-19 screening programme (PSC-19) gave 275 people a good fright before they heaved a sigh of relief in the subsequent test.
As of Monday, a total of 401 people first tested positive using the antigen rapid test kits (RTK-Ag), before subsequent tests using the reverse transcription polymerase chain reaction (RT-PCR) showed only 126 confirmed cases.
So far, 11,330 people have taken part in the programme.
Clinical pharmacologist Prof Dr Lee Toong Chow explained that RTK-Ag kits generally have an inaccuracy rate of between 2% and 10%.
He said the RTK-Ag tests conducted on the asymptomatic community that factored in low prevalence rate could lead to higher false positive cases.
“Depending on the test methods used, those with lower accuracy rate means the probability of recording false positives will become higher.
“Three factors influence the false positive diagnosis of accuracy, sensitivity and prevalence rate.
“If the prevalence rate is 0.3%, the false positive rate of 5% may grow up to 35% down the chain using RTK-Ag tests.
“The numbers are only confirmed when using RT-PCR tests with a 99.7% accuracy which are currently the gold standard of diagnosing Covid-19,” he said.
Dr Lee, who is also Penang Safe project coordinator, claimed that the RTK-Ag kits used by them have a 2% inaccuracy rate.
“The higher the prevalence rate, the lesser the error in tests as this group has higher viral load.
“In Penang, the prevalence rate in non-red zones is below 0.3% while in red zones, positivity may be as high as 10% to 20%.
“Hence, RTK-Ag tests are mostly used on people with Covid-19 symptoms,” he said.
“The RT-PCR tests are more accurate because they use three markers to conclude the results, while RTK-Ag tests use antigen against N-gene, a plasmid that contains the SARS-CoV-2 N-gene.
“RT-PCR tests look for the presence of viral genetic material (RNA) in a sample taken from the patient, while RTK-Ag tests look for viral antigens in a sample.
“Despite being highly sensitive and specific, RT-PCR tests are slow with results taking a day to produce, besides being labour- intensive and costly.
“RTK-Ag tests are cheaper but less sensitive, and results can be obtained within 10 to 15 minutes to speed up isolation work.
“RTK-Ag tests are more suitable for screening but RT-PCR tests are confirmatory,” he added.
As daily cases remain high, Dr Lee urged the people to be disciplined.
“Many people, especially those in the industrial sector, have at least received their first dose of the vaccine.
“Being vaccinated does not ensure full immunity to Covid-19, but only reduces the severity during Covid-19 infection.
“People may still spread the virus due to poor discipline in adhering to standard operating procedures meant to break the infection chain,” he said.