Beware the false negatives

Rush before the dinner bell: Penangities at Ayer Itam market shopping for last minute Chinese New Year essentials. — LIM BENG TATT/The Star.

PETALING JAYA: Never mind if the test kit says you don’t have Covid-19. If you have symptoms, just stay home and avoid contact with others this Chinese New Year.

That is the message from health experts, who say new variants can evade antibodies and may not be easily identified by tests. Incubation time also plays a role in virus detection.

Those with symptoms must mask up and refrain from going to crowded places even if they test negative, says the Medical Practitioners Coalition Association of Malaysia (MPCAM).

They should also avoid family gatherings and the annual reunion dinners just to be on the safe side, said MPCAM president Dr Raj Kumar Maharajah.

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“False negatives are present in every test. The best protection is still to wear masks, practise physical distancing, avoid crowded places, and maintain hygiene,” he said.

Public health expert Prof Dr Sharifa Ezat Wan Puteh of Universiti Kebangsaan Malaysia said false negative Covid-19 results can occur for many reasons, and it was best to retest after a few days.

“The new variants have developed mutations. They may evade previous antibodies and may not be easily recognised by tests,” she said.

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The Rapid Test Kit Antigen (RTK-Ag) used for self-tests also has a lower accuracy, estimated at 75%, compared to the gold standard Polymerase Chain Reaction (PCR) test, which has a 95% accuracy, she said.

“PCR detects nucleic acid, but RTK detects antigen only, and you may or may not be infected.

“Incubation time also plays a role in virus detection.

“If the test is done too early, the virus may not be detected,” she said.

Prof Sharifa Ezat also cited a 2022 study in the British Medical Journal, which mentioned that the overall median incubation time of the Delta variant is almost three days, with 95% testing accuracy only after about eight days.

The incubation times found in this study for Covid-19 were shorter than those observed with the earlier variants.

“So if you test too soon, your body may not develop enough viral load to test positive.

“It is best to retest if negative. Retest within 48 hours after the first test, and you may repeat the test if symptoms persist or you can test using the PCR method,” she said.

Universiti Malaya professor of epidemiology and public health Prof Dr Sanjay Rampal said those who have come in contact with a positive case during the infectious period should self-monitor for symptoms and seek care immediately if symptomatic.

“When asymptomatic, wear a mask,” he said.

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