The tests should be carried out for at least three consecutive days to guard against the Omicron variant and four straight days against the Delta strain, said Kumitaa Theva Das of Universiti Sains Malaysia.

Kumitaa told FMT that Omicron could be detected three days after exposure, the Delta variant on the fourth day and the original Wuhan strain on the fifth day.

She said a once-a-week self test would be sufficient for those who were not going out regularly but were living with those who did.

She warned against trusting negative results in early testing.

Kumitaa also said a Twitter user’s experience of testing negative on her self-test kit but positive on her RT-PCR test was to be expected.

She said PCR testing relied on multiplying the virus load in the swab sample to enable detection.

“With a saliva test through the RTK-Antigen kit, if it is early and the virus amount is too little, it might not be picked up,” she said. “Hence, a three-days-in-a-row testing is key.”

The Twitter user’s PCR results showed she registered a Ct value of 36 to 37, which Kumitaa said indicated that it might be a recent infection or the end of the infection. Ct stands for “cycle threshold”, which measures the viral load.

She said low Ct numbers might not be picked up through antigen testing.

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