Early tests mean Covid-19 patients detected at 'more infectious' phase - DG

 

COVID-19 | Health Ministry director-general Dr Noor Hisham Abdullah said swift action to detect Covid-19 cases may have led to patients being detected at an earlier, highly infectious phase of the disease.

However, he does not rule out the possibility that the high level of infectivity may be due to the virus’ D614G mutation.

He said this in response to a question from the media asking why patients’ samples from the current third wave have a lower cycle threshold value (C_t) when tested, compared to samples from the first two waves of the outbreak in Malaysia.

“If you look at the cases in Sabah, the C_t value detected may be because we have conducted screening quickly. If we do it quickly, we are detecting cases at its beginning compared to the first and second waves where the C_t value corresponds to a later stage of the infection.

“So, what we have done is active case detection. We identify cases sooner, so their C_t value is lower and the amount of virus at the time is high. When the amount of virus is high in a person’s body, his infectivity is higher […]

“Another possible reason is because of the D614G mutation is causing the body to have a higher viral load. This causes the virus to become more infectious,” he said at his daily briefing today, while still in quarantine at home.

C_t is one of the key parameters in a reverse transcription-polymerase chain reaction (RT-PCR) test, which is regarded as the gold standard diagnostic test for Covid-19 and is the only test used for final diagnosis of suspected Covid-19 patients in Malaysia.

RT-PCR tests are more sensitive than other tests because it involves an “amplification” step, where viral particles in the sample are artificially doubled. This step is repeated again and again until the number of viral particles in the samples reach detectable levels.

The number of cycles needed for the sample to be detectable as positive is referred to as the C_t value.

Hence, a lower C_t value means fewer cycles of amplification are needed to detect the virus’ presence, thus implying that it is already present in high numbers in the original sample. It should be noted that C_t values are also affected by variations in the equipment and reagents used.

Noor Hisham said laboratories in Malaysia normally run about 40 cycles of RT-PCR on its samples before moving on to the next batch of samples.

According to the question posed to him, the Ct value of samples during the first and second waves of the outbreak in Malaysia is usually between 23 to 30.

In comparison, the Ct values in the third wave are generally between 16 to 18.

For the record, previous studies showed that Covid-19 patients are most infectious about several days after acquiring the infection, just before symptoms appear.

The viral load of these patients is also at its highest just before symptoms appear and then declines until the patients are unlikely to be infectious 10 days after symptoms appear, even if they still feel ill or test positive on RT-PCR tests.

Meanwhile, Noor Hisham said 61 laboratories nationwide conducted 24,952 RT-PCR tests yesterday, out of a combined capacity of 54,706 tests per day.

The three laboratories in Sabah are running over capacity with 3,032 tests conducted out of a capacity of 2,500 tests per day. They received 3,296 samples yesterday, some of which would be flown to Peninsular Malaysia for testing instead.

As for antigen rapid tests (RTK-Ag), Noor Hisham said a total of 289,400 tests have been conducted since the beginning of the pandemic, of which 2,338 were positive.

He said 115,000 RTK-Ag kits are being shipped to Sabah, including 75,000 that have been delivered yesterday and another 25,000 to be delivered next week.

“With the increase (in number of test kits) we hope to detect more cases in Sabah,” he said.

The RTK-Ag kits are less sensitive than RT-PCR, but can be done in minutes rather than hours and only requires basic laboratory facilities. - Mkini