While only around 20% of the daily COVID-19 tests done in the State are Reverse Transcription Polymerase Chain Reaction (RT-PCR) tests, which are considered to be more sensitive than more widely-used antigen tests, experts say that increasing RT-PCR testing might not be useful from a clinical point of view, though it would place an additional burden on the system.
“The sensitivity of an antigen test is around 50%, while the sensitivity of an RT-PCR test is around 75%. But the cost of an RT-PCR test is several times that of an antigen test. If RT-PCR testing is increased, we might be able to capture more people who might not have a high viral load. But their viral load is low and chances of disease transmission from them are low,” said Dr. Anish T.S., Associate Professor of Community Medicine, Government Medical College, Thiruvananthapuram.
“Usually, in an antigen test, a false negative is possible when the viral load is low. Identifying people with high viral load is more important,” he said.
Dr. A.P. Sugunan, scientist and officer-in-charge at NIV Alappuzha, said that the detection format in an RT-PCR test, which detected the RNA sequence, was more sophisticated than an antigen test which detects the antigen-antibody reaction which was visible to the naked eye. “The sensitivity of an RT-PCR test means that it can detect even RNA fragments which need not represent viable virus. They might be dead or residual fragments. Whether this sort of positivity is useful clinically or in view of public health is debatable. Testing people who have a reasonable load of virus is important clinically since they are more likely to transmit it, and for this, an antigen test might be a good approximation,” he said.
Data on the COVID-19 dashboard of the Directorate of Health Services shows that the number of antigen tests done has been growing since early July when they were introduced. On the 13th of August, 11,130 RT-PCR tests and 16,369 antigen tests were done. On October 30, the number of antigen tests had increased to 39,944, while 11,812 RT-PCR tests were done.
“RT-PCR tests are roughly 20% of the total test figures with slight daily variations. When the rapid antigen tests are nearly 80% of the testing figures, the PCR capacity would also have to be increased in order to retest the negatives from the antigen tests,” said Rijo John, health economist.
Facility augmented
The number of RT-PCR systems in the State was ramped up from 13 in pre-COVID-19 times to 58 in government facilities. Thirty new machines were procured and 15 were arranged to be used for testing through inter-department transfers at institutions, according to an official at the Kerala Medical Services Corporation Limited.
For RNA extraction and testing, samples are processed in batches of around 94, and it can take about four hours to process a single batch, Dr. Sugunan said.
Districts have varying capacities for RT-PCR testing in government facilities, from a single system in Kollam to four machines in Ernakulam that can process about 200 samples each everyday.
“When we were doing only RT-PCR tests, the system was burdened and there was a problem of delayed results since the process is more tedious,” Dr. Anish said.
On the ICMR’s list, 24 government and 30 private labs are approved for RT-PCR testing.
