The Tamil Nadu government has decided to go in for pool testing of COVID-19 samples. The protocols are expected to be ready in a few days.
Pool testing is a method by which multiple samples, up to a total of five, are tested together, in one single RT-PCR test. In mid-April, the Indian Council of Medical Research (ICMR) put out an advisory, recommending pool testing as the number of positive cases began to rise rapidly.
Pool testing can be done only in a population where the positivity is known to be low. Five samples are taken together and analysed, and if the result of a pool is positive, individual samples are tested again.
Health Secretary J. Radhakrishnan said the government had decided to go ahead with the testing methodology in areas where it was sure that case positivity rate was under 2%. “It will not work in an area like Chennai, where the positivity rate is about 20%. But there are several areas where the positivity is way lower than that, which will qualify. One area we are directly looking at is people returning from abroad. That category of people has only about 1.5% positivity. Pool testing would be ideal in that situation.”
Pool testing is recommended as a tool to increase the number of tests quickly, and a cost-saving measure while doing so. As per the ICMR advisory, it must be used only in areas with low prevalence of COVID-19. In areas with positivity of 2-5%, sample pooling for PCR screening may be considered, only in community survey or surveillance among asymptomatic individuals, strictly excluding the pooling samples of individuals with known contact with confirmed cases, and health care workers in direct care of COVID-19 patients. It says that it is preferable that the number of samples to be pooled is five, though more than two samples can be pooled, because of the higher possibility of missing positive samples with a low viral load.
No antigen testing
The State government has also decided that since it has ramped up its RT-PCR testing and will go in for pool testing as well, antigen testing is not necessary. Antigen tests involve drawing a blood sample and testing it to show if a person has antibodies for COVID-19, indicating that he/she has, at some point, been positive.
“In this scenario, we feel that antigen tests are not necessary here. Given that its sensitivity (correctly identifying those with disease) and specificity (those without the disease) is lower than RT-PCR, we would rather go for the latter,” Dr. Radhakrishnan explained.
