Let’s say a cluster has 100 people to be tested. The samples could be divided into groups and a sample containing specimens of multiple people can be run through the PCR system simultaneously. If the collective sample tests positive, individual samples in that group will be taken up for testing. If the collective sample tests negative, it can be assumed that no one in that group is Covid-19-positive and there is no need to run individual tests.
Such a methodology helps to save reagents used for testing. A Velumani, chairman of Thyrocare, said, “Reagents comprise around 30 per cent of the test cost.” He felt there can be savings of 70 per cent of the reagent cost when testing in a random population, while when testing in a suspect population, the savings on reagents would be around 20 per cent or so.
The private diagnostic laboratories have already approached the respective state governments and the municipalities where they are operating to approve protocols for such testing in clusters. Velumani said that Thyrocare and Mumbai’s Kasturba Gandhi Hospital are already working on the same and a protocol is in place.
He added that in this case, however, the testing methodology and protocol can be different for different clusters. “In hotspots, we cannot take large sample pools as we cannot afford dilution,” said Velumani.
Executives at private labs feel this is one of the best ways to expand testing into the hinterland. “In a village of a few thousand people, the testing can be done on sample pools of 15-20 people or more. This would save time and cost of tests,” said an executive at a private diagnostic chain.
The ICMR, on its part, has suggested that pooling of samples of more than five people should be avoided, as there is a possibility that the effect of dilution will lead to false negatives. In simpler terms, positive samples with low viral load can be missed out if the sample pool is more than 5. The ICMR has tested this method at its labs in Lucknow.
Another method of testing the government is considering is faecal screening. Several studies done globally have indicated that testing excrement can help detect Covid-19 in patients much before they show symptoms. Researchers in Australia and the Netherlands, for example, have found that people infected with the virus start excreting traces of Covid-19 almost a week before their flu-like symptoms emerged.
An industry source said the government is considering the same as a screening tool here. “Testing from sewage samples of housing colonies can be done. The sewage samples from each building in a housing colony can be tested. The pooled sample testing can be done for people living in the identified building,” explained an industry official.
In cities like Mumbai which have been badly hit, such a screening is likely to be implemented soon, as the government runs dry of test kits and reagents.