Pooled sampling guidelines for returning migrant workers contradict ICMR directions
New Delhi: The Ministry of Health and Family Welfare guidelines issued on Thursday for the pooled sampling of returning migrant workers seems to be in direct contradiction of the recommendations made by the Indian Council of Medical Research with respect to the effectiveness of this testing method in a study.
While the MoHFW guidelines for testing returning migrant workers directed that specimens from 25 people be pooled for conducting RT-PCR (Reverse transcriptase polymerised chain reaction) tests, a detailed feasibility study conducted by a Department of Health Research (DHR) facility has shown that pooling specimens from more than five people would be counterproductive and not yield efficient results.
According to the study, conducted by researchers at the DHR/ICMR Virus Research and Diagnostic Laboratory (VRDL) in King George's Medical University, Lucknow, it was reported that pooling of more than five specimens would lead to a "higher possibility of missing positive samples with low viral load" and that it is "strongly discouraged to pool more than more than five samples, except in research mode".
The study had explained that some individual specimen may contain a relatively low viral load and that if more than five specimen are pooled for testing, there is a much higher of chance of the virus being diluted, resulting in higher risks of false negative results. According to the Ministry's guidelines for pooled sampling of returning migrant workers, if the pooled sample tests negative for SARS-CoV-2, then no individual RT-PCR tests would be needed.
Experts in the field have said that such false negative results may lead to situations where many COVID-19 patients fall through the cracks and then go on to transmit the virus to others.
In addition, the KGMU study showed that pooled sampling is only effective in populations where the positivity rates are low (below 5 per cent). And while the Ministry's guidelines seemingly consider this aspect and direct that such pooled sampling be conducted only in "green zone" districts; it does not consider the population of returning migrant workers.
One doctor from West Bengal said that there is no way of knowing what the positivity rate among these migrant workers is, so it would be impossible to determine if they have low or high prevalence rates, given that they have never been tested before.
With the Shramik Special trains now having started and walking migrant workers finally starting to reach their home states after many of them perished on the way, states like Uttar Pradesh, Madhya Pradesh, Bihar and West Bengal will have to prepare to quarantine and test tens of lakhs of returning workers.
