The Indian Council of Medical Research (ICMR), in an advisory, has advised the feasibility of using pooled samples for molecular testing of COVID-19 citing the rise of cases in India, The Hindu has reported.
According to the report, the advisory states that it is crucial to increase the number of tests being done in laboratories in India.
"Positivity rate in cases is still low. Hence, it may help to use the pooled samples for screening," the advisory noted. It said that the aim was to increase the capacity of laboratories to screen an increased number of samples for the purpose of surveillance, although it has noted that pooling of samples is "not recommended" in "areas or population with positivity rates of >5 percent for COVID-19".
"Preferable number of samples to be pooled is five, though more than two samples can be pooled, but considering higher possibility of missing positive samples with low viral load, it strongly discouraged to pool more than 5 samples, except in research mode,' the advisory said, according to the newspaper.
The ICMR said that the feasibility study was conducted at DHR/ICMR Virus Research and Diagnostic Laboratory (VRDL) at King George’s Medical University (KGMU), Lucknow.
"It has been demonstrated that performing real-time PCR for COVID-19 by pooling 5 samples of TS/NS (200 ul/sample) is feasible when the prevalence rates of infection are low," the advisory said.
"All individual samples in a negative pool to be regarded as negative. De-convoluted testing is recommended if any of the pool is positive. Pooling of more than 5 samples is not recommended to avoid the effect of dilution leading to false negatives," the advisory further noted, according to the newspaper.
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What is pooled sample testing?
The advisory has said that a pooled testing algorithm "involves the PCR screening of a specimen pool comprising multiple individual patient specimens, followed by individual testing (pool de-convolution) only if a pool screens positive".
"As all individual samples in a negative pool are regarded as negative, it results in substantial cost savings when a large proportion of pools tests negative," the advisory has noted.
In other words, a pooled testing method involves putting multiple swab samples in a test tube and testing them using a single RT-PCR test.
So, if the test is negative, all the people tested are negative.
If the test is positive, all persons are tested individually. This method can greatly enhance the capacity to test in a low-resources setting, as RT-PCR test is reliable but not scalable.
The number of samples that can be pooled will be determined by the infection rate. More samples can be pooled with less infection.
Testing samples from multiple patients with a single PCR test, also known as pooled sampling, has been used previously in the early stages of the HIV epidemic when PCR costs were high.
