Coronavirus (COVID-19): Faced with criticism about insufficient testing and the reality of limited resources, India has decided go for pooled testing in districts where no cases have been reported. This would on the one hand give data on whether these really are zero-case districts, and on the other, save resources.
What is pooled testing?
It is a method of testing several samples together with the same technique (RT-PCR) that is being currently used to test individual samples. Samples taken from the nose or throat are pooled together by suspending them in a solution in laid-down proportions. The remaining samples are then labelled and kept separate. Pooled samples are subjected to a test; if it is negative then all samples in the pool are cleared. If it is not, then individual samples are tested to find out which one is positive.
What does the Indian Council of Medical Research say on pooled testing?
Last week, the ICMR brought out an advisory on pooled testing, ideally in districts where incidence of COVID-19 is low.
“Number of COVID-19 cases in India is rising exponentially. In view of this, it is critical to increase the numbers of tests conducted by laboratories… Hence, it may help to use the pooled samples for screening… 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 said. The idea, it said, is to increase the capacity of laboratories to screen more samples for surveillance rather than diagnostic purposes.
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The advisory followed a feasibility study at the Virus Research & Diagnostic Laboratory at King George’s Medical University, Lucknow. The study showed that “performing real-time PCR for COVID-19 by pooling 5 samples of TS/NS (200 microlitres/sample) is feasible when the prevalence rates of infection are low.”
The ICMR has also set an upper limit of five samples that can be pooled; this is to avoid false negatives because of excessive dilution. More samples can, however, be pooled if it is being done only for research purposes.
Where can pooled testing take place?
The ICMR document is clear that pooled testing is only to be used in “areas with low prevalence of COVID-19 (initially using proxy of low positivity of <2% from the existing data). Still a watch should be kept on increasing positivity in such areas”. It adds: “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 pooling samples of individuals with known contact with confirmed cases, Health Care Workers (in direct contact with care of COVID-19 patients). Sample from such individuals should be directly tested without pooling.” The ICMR does not recommend pooling in areas where positivity rates exceed 5%.
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Government sources, though, say pooled testing is currently being considered only in the districts from where no cases of COVID-19 have been reported so far. Reacting last week to reports of private hospitals resorting to pooled testing, Dr R R Gangakhedkar, head of epidemiology and infectious diseases at ICMR, said: “This is to be done in areas where the rate of seropositivity is less than 2%… five samples can be pooled. This helps clear more people with less tests. So far private hospitals are concerned, this is not exactly for individual diagnosis. Using it like that should affect the testing charges. Private hospitals should think about this.”
Is pooled testing being used elsewhere?
In the US, pooled testing is being used in some areas to test population-level infection rates. For example, scientists from Stanford Medicine used it to estimate the prevalence of the disease in the San Francisco Bay Area while conserving scarce testing resources.
It can be used for any disease whose test involves, like COVID 2019, an RT-PCR test, and is routinely used for HIV screening purposes. In a 2010 article in The Journal of Acute Immunideficiency Syndrome, US researchers noted: “Pooling strategies have been used to reduce the costs of polymerase chain reaction-based screening for acute HIV infection in populations in which the prevalence of acute infection is low (less than 1%). Only limited research has been done for conditions in which the prevalence of screening positivity is higher (greater than 1%).”
It is a well-recognised “screening tool” for epidemiologists.
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