Explained: ‘Pool Testing’ for COVID-19 in India

How is pool testing conducted and what does it aim to achieve? 

4 min read
Explained: ‘Pool Testing’ for COVID-19 in India

In an advisory issued on 13 April, the Indian Council of Medical Research (ICMR) suggested using ‘pooled samples’ for screening for COVID-19 in areas with a low positive rate of the disease.

The Andaman and Nicobar administration was the first in India to have tried the technique in order to increase testing and save up on time. Chief Secretary Chetan Sanghi tweeted that the union territory has been pooling samples using just one-fourth of the test kits.

A private hospital in Delhi will also be conducting pool testing for all its employees, as FIT had reported earlier.

As more states are expected to follow suit, we explain what the method means and how it could help with COVID-19 containment.


What is Pool Testing?

The ICMR explains the pooled testing algorithm in the following manner:

“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.”

In simpler words, this means that instead of conducting singular tests for the novel coronavirus, samples of several individuals are pooled together, and this pool is then tested using one RT-PCR kit. If this pool tests negative, it is concluded that the entire group is COVID-19 negative.

This way, the number of test kits used comes down.

If, however, the pool tests positive, subsequent PCR tests will have to be conducted for each individual part of the pool to identify the positive case.

The method is not new. In fact, it was introduced back in the 1940s and has been used for surveillance and screening of transmittable infections such as HIV and syphilis ever since.

Where is Pool Testing Advised?

This testing in India is advised only in areas with a low prevalence of COVID-19 (under 5%). Even among them, there are specific guidelines to be followed:

  1. Use only in areas with a low prevalence of COVID-19 (initially using proxy of low positivity of less than 2% from the existing data). A watch should be kept on increasing positivity in such areas.
  2. In areas with the 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, such as the Health Care Workers (in direct contact with the care of COVID-19 patients). Sample from such individuals should be directly tested without pooling.
  3. The pooling of the sample is not recommended in areas or population with positivity rates of more than 5% for COVID-19.
  4. The preferable number of samples to be pooled is five, though more than two samples can be pooled. Considering the higher possibility of missing positive samples with low viral load, the ICMR strongly discourages to pool more than 5 samples, except in research modes.

Going by these guidelines, Maharashtra cannot use the testing technique for now, because of its positivity rate of 5.7%, officials have said. Similarly, any state with a high rate will not be able to test in this way.

Why Pool Testing?

By testing more people at once, the method helps increase the capacity of laboratories, brings down the costs, test kit usage and cuts down on time. The only exception is when a pool tests positive, which is expected to be relatively rare in areas with low infection rates.

Speaking to Business Standard, A Velumani, chairman of an ICMR-approved private lab said, “ “Reagents comprise around 30 per cent of the test cost.” This method can help save 70% of the reagent cost in a random population of 20% in a suspect population, he believed.

The ICMR advisory is based on a study conducted at DHR/ICMR Virus Research and Diagnostic Laboratory (VRDL) at King George’s Medical University (KGMU), Lucknow. It found that performing real-time PCR for COVID-19 by pooling five samples is feasible when the prevalence rates of infection are low. The recommendations made for India are based on these findings.

The method has already been studied in Germany, US and Israel; wherein the latter even showed pooling’s efficiency for testing over 60 patients simultaneously.

Researchers at German Red Cross Blood Donor Service and the Institute for Medical Virology at the University Hospital Frankfurt at Goethe University are among the first ones to have worked on the procedure for SARS-CoV-2.

They estimate that pooling can help increase the testing rate in Germany from 40,000 per day to 2,00,000 to 4,00,000 daily.

“This makes it possible for the implementation of expanded testing in larger population groups as demanded by all scientists and also politicians to be carried out sooner than previously thought, even in view of limited test kit resources,” said Professor Erhard Seifried from Frankfurt.

Will It Be Efficient for India? An Expert’s Take

FIT spoke to Dr Shahid Jameel, a well-known Virologist and CEO of Wellcome Trust DBT India Alliance who explained that for areas with a low infection rate, pooled testing could work well. In fact, a particular algorithm for pooling, called ‘matrix’, has been used intensively for HIV screening and testing, he informs.

The only potential drawback of this method can be a reduction in ‘sensitivity’ for the test to detect the virus.

“The challenge with pooling is that sometimes it reduces the sensitivity of the sample because every sample undergoes a dilution. So if you use 10 samples for one pool, then you can only put 1/10th of each to maintain the volume consistency. Now 1/10th is not a problem if the viral load is good. But if it is low, then it can compromise the sensitivity.”
Dr Shahid Jameel

He adds that since the ICMR guidelines only recommend it for areas with low rates of infection, the strategy could be sound for India, considering the many advantages it offers. Nevertheless, it still is important to be aware of the challenges before formulating or modifying any of these strategies, he advises.

(Subscribe to FIT on Telegram)

Stay Up On Your Health

Subscribe To Our Daily Newsletter Now.

Join over 120,000 subscribers!