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Explainer: What is ‘Pooled Testing’ and how is it being done in India?


Pooled testing has been followed worldwide as an effective tool to assess the spread of any infectious disease. Even for COVID-19, pooled testing has been used in multiple countries. The Health Ministry recently issued guidelines for pooled testing in India.

The Director-General of the World Health Organization has repeatedly emphasized on extensive testing for COVID-19 in various countries.  He asked each country to detect, test, isolate, and treat every case, and trace contacts. The need for rigorous & optimal testing has been emphasized by one and all.

The testing strategy around the world is also evolving, to meet with the ever changing situation & increase in the number of cases. As on 25 May 2020, India has reported over 1.38 Lakh confirmed COVID-19 cases with 4021 deaths, according to the data released by the Ministry of Health. Around 30.3 Lakh samples are reported to have been tested as of 25 May 2020. The testing capacity around the country has increased substantially in the last few weeks.

Pooled testing is one such testing method which helps in increasing the testing capacity. Introduced in 1940s, pooled testing has been used widely for testing infections such as influenza, chlamydia, malaria, and even during the early stages of the HIV pandemic. Now, this method has been employed for detection of SARS- COV-2 (COVID-19).

What is pooled testing method?

Currently, in order to test an individual sample for COVID-19, Reverse Transcriptase Polymerase Chain Reaction Test or RT-PCR Test is used. Pooled testing method involves testing samples from multiple individuals together using the same method. That is, under pooled testing, multiple samples are tested using one RT-PCR test kit. If this pool of samples gives a negative result, one can arrive at a conclusion that all the samples tested, are free of the virus. In case the result turns out positive, it means that at least one person has been infected with the virus. Thus, each of the sample in the pool is tested individually to identify who is/are infected.

Recommended pool size is five samples at a time

Pooled testing not only increases the testing capacity, but also helps in saving limited resources including kits, reagents, expense etc. However, the optimal pool size, or the optimal number of samples that can be tested at a time, to get accurate result, is restricted to five samples in USA, Israel, and Germany. Even in India, studies conducted in DHR/ICMR Virus Research and Diagnostic Laboratory (VRDL) at King George’s Medical University (KGMU), Lucknow recommended pooling of at most five samples. If more than five samples are used, the concentration of virus might end up being low, which will result in false negatives. However, use of more than five samples in each pool is permitted only for research.

Pooled testing is recommended only in areas with low prevalence of COVID-19

ICMR, as per the advisory released on 14 April 2020, recommended the use of pooled testing only in areas where the prevalence of COVID-19 is low, with positivity less than 2% (positivity refers to number of positive samples out of every 100 samples tested). In those areas with positivity between 2 to 5%, pooling of samples should be considered only for community survey or surveillance among asymptomatic individuals. Samples of those individuals who have had contact with confirmed cases and health care workers in direct contact with COVID-19 patients should be tested individually without pooling. In areas with positivity more than 5%, pooling of samples is not recommended. Thus, it is necessary that identification of hotspots and non-hotspots be carried out prior to implementing pooled testing.

Pooled testing suggested for migrants & returnees in quarantine facilities in green zones

On 14 May 2020, the Health Ministry released guidelines on using pooled testing for migrants/returnees from abroad/ green zones. As per the guidelines, the Ministry has decided that one time RT-PCR based pooled sampling for surveillance purposes is to be used for migrant workers and international passengers in institutional quarantine facilities and hotels designated for quarantine in green zones and in districts where no case has been reported in 21 days.

What is the process?

A cohort of 25 persons will be identified. Throat/Nasal swabs will be collected from them by trained laboratory personnel wearing appropriate protective gear. The samples are to be aliquoted and pooled samples from 25 specimens will be tested in the laboratory by RT-PCR method. The result is to be conveyed to the concerned facility within 24 hours. If any pooled sample is tested positive, then individual test of each sample will be conducted as mentioned earlier.

Pooled testing is useful for determining the prevalence of infection in a community or group

The method of pooled testing has been found to be effective in curbing spread in some countries like Germany, Israel, and the USA. Germany has been lauded for its aggressive testing and low mortality. In San Francisco Bay Area of the USA, testing of pooled samples helped the researchers at Stanford to identify the prevalence of COVID-19, according to a medical news report published by Stanford.

The Stanford Clinical Virology Laboratory tested the nasal/oral swabs collected from 2,888 people who had sought care at Stanford Health Care between 01 January and 26 February 2020 but were tested negative for the common respiratory viruses. The researchers grouped the samples into 292 sets of nine or 10 and conducted pooled testing for SARS-COV-2. Of these, two sets were tested to be positive. Conducting individual tests of the samples falling in these two categories showed that two individuals, one from each set, were infected with the coronavirus. The prevalence of the infection helped the researchers prepare diagnoses.

Why India needs pooled testing?

In the initial days, there was all round criticism about conducting fewer tests. A study by ORF reveals that, as on 24 May 2020,  the number of COVID-19 tests per million population shows that UK conducted over 49,352 tests per million population, USA conducted 43,404, and Germany conducted 42,923 tests. France conducted only 21,218 tests whereas India conducted only 2,135 tests per million population. In the past few weeks, as the number of tests conducted in India increased, the percentage of positive cases has also increased.  Considering India’s large population, there is a need for increased testing. Pooled testing is one method which would be useful in areas which are less affected thus saving valuable testing kits for areas which are highly affected.

Some states and union territories have started using pooled testing method

Andaman and Nicobar Islands is among the first in the country to start pooled testing of samples from suspected patients. The chief secretary of the union territory tweeted that only a quarter of the test kits were used by pooling samples. Uttar Pradesh has also been reported to have started pooled testing of samples. Gradually other states are also adopting this method.  

Featured Image: Pooled Testing


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