- Indian Council of Medical Research (ICMR) has recently issued an advisory for using pooled samples for testing of COVID-19.
- The objective is to increase the number of tests conducted by laboratories across the country.
What is pooled testing?
- In a pooled testing algorithm, samples of multiple individuals are put together in a tube and screened through the PCR test.
- Following image shows the steps to be taken depending on the results of those tests
- Substantial cost savings and reduction in requirement of testing kits:
- For example, if a pooled sample consists of the samples of five individuals and it tests negative, the cost of four testing kits is saved and more number of people are covered with fewer resources.
- The “door to door” approach: Under this approach collecting pooled samples requires around 56-93 percent fewer tests.It will be applicable in areas where the prevalence of the disease is low to moderate.
- Increment in overall people screened:
- It is critical to increase the numbers of tests conducted by laboratories.
- And as the positivity rate in COVID-19 cases in India is still low,it may help to use the pooled samples for screening.
- Tracking down the asymptomatic cases:
- Pooled screening can also help in tracking down the asymptomatic cases of the disease, thereby tracking community transmission.
- Containing foreseeable second wave outbreaks:
- In particular, the “door-to-door” pooled-sample approach can facilitate mass screening in early stages of COVID-19 outbreaks, especially in low- and middle-income settings, and in containing foreseeable second wave outbreaks worldwide.
Recommendations of ICMR for pooled testing
- Fixed Maximum number for pooling:
- While more than two samples can be pooled together, the number should not exceed five samples to avoid sample dilution, which can lead to false negatives.
- Ideal areas or populations for its use
- Use only 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 2.
- In areas with positivity of 2-5%:
- Sample pooling for PCR screening may be considered only in community survey or surveillance among asymptomatic individuals.
- Pooling of sample is not recommended in areas or population with positivity rates of >5% for COVID-19
- Non inclusion of likely positive cases:
- Samples of individuals with known contact with confirmed cases or healthcare workers should not be included in the pooled samples.
About Polymerase Chain Reaction (PCR)
- The standard test for COVID-19 so far has been the Polymerase Chain Reaction (PCR).
- It is done after taking nasal or throat swabs of the patient, from which RNA is extracted and converted to DNA, which is then amplified before being checked for confirmation of the virus.
- The PCR test takes up to nine hours.
- The long time is also because the swab is first screened to decide the family of the virus; if that confirms it is a coronavirus, a second test is done to ascertain if it is indeed the novel coronavirus SARS-CoV2.