The Indian Council of Medical Research (ICMR) issued an advisory for using pooled samples for testing of COVID-19 in order to increase the number of tests conducted by laboratories across the country. “It is critical to increase the numbers of tests conducted by laboratories. Positivity rate in cases is still low. Hence, it may help to use the pooled samples for screening,” the ICMR advisory stated.
Coronavirus (COVID-19): 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. In case the pooled test turns out to be positive, individual samples are tested, which is referred to as pool de-convolution. If there’s no positive result, all individual samples in the pool are regarded as negative, resulting in substantial cost savings.
Pooled testing: What the ICMR has recommended
- ICMR has advised that 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.
- This method can be used in areas where the prevalence of COVID-19 is low, which means a positivity rate of less than two percent. In areas with a positivity rate between two to five percent, sample pooling of PCR screening may be considered in a community survey of surveillance among asymptomatic individuals, ICMR has said.
- A paper published on the preprint server MedRxiv says that testing pooled samples is most effective when the chance of positive detection of the target is low. “In such cases, large groups of samples can be conclusively classified as negative with a single test, with no need to individually test every sample,” it says.
- Samples of individuals with known contact with confirmed cases or healthcare workers should not be included in the pooled samples. Also, ICMR has said pooling of sample is not recommended in areas or population with positivity rates of over five per cent.
Why pooled testing?
- Using this method, substantial costs and testing kits are saved. For instance, 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. But in case the sample tests positive, all individuals need to be tested separately.
- Significantly, pooled screening can also help in tracking down the asymptomatic cases of the disease, thereby tracking community transmission.
- Another preprint research appearing on Research Gate says, “Pooled-sample PCR analysis strategies will save substantial resources for COVID-19 mass testing. 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.”
- The paper says that “door to door” approach of collecting pooled samples requires around 56-93 percent fewer tests, in areas where the prevalence of the disease is low to moderate.