The Indian Council of Medical Research on Thursday updated its testing strategy for Covid-19 to allow tests for people in health ministry-designated “hotspots” or “clusters” who show influenza-like symptoms.

The research organisation, which is overseeing India’s Covid-19 testing, said it will conduct the rRT-PCR or real-time reverse transcriptase polymerase chain reaction tests on nasal or throat swab samples of people who show symptoms like fever, cough, sore throat and runny nose within a week of symptoms appearing. Beyond that period, an antibody test will be done.

These tests will be applicable to all those who show influenza-like symptoms in hotspots, clusters and in “large migration gatherings or evacuee centres”.

So far, India has used the RT-PCR or reverse transcriptase polymerase chain reaction test on nasal or throat swab samples of suspected patients to test for Covid-19. The RT-PCR tests look for the presence of the virus in the DNA. They can take anywhere between 12-24 hours to show results.

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In comparison, the rapid antibody tests identify disease-fighting antibodies in blood samples and can deliver results in 45 minutes to two hours. However, since antibodies are usually detectable only after around seven to 10 days of being infected by the virus, such tests could throw up false negatives – an infected person may appear normal in the blood test.

ICMR’s testing strategy so far had included five categories of people – symptomatic individuals who undertook international travel in last 14 days, symptomatic contacts of laboratory confirmed cases, symptomatic healthcare workers, patients with acute respiratory illness and asymptomatic direct and high-risk contacts of confirmed case.

The revision in testing strategy to include hotspots and clusters came as governments in Delhi, Uttar Pradesh and Madhya Pradesh sealed infection hotspots and tightened restrictions to slow down the spread of the coronavirus.

The number of confirmed coronavirus cases in India rose to 5,865 on Thursday. The toll in the country is now 169, said the health ministry. At an all-party meeting on Wednesday, Prime Minister Narendra Modi hinted that the nationwide lockdown to slow down the spread of infection may be extended beyond April 15.

On Thursday, Odisha became the first state to extend the lockdown till April 30. The chief ministers of several states have either favoured the lifting of the lockdown in a phased manner or an extension.