Antigen tests: Positivity rate varies from 1% to 23% in Maharashtra

MUMBAI: The wide variation in the positivity rates of antigen tests among districts has raised questions about the utilization of the rapid test and the selection of population being screened. Data collated by TOI shows the difference in antigen positivity rates, or the percentage of positive cases among the tests processed, ranges between 1% and 23% in the state.

Though the state’s overall antigen positivity is around 12.5%, some districts have double that rate.

‘Positivity rate can vary as spread of virus not uniform across state’

Some others have one-tenth or one-fifth of the number. Pune, which has carried out the highest number of antigen tests in the state at 1.2 lakh, has a positivity of 23%. The PR, though, drops drastically to 5% for Aurangabad that has performed the second-highest number of antigen tests. What explains the variation could be that Pune is largely using it in containment zones while Aurangabad screens people entering the district. The contrast gets starker for low-Covid prevalence districts such as Chandrapur and Gondia where positivity is as low as 1.2% and 2.7%, respectively.

Within the Thane division itself, the difference can be seen between Mumbai that has a PR of 5% and Thane city’s 15% or Palghar ’s 16%. In Mumbai, a civic official said the PR is around 25-30% when used in hospitals’ fever clinics but as low as 2-4% when used in the community to screen frontline workers. “The key here is to select the population that you want to screen using antigen,” said the official.

Dr Pradeep Awate, state surveillance officer, said transmission is not uniform across districts and therefore positivity rates could vary.

While RT-PCR is considered the gold standard for detection of the Sars-CoV-2 virus, which causes Covid-19, the rapid antigen test was allowed by the Indian Council of Medical Research (ICMR) in June to boost testing. Unlike RT-PCR, rapid antigen tests give results within 30 minutes. They come with a rider, though, of higher false negativity and can miss up to 50% cases. Due to the lower accuracy, the ICMR advises its use in symptomatic patients or in containment zones.

“Districts have bought kits in huge numbers and some may be using it in the wrong population,” said a state official.

Aurangabad, for instance, had made it mandatory for traders to conduct antigen tests for their employees before starting businesses. Mobile testing vans have been deployed at six entry points in the industrial town.

“We are testing all the people who come here for work. The movement inside the district is high as it’s the biggest city in Marathwada and a big industrial belt,” said an official from the district.

In Chandrapur, where the PR rate is as low as 1.2%, district health officer Dr Raj Gehlot said they have set up 15 antigen testing centres in talukas and two in corporations. “We did use the kits to test people coming from other districts but only symptomatic,” he said. A government official admitted the decline in positivity due to the use of antigen kits may not give an accurate picture of the disease progression in the state.