Antigen test shows 7.46% positivity rate in containment zones
Experts said the rapid antigen tests are providing a better picture of the spread of the infection since so far only those with symptoms in containment zones were being tested.Updated: Jun 30, 2020, 04:00 IST
Of the 99,274 people surveyed using the rapid antigen test, which began in containment zones and neighbouring areas on June 18, 7,414 tested positive for the coronavirus disease (Covid-19) till June 26, suggesting a positivity rate of 7.46%,a senior Delhi government official said on Monday.
Experts said the rapid antigen tests are providing a better picture of the spread of the infection since so far only those with symptoms in containment zones were being tested. With random sampling in containment zones and areas around them, and rapid testing, the outcome of these tests provide a more representative picture, they said.
The rapid antigen test, a much faster way to confirm positive cases, was started as part of the Centre’s and the state government’s efforts to ramp up testing in Delhi. “We are conducting close to 7,000-12,000 rapid tests per day. These tests are being done mostly in containment zones and its neighbouring areas. It is also being done in some other areas. It is helping us isolate positive cases early, as the test results are out within 30 minutes,” said a senior Delhi government official with the revenue department, which is compiling the testing data.
The results, a district magistrate said, were used to identify new clusters of Covid-19 cases and redraw boundaries of the containment zones. On Sunday, the Delhi government increased the number of containment zones from 280 to 421.
Tests that detect presence or absence of an antigen in the body are called antigen detection tests. An antigen is a foreign molecule that induces an immune response within the body, especially the production of antibodies, and detecting its presence determines infection.
Delhi is the first state to roll out antigen-based testing as Covid-19 cases in the city have seen unusual surge in the number of cases, making it imperative to conduct large-scale quick testing to trace, treat and isolate the infected individuals. Till Monday, Delhi reported 85,161 cases and2,680deaths due to the infection.
The test has high specificity (true negative rate) of 99.3% to 100%, which rules out people who are not infected. Tests with a high specificity are most useful when the result is positive. Sensitivity (true positive rate) is lower at 50.6% to 84%, which makes it less accurate in correctly diagnosing a positive case. This is why people with symptoms who test negative have to undergo an RT-PCR test to rule out active infection.
“The antigen test helps us understand the prevalence of the infection. We could identify clusters where the number of cases is more and have redrawn the boundaries and added new areas accordingly,” the district magistrate said.
This, another district magistrate said, will help in de-sealing containment zones at the earliest. “With this, we are able to identify positive cases and take them out of the zone or place them in home isolation. This way we can scale down the containment measures as soon as cases will go down,” said another official.
Dr T Jacob John, professor emeritus and former head of the department of virology at Christian Medical College at Vellore, “It (the positivity rate) reflects the situation at the ground. If it is 7.46% in containment zones and neighbouring areas, then chances are that the positivity rate in non-containment areas would be far less.”
A doctor with the district surveillance team said the rapid antigen test has expedited the Covid management process, as the cases are detected early. “The test has proved to be helpful in faster detection of positive cases in containment zones. We are able to identify and isolate these cases early. The RT-PCR test takes longer than a rapid test, so the detection of a case, its contact tracing, etc used to take time. But now we are able to take prompt action and check the spread of infection,” said the doctor, who didn’t wish to be named.
The conventional real-time polymerase chain reaction (RT-PCR) test requires five to six hours to give results and specialised laboratory facilities in terms of equipment, biosafety and biosecurity.
Early detection, doctors say, will help in keeping fatality rate in check, as timely diagnosis and treatment can be provided to patients. “When you pick up the infection early, it brings down the fatality rate since timely treatmentis key in managing and containing this disease,” said Dr Lalit Kant, former epidemiology and communicable diseases head at the Indian Council of Medical Research.
Dr Jugal Kishore, head of community medicine at Safdurjung hospital, said, “The positivity rate through RT-PCR is high as those with symptoms or contacts of positive cases were getting tested. But now as we are testing people in the community using the rapid antigen test, the prevalence rate would be less as we are testing even those who don’t have any symptom.”