Nearly third of Delhi’s cases in 30 days detected in rapid tests

The 65,810 cases account for 54.79% of the 120,107 cases of Covid-19 Delhi registered till Thursday.

Around 30% of the 65,810 Covid-19 cases diagnosed in Delhi between June 14 and July 12 were done using rapid antigen tests, according to data collated by the Delhi Disaster Management Authority (DDMA )—highlighting the importance of these tests in identifying infections.

A man rides his bicycle past a graffiti, amidst the spread of the coronavirus disease (Covid-19), in New Delhi. (REUTERS)

While Delhi’s use of antigen tests, which provide results quickly, and on anyone in a containment zone (and not just people who met the stringent criteria for testing laid down by the Indian Council of Medical Research), was known, this is the first time data on the number of people diagnosed positive using these tests is being publicised. HT has reviewed the data. From July 3, the government also allowed the antigen tests to be used more widely—in hospitals, dispensaries and testing centres.

The 65,810 cases account for 54.79% of the 120,107 cases of Covid-19 Delhi registered till Thursday.

In the same period (June 14-July 12) 201,950 individuals went through RT-PCR (Reverse Transcription - Polymerase Chain Reaction) test, and of this, 151,690 test results were received, the data showed. A senior government official who asked not to be named said that laboratories, in the light of the workload during the mentioned period, took up to three days to release results. The results of 50,260 tests were awaited as on 8 pm July 12 (the cut off for the data).

Out of the total test results received, 46,320 individuals tested positive, translating into a positivity rate of 30.54%. The 46,320 individuals who tested positive through RT-PCR tests account for 70.38% of the total caseload of 64,810 recorded in the mentioned period.

Between June 14 and July 12, 299,699 individuals went through rapid antigen tests and as many results were received. A total of 19,490 individuals tested positive,translating into a positivity rate of 6.50%. This is understandable—the rapid tests were applied indiscriminately on anyone in a containment zone (and later outside containment zones) who wanted to be tested; the RT-PCR tests are only applied to symptomatic individuals who meet certain criteria. They also have to be prescribed by a doctor.

The data summarised by the DDMA—whose chairperson is lieutenant governor Anil Baijal and vice-chairperson is chief minister Arvind Kejriwal—was submitted to the Union home ministry earlier this week, senior government officials said.

While RT-PCR tests are considered to be more accurate, it may take anything between 4 to 6 hours to show results (and labs could take even longer given the backlog). Rapid antigen tests, though less accurate, can show results between 30 to 45 minutes.

At present, antigen tests are carried out by the sole kit approved by the Indian Council of Medical Research (ICMR)—Standard Q COVID-19 Ag detection kit developed by South Korean company SD Biosensor. In evaluations by ICMR, the test kit was found to have moderate sensitivity at 50.6-84%, and high specificity of 99.3-100%.

A highly sensitive test will detect all positive cases but will also throw up several false positives. A highly specific test means that there are few false positive results, which is why any test that shows positive in the Standard Q kit is regarded as a positive infection. Since the kit has low sensitivity, there is a chance that some positive cases could go undetected, which is why people who symptoms and do not trigger a positive result are advised to take a second lab test.

The Delhi government started using rapid antigen tests in mid-June. Initially these were used only in containment zones. From July 3, the government started rapid antigen tests in hospitals, dispensaries and centres set up in several parts of the city (the tests are free in the centres).

Delhi’s deputy chief minister and interim health minister Manish Sisodia said, “Testing has always been one of the key focus areas of the Delhi government. We have increased testing in Delhi manyfold on the directions of chief minister Arvind Kejriwal. We are using a mix of rapid antigen testing and RT-PCR to identify people with Covid-19 at an early stage, so that we can isolate them in time and provide them with proper treatment. This has been a very successful strategy in restricting the spread of the disease in Delhi. As per ICMR guidelines, the patients whose rapid antigen test is negative but they still exhibit Covid-like symptoms, need to be re-tested through RT-PCR. The Delhi government is following this guideline.”

Experts said that the numbers suggest that Delhi is on the right track in terms of testing.

Dr Lalit Kant, former head of epidemiology at the Indian Council of Medical Research (ICMR), said: “It is good that a large number of positive cases are being identified quickly through rapid antigen tests. The infected individuals can be isolated without any delay. However, individuals who test negative should also be diagnosed through RT-PCR tests in order to trace the maximum possible number of individuals who are infected. The sensitivity is lower than the specificity. So, retests help in detection of false negatives.”

Dr T Jacob John, virologist and former professor at Christian Medical College in Tamil Nadu’s Vellore, said: “The number suggests that around 20,000 people were quickly tested positive with much less efforts in a period of around four weeks through rapid antigen tests. The government did not have to invest RT-PCR kits on them. The RT-PCR tests consume more time and effort. This also helped reduce burden on the laboratories. It is also good that the government has adopted a strategy which relies on a mix of both kinds of tests. However, the government must ensure that individuals who test negative in rapid antigen are sent for testing again through RT-PCR method.”

As per ICMR guidelines, symptomatic persons who test negative in the rapid antigen tests should be followed up with an RT-PCR test.

The data shows that of the 280,209 individuals who tested negative through rapid antigen tests till July 12, 1,513 were re-tested through RT-PCR tests. Of these, around 249 were diagnosed positive.