More than 180 people took the rapid antigen test conducted by the Calcutta Municipal Corporation between Thursday and Friday and 23 of them have tested positive for Covid-19, a CMC official said.

This civic body launched the rapid antigen test on Thursday and 50 people were tested at the Chetla market and more than 30 near Hatibagan, the official said. On Friday, 50 more people were tested at Chetla and an equal number at Kankurgachhi.

Four people who tested negative in the antigen tests but had symptoms of Covid-19 gave their samples for RT-PCR tests, the CMC official said. The antigen tests and RT-PCR tests are two types of tests to detect the presence of the antigen — the novel coronavirus — in a person’s swab sample.

The rapid test is known to show many false negatives, which means that someone who tests negative in the test could actually be affected by the virus. A symptomatic person who tests negative should undergo a RT-PCR test to confirm if she/he is actually negative, according to the guidelines for rapid tests. Someone who tests positive need not undergo further tests and should be considered as Covid positive.

Despite its deficiency, the rapid tests have been approved because they give results in 30 minutes. The RT-PCR tests take at least a day to give results.

The civic body will continue the antigen tests across the city. The tests will be conducted daily in predetermined locations, a CMC official said.