District residents can now get free antibody testing as part of the city’s effort to track COVID-19 transmission in the community, officials announced Monday.

The tests, also called serology screenings, identify antibodies in blood that show if someone has fought off COVID-19. The city’s health department has partnered with the Department of Forensic Sciences’ Public Health Lab to offer them.

District residents six years old and above can qualify for free tests. Residents should call 1-855-363-0333 to make an appointment. All testing will occur at the Navy Yard Serology Testing Site in Southeast.

Some doctors say the presence of antibodies might protect people from catching the virus again. However, health experts say a positive test does not guarantee immunity.

LaQuandra Nesbitt, the director of D.C.’s Department of Health, stressed that point at a Monday press conference. She urged District residents to continue following social distancing guidance, wearing face masks and washing their hands, even if they tested positive for antibodies.

Private doctors and labs in the region also provide antibody testing, but it isn’t always free. The federal CARES Act requires insurance companies to cover the cost, but many labs do not accept insurance.

The city also announced Monday that it is expanding its free COVID-19 testing program. Four firehouses (up from two) will be open every day except Sunday for walk-up testing.

The Bowser administration said that more than 2,300 people have been tested at firehouses since the program began two weeks ago. Personnel from the DC Fire and EMS Department, the city’s Public Health Lab, and the D.C. Department of Health administer the swab tests.

A full list of District testing sites can be found here.

This story was updated at 5:45 p.m. with additional details about the District’s testing partner.