National Jewish Health in Denver will offer tests for antibodies to the new coronavirus to anyone who wants one starting Friday, though people who want to be tested for a current infection still will need a doctor’s referral.

The immune system develops antibodies that are specific to a type of virus or bacterium. If the same virus enters the body again, the antibodies essentially tag it for other cells in the immune system to destroy. That means that if a person has antibodies for a particular virus, like COVID-19, they know they were infected with it at some point.

Such testing can reveal whether people who never showed symptoms were infected with the coronavirus. While more than 10,000 people now have tested positive for COVID-19 in Colorado, public health officials believe the number of people actually infected ranges from 65,000 to 75,000.

“Moving forward with antibody testing provides important information about who has had COVID-19,” Dr. Michael Salem, president and CEO of National Jewish, said in a statement. “It adds an additional invaluable tool to our high-capacity virus testing towards understanding the extent of the COVID-19 pandemic in the broader population and in charting a path to renewed social and economic activity.”

National Jewish has blood tests available for two types of antibodies: IgG and IgM. IgG antibodies take at least a week to develop, and stick around after the body defeats the infection. IgM antibodies appear sooner, but disappear shortly after the infection ends — so if you have those, you may still be contagious.

The hospital cautioned that not everybody develops a strong antibody response, and people with compromised immune systems may not have any antibodies specific to COVID-19. If you believe you could be infected now and want to know if you need to isolate yourself, a swab test looking for virus particles in your nose and throat would be more accurate, though it’s still possible to get a false negative result.

The U.S. Food and Drug Administration urged the public not to make decisions based on the results of antibody testing. It isn’t clear how long antibodies specific to the new virus will stay in people’s blood, or whether they provide meaningful protection from getting the virus again.

For now, the FDA guidance focuses on antibody testing as a way to get a general sense of how much of the population has been infected, and to identify people researchers can follow to find out if they have developed immunity. If people who’ve had the virus once are immune in the medium or long term, they could go back to work, lessening the economic impact of the shutdown.

People who have antibodies also could donate blood plasma, which could be purified and given to people still battling the virus to help boost their immune systems. That strategy has worked for some viruses, but there isn’t enough testing yet to know if it will be helpful with COVID-19.

If you want to get antibody testing, you can schedule an appointment online without a doctor’s note, provided you aren’t showing symptoms of COVID-19. The test costs $94, and results should be available in two days. All appointments for this weekend are booked, but slots are available starting Monday.

If you have COVID-19 symptoms and want to be tested, you should ask your doctor to send a referral for swab testing. Those tests are performed at a drive-thru station in National Jewish’s parking lot, with results available in about two days.

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