The Food and Drug Administration has issued its first authorization for a COVID-19 test that looks for antibodies in the blood, rather than for the virus in the nose or throat. While the antibody approach means this test will have limitations, it’s an important tool that could help in the response to the pandemic.

The test is produced by the biotechnology company Cellex. Health care providers have to draw blood from a patient’s vein to run the test, and it can only be done in certified labs — not a doctor’s office. It takes 15 to 20 minutes to get a result.

When someone is infected with a virus for the first time, their immune system begins producing antibodies specific to that virus. Checking to see if someone has coronavirus-specific antibodies is good evidence that they’ve been infected. However, the body doesn’t start to make these immediately, and they might not appear until someone’s illness has run its course. As a result, antibody-based tests can tell doctors whether someone has already had COVID-19, but they’re not as good at testing if the patient currently has it.

Still, the FDA authorization indicates that the agency thinks the benefits of having the test available outweigh the risks. “It is reasonable to believe that your product may be effective in diagnosing COVID-19,” the agency wrote in the letter, which authorized the test for emergency use.

The FDA was already allowing companies to make and distribute antibody tests for COVID-19 without formal authorization. However, without approval, they were not able to claim that their test could diagnose the disease.

Even if they’re not great diagnostics, antibody tests are critical in the response to the pandemic. They’d help figure out who has already been sick — whether they had significant symptoms or not — and therefore, who is probably immune enough to the virus to safely move around the world normally. They’ll also help public health officials understand how much of the population has been infected by the coronavirus. Dozens of companies are working to develop antibody tests, as are researchers at the Centers for Disease Control and Prevention.

The US struggled to ramp up COVID-19 testing, and it’s still hard for people to access tests. Though there have been over 1 million tests run in the US, a milestone President Trump highlighted earlier this week, the testing rate per capita is still far behind other countries. An easily available antibody test would give people who were not able to get tested when they were sick a chance to know if they actually had COVID-19.