Antibody tests are meant to recognize a past infection. Many of these have hit the market in recent weeks and are being offered at local clinics. Officials have touted the tests as crucial for reopening the economy and developing public health strategies to contain the virus.

But there are still questions about how accurate they are. And even with a very good test, it’s possible to test positive for antibodies even when you don’t actually have them. Watch our new video to learn why.