Some of these at-home tests are ready to go. But they have been sitting on the shelf, unused, for months. Why? The holdup is that rapid, over-the-counter antigen tests are not nearly as sensitive to the coronavirus as the molecular PCR (polymerase chain reaction) tests currently used for most Covid-19 testing. In fact, by PCR standards, some at-home antigen tests might catch only half or a third of people who test positive on a PCR test. But that’s because they’re looking for different things in different ways.