| For months, there has been a debate about whether the U.S. can test, and test sufficiently, to track Covid-19 infections. Sick people struggled to get tested, while labs couldn't get their hands on tests, or were short of swabs to take patient samples and chemicals to process tests. As the country's capacity has increased, these problems haven't entirely gone away. But lately a new concern has emerged about how well the U.S. is counting and reporting the tests it does perform. There are two kinds of tests that can be done now for the coronavirus, and though they were developed for the same pandemic, the differences are vast. Most importantly, they tell test-takers different things. Diagnostic tests look for active infections and present a picture of current Covid-19 cases. Antibody tests, meanwhile, detect markers in the blood that reveal whether a person has been exposed to the virus in the past and help determine the level of community exposure. In their reports and questions on testing capacity, decision-makers and public-health experts have largely focused on the diagnostic form, because it measures active illnesses and the tests are considered particularly reliable.  Residents of Chelsea, Massachusetts, line up to get a rapid fingerstick antibody blood test for the coronavirus Photographer: Stan Grossfeld/The Boston Globe /Getty Images That's what made the revelation that the U.S. Centers for Disease Control and Prevention and some states combined completed diagnostic tests and antibody tests in public datasets surprising. By mixing two test results, the CDC and the states presented a distorted picture of Covid-19 cases and the country's overall capability. The practice was first reported by The Atlantic. The CDC says it's working to distinguish the two types of tests, while some states recently stopped the practice of combining them. Some of the states also said antibody tests made up a small share of their counts. The whole situation, while messy, underscores the uphill battle not just to test but to build the infrastructure to track test results. Results that, of course, have enormous implications for when places reopen and how safe it will be to do so.—Emma Court | 
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