More than 800,000 rapid COVID-19 tests passed their expiration date recently in a Florida warehouse, prompting concerns that they would go to waste. But the manufacturer, Abbott, obtained a three-month extension on the expiration date after supplying additional data to the FDA. Such tests — which work by detecting antigens (pieces of viral proteins) on the patient’s swab sample — will eventually expire. But the exact date in each case is an open question, in part because some of the manufacturers are new at this game. Even Abbott, which has years of experience making similar tests for other pu…