The COVID-19 diagnosis of the president, first lady and a number of staffers is once again calling into question the use of rapid tests that the White House has screened people with for months. The tests, made by Abbott Laboratories, give results in just 15 minutes. But there are also growing concerns about the test’s accuracy. One study found it had an unacceptable “false negative” rate of nearly 20%. That means someone close to the president may have tested negative but was in fact positive.