There are just so many variables. Testing may come either too soon, before enough virus is present to detect, or too late, after a person has already spread the virus to others. And most rapid tests, even according to their instructions, are meant to be used in pairs — generally a day or two apart — for increased accuracy. Despite that, a few brands are sold one to a box and, with the tests sometimes expensive and in short supply, families are often relying on a single screening.