They test ALL BLOOD and they hold it for a window of time as part of the testing to make sure there's nothing wrong with it so the blood doesn't act as a spreader. Unusable blood is destroyed and the donor is promptly notified thereafter. They were just bigoted assholes until now.

This is how they’re catching Covid persisting in the blood of folks post infection. Donations are showing up tainted with C-19 during the test and quarantine phase.