The philosophy behind the maintenance of safety of the blood supply is a layered safety process. The first layer is the questionnaire. What we're trying to do in that process is to have only people come in to donate who are the least likely to test positive in any of the tests we're doing. Why do we do that? We do it in part because of this window period that Dr. Sher was speaking about. We do it also because none of our tests are completely perfect. We know that there can be errors in a test when it's conducted, because there are humans involved in the process. We also know that the pathogens themselves—the viruses, parasites, or bacteria that we're testing for—are living organisms, and their DNA mutates. They may mutate to the point where they're not picked up by the test, and we have actually seen that phenomenon happen.
All blood operators start with this screening process initially in trying to get donors who are least likely to be positive in their testing. That's the philosophy behind the screening assessment.