Actually, testing all samples is quite inefficient : blood tests are quite expensive. They simply mix a large quantity of samples together, and test the result. The downside is having to trash the large amount of blood if the test is positive. That's why they use statistically relevant information in deciding who donates blood. Of course, quite often the prejudices of the doctor or of the agency get in the way.

Un roi sans divertissement est un homme plein de misères