If i would like to give blood but if there were a problem with it, say leukemia, would they know before i donated?

Blood donation tests: Donating blood is one of the best ways to help your community. Before it is used, it is tested thoroughly for abnormalities and some infectious diseases. These tests include doing a white blood cell count (which could identify leukemia), liver function tests, hiv, hepatitis a, hepatitis b and hepatitis c, as well as other tests. You would also be notified if an abnormality were found. Go donate!