Donors are screened for health risks that might make the donation unsafe for the recipient. Some of these restrictions are controversial, such as restricting blood donations from men who have sex with men for HIV risk. Autologous donors are not always screened for recipient safety problems since the donor is the only person who will receive the blood. Donors are also asked about medications such as dutasteride since they can be dangerous to a pregnant woman receiving the blood. Donors are examined for signs and symptoms of diseases that can be transmitted in a blood transfusion, such as HIV, malaria, and viral hepatitis. Screening may extend to questions about risk factors for various diseases, such as travel to countries at risk for malaria or variant Creutzfeldt - Jakob disease (vCJD). These questions vary from country to country. For example, while Québec may defer donors who lived in the United Kingdom for risk of vCJD, donors in the United Kingdom are only restricted for vCJD risk if they have had a blood transfusion in the United Kingdom.

The donor's blood type must be determined if the blood will be used for transfusions. The collecting agency usually identifies whether the blood is type A, B, AB, or O and the donor's Rh (D) type and will screen for antibodies to less common antigens. More testing, including a crossmatch, is usually done before a transfusion. Group O is often cited as the "universal donor" but this only refers to red cell transfusions. For plasma transfusions the system is reversed and AB is the universal donor type. Most blood is tested for diseases, including some STDs. The tests used are high-sensitivity screening tests and no actual diagnosis is made. Some of the test results are later found to be false positives using more specific testing. False negatives are rare, but donors are discouraged from using blood donation for the purpose of anonymous STD screening because a false negative could mean a contaminated unit. The blood is usually discarded if these tests are positive, but there are some exceptions, such as autologous donations. The donor is generally notified of the test result.

A donor is arm at various stages of donation. The two photographs on the left show a blood pressure cuff being used as a tourniquet. There are two main methods of obtaining blood from a donor. The most frequent is simply to take the blood from a vein as whole blood. This blood is typically separated into parts, usually red blood cells and plasma, since most recipients need only a specific component for transfusions. The other method is to draw blood from the donor, separate it using a centrifuge or a filter, store the desired part, and return the rest to the donor. This process is called aphaeresis, and it is often done with a machine specifically designed for this purpose.