Blood from a donor is collected as a unit of whole blood containing both cellular and plasma components. In Canada, blood is collected and tested by the Canadian Blood Services and Héma-Québec . The collected blood must be ABO grouped, Rhesus typed, tested for compatibility and screened for transmissible diseases before use in a transfusion.

A sample of blood from a patient who may require a blood product undergoes a Type and Hold. In this procedure, ABO and Rh grouping as well as an antibody screen are performed. In addition, the plasma from the sample is stored for crossmatching when a transfusion becomes necessary.