A general term for a testing (or evaluating) one person’s serum/plasma against another person’s red blood cells to check and see if there is an incompatibility (primarily in the ABO and Rh systems). There are actually two main types of crossmatch: “Major” and “Minor.” Major crossmatches test donor RBCs against recipient serum/plasma, and are required any time a transfused blood product contains over 2 mL of RBCs. Minor crossmatches (not required in routine testing) assess compatibility between donor serum/plasma and recipient RBCs. The major crossmatch, as a core part of pretransfusion testing, serves as the final check of ABO compatibility between a donor and a recipient. Finally, I said “or evaluating” above because not all crossmatches require actual testing of plasma against RBCs; specifically, the computer crossmatch evaluates compatibility virtually (click the link for more details).