Even if major blood group typing has been done, transfusion reactions can occur due to unexpected antibodies in the serum of the recipient or due to incorrect RBC typing. To prevent reactions, the concept of a crossmatch evolved. The basic procedure is the “major” crossmatch, in which RBCs of the donor are placed into the serum of the recipient under standard conditions. Previously, the “minor” crossmatch (RBCs of the recipient matched with the serum of the donor) was also a standard procedure, but most blood banks no longer perform this test, and in addition, the AABB recommends that the test not be done. Even if the donor serum contains antibodies to one or more of the RBC antigens of the recipient, the relatively large blood volume of the recipient in most cases should dilute the relatively small volume of donor serum to a point at which it is harmless. The minor crossmatch has been replaced by an antibody screen on recipient serum. The antibody screen consists of the recipient serum and a commercially supplied panel of different blood group O RBCs that contain various other blood group antigens. If there is free antibody inthe patient serum (other than ABO antibody), it either will coat or will agglutinate the test RBC so that the presence of the antibody will be detected. Detection is carried out by incubation at 37°C (usually with some enhancing agent, e.g., albumin or low ionic strength medium), followed by a direct Coombs’ test. The antibody screen does not detect errors in ABO typing, since the reagent RBCs are all group O and thus nonreactive with anti-A or anti-B antibody. Once an unexpected antibody is discovered, the type of antibody has to be identified. This can usually be deduced by the pattern of reaction with the various RBCs of the test panel. In a few cases the specimen must be sent to a referencelaboratory for special tests.

In summary, antibody screening is designed to demonstrate unexpected antibodies in the serum of the recipient that may destroy donor RBCs that were thought to be compatible on the basis of ABO and Rh blood group typing.