Routine pre-transfusion testing must include an indirect antiglobulin screen for clinically significant alloantibodies. The intended recipient's plasma or serum is tested against 2 or 3 examples of red cells from group O donors whose antigenic makeup is confirmed relative to the blood groups considered critical to the safety of red cell transfusion (e.g., Rh, K, Fy, Jk, MNS).
The indirect antiglobulin test is preceded by an immediate spin reading, addition of an enhancement medium such as LISS (low ionic strength saline) or PEG (polyethylene glycol), and incubation at 37o C according to the manufacturer's instructions. At the end of the incubation, some methods require a saline wash of the cells/serum or plasma mixture, followed by the addition of antiglobulin antisera. Negative antiglobulin tests must be followed by addition of antiglobulin control check cells. Other methods may not require the saline wash step; therefore, it is important to consult the facility's procedures (SOPs) and the manufacturer's direction circular for the method or reagents in use.
Results of each phase of testing are recorded, interpreted, and maintained as part of the recipient's history. If the antibody screen is positive, the recipient's serum or plasma must be tested against more group O donor cells to identify the alloantibody. Based on the alloantibody(ies) determined to be present in the recipient's serum or plasma, red cell units that are negative for the corresponding antigen(s) must be provided. For example, if testing confirms the presence of anti-E (an antibody in the Rh system), red cell units that are negative for the E antigen must be identified and crossmatched with the recipient's plasma or serum through the antiglobulin phase.
Antibody screening and subsequent identification of alloantibodies, as described above, is often part of routine prenatal testing.
