Blood cross matching is the final safety verification step performed immediately before a patient receives a blood transfusion. This laboratory procedure ensures the specific unit of donor blood is harmonious with the recipient’s immune system. Its purpose is to confirm that the recipient’s antibodies will not recognize and destroy the antigens on the donor’s red blood cells (RBCs). By simulating the transfusion in a test tube, the crossmatch verifies the compatibility established during earlier testing, preventing a potentially fatal immune reaction.
Essential Pre-Transfusion Testing
Before the cross matching process begins, a series of preparatory laboratory steps are mandatory to identify the most suitable donor units. The initial and most fundamental step is determining the ABO and Rh blood groups for both the patient and the donor blood product. The ABO system is based on the presence or absence of A and B antigens on the red cell surface, while the Rh system focuses on the D antigen.
This grouping process involves both forward typing, which tests the red cells for antigens, and reverse typing, which tests the patient’s plasma for expected anti-A or anti-B antibodies. While these tests narrow the selection to an ABO and Rh-compatible unit, they do not account for every possible immune mismatch. The vast majority of severe transfusion reactions are caused by errors in the ABO group, which this initial testing is designed to prevent.
The second preparatory step, known as the antibody screen, involves testing the recipient’s plasma against a panel of commercially prepared screening cells. These cells possess a wide range of clinically relevant red blood cell antigens besides A, B, and D. This test detects “unexpected” antibodies, which are often produced after previous transfusions or pregnancies, and could cause a delayed or immediate reaction if the donor unit carries the corresponding antigen. Only after the patient’s ABO/Rh type is confirmed and any unexpected antibodies are identified can the final crossmatch proceed.
The Core Cross Matching Process
The major crossmatch involves physically combining a sample of the recipient’s plasma with a sample of the donor’s red blood cells. This mixture is then tested through a sequence of phases to detect different classes of antibodies that react under various conditions. The first phase is the immediate spin (IS) crossmatch, where the mixture is centrifuged immediately after mixing at room temperature.
This initial spin primarily detects ABO incompatibility, which is caused by cold-reactive antibodies, specifically the naturally occurring anti-A and anti-B antibodies. If agglutination, or clumping, occurs at this stage, the unit is immediately deemed incompatible because of a major ABO mismatch. If the immediate spin is negative, the mixture proceeds to the second phase, which is incubation at 37°C.
Incubating the mixture at body temperature allows warm-reactive antibodies, typically Immunoglobulin G (IgG), to bind to antigens on the donor red cells. These IgG antibodies are often the unexpected antibodies identified during the antibody screen. Since IgG molecules are too small to cause visible clumping on their own, the final step, the Antiglobulin Test (AHG), is necessary. This test involves washing the mixture to remove unbound proteins and then adding a reagent known as anti-human globulin, which acts as a bridge to link the sensitized red cells. This results in detectable agglutination if IgG antibodies were present.
Understanding Compatibility and Incompatibility
The presence or absence of agglutination or hemolysis determines the unit’s safety for transfusion. A compatible result is achieved when there is no visible clumping of red blood cells (agglutination) and no destruction of the red blood cells (hemolysis) observed in any phase of the test. This outcome signals that the recipient’s plasma does not contain antibodies that react with the donor’s red cell antigens, and the unit is safe to release for transfusion.
Conversely, an incompatible result is indicated by the presence of agglutination or hemolysis at any point during the crossmatch procedure. This positive reaction means the recipient’s immune system has antibodies ready to attack the donor cells, and the blood unit must not be transfused. The specific phase in which the incompatibility occurs helps the blood bank technician determine the type of antibody involved, guiding the selection of a different unit. Transfusing incompatible blood can trigger an Acute Hemolytic Transfusion Reaction (AHTR), where the recipient’s antibodies rapidly destroy the transfused red cells.
Symptoms often begin within minutes of starting the transfusion and can include fever, chills, low back pain, and a feeling of impending doom. The massive destruction of red cells releases hemoglobin, which can severely damage the kidneys, leading to acute kidney failure and potentially death if the transfusion is not stopped immediately.