Antibody screening (indirect antiglobulin test, IAT) and crossmatching are essential pre-transfusion compatibility procedures. These tests detect clinically significant antibodies in patient plasma that could cause hemolytic transfusion reactions or hemolytic disease of the fetus and newborn.
The Indirect Antiglobulin Test (IAT)
The IAT detects unexpected RBC antibodies in patient plasma. Patient plasma is incubated with reagent screening RBCs of known antigen phenotypes (typically two or three cell sets covering clinically significant antigens including D, C, c, E, e, K, k, Fyᵃ, Fyᵇ, Jkᵃ, Jkᵇ, Leᵃ, Leᵇ, M, N, S, s, P1). After incubation at 37°C to allow antibody binding, the cells are washed to remove unbound immunoglobulins. Anti-human globulin (AHG, Coombs reagent) is added, which bridges cell-bound IgG antibodies, causing agglutination if antibodies are present. The IAT can be performed in tube, gel column (microcolumn), or solid-phase methods, with gel and solid-phase offering improved standardization and sensitivity.
Antibody Identification
When the antibody screen is positive, identification is performed by testing the patient plasma against a panel of 11–16 phenotyped RBCs. The pattern of reactivity is analyzed to determine the antibody specificity. Multiple antibodies may be present simultaneously, requiring careful exclusion. Characteristics of common clinically significant antibodies include: anti-K (IgG, strong anamnestic response), anti-Fyᵃ (IgG, dosage effect), anti-Jkᵃ (IgG, difficult to detect, associated with delayed hemolytic reactions), anti-M (often IgM, cold-reactive, usually clinically insignificant at 37°C), and anti-E (IgG, commonly encountered). Antibody titers are measured for clinically significant antibodies in pregnancy to assess HDFN risk.
The Crossmatch
The crossmatch is the final check of compatibility between donor RBCs and the intended recipient. Three methods exist. The immediate spin crossmatch detects ABO incompatibility by mixing recipient plasma with donor RBCs at room temperature and checking for agglutination. The antiglobulin crossmatch (AHG crossmatch) is the most sensitive, incubating recipient plasma with donor RBCs at 37°C followed by AHG, detecting antibodies to all clinically significant antigens. The computer (electronic) crossmatch is used when the patient has a negative antibody screen and no history of clinically significant antibodies, relying on computer verification of ABO compatibility between the donor unit and recipient type.
Clinically Significant Antibodies
Antibodies are classified as clinically significant if they can cause hemolytic transfusion reactions, HDFN, or reduced RBC survival. The most significant are those against ABO (IgM, intravascular hemolysis), Rh (D, C, c, E, e — IgG, extravascular hemolysis), Kell (K, k — IgG, strongly immunogenic), Duffy (Fyᵃ, Fyᵇ — IgG, associated with delayed reactions), Kidd (Jkᵃ, Jkᵇ — IgG, implicated in delayed hemolytic reactions, prone to falling to undetectable levels), and MNS (S, s — IgG). Clinically insignificant antibodies (usually IgM, cold-reactive, not reactive at 37°C) include anti-Leᵃ, anti-Leᵇ, anti-I, anti-P1, and most anti-M. Distinction requires thermal amplitude testing and antigen typing.
Special Considerations
Patients with a positive antibody screen require antigen-negative RBC units for transfusion. The blood bank must maintain an inventory of phenotyped donor units for alloimmunized patients. Patients with warm autoimmune hemolytic anemia (AIHA) pose special challenges as the autoantibody causes pan-reactivity in the IAT and crossmatch. Techniques to resolve autoantibody interference include autocontrol testing, cold adsorption, and alloadsorption to detect underlying alloantibodies. Patients with a history of recent transfusion or pregnancy within 3 months may have a negative antibody screen but remain at risk for delayed hemolytic reactions due to low-level alloantibodies below detection threshold.
Quality Control and Documentation
All antibody screening and crossmatching procedures require daily quality control: verification of reagent integrity (AHG, screening cells, enhancement media), positive and negative controls, and documentation of all results. The patient’s historical record must be checked for previous antibodies. A transfusion reaction investigation requires post-transfusion antibody screening, direct antiglobulin test, and repeat crossmatch with the transfused unit.
