Blood Banking / Immunohematology
Blood Banking and Immunohematology
This domain tests ABO/Rh typing, antibody detection and identification, compatibility testing, and the resolution of serologic discrepancies - the decision points that determine whether a unit can be released.
Core rules
- The indirect antiglobulin test (IAT) detects IgG red cell antibodies in plasma; the direct antiglobulin test (DAT) detects antibody and/or complement already bound to the patient's cells in vivo.
- Anti-A and anti-B are naturally occurring IgM; anti-D is clinically significant IgG, so weak D testing is performed on all apparent D-negative patient samples in tube or as validated.
- Reverse (back) typing is invalid before roughly 4–6 months of age because anti-A/anti-B have not yet formed; forward typing alone is used.
- Antibody identification panels use rule-out logic: an antibody is excluded when its corresponding antigen-positive cells fail to react at IAT.
- Crossmatch incompatibility in a patient with a negative antibody screen usually signals an antibody to a low-incidence antigen, an A2/A2B subgroup, or rouleaux.
Worked example
Forward type: 4+ with anti-A, 0 with anti-B. Reverse type: 2+ with A1 cells, 0 with B cells. The discrepancy suggests anti-A1 in an A subgroup. Dolichos biflorus lectin agglutinates A1 but not A2 cells; if the patient's cells are nonreactive with the lectin, the patient is group A2 with anti-A1, and A1-negative (A2 or O) units are selected if transfusion is needed.
Traps that cost marks
- Calling rouleaux a true agglutination; saline replacement disperses rouleaux but not antibody reactions.
- Ignoring a positive DAT when the eluate explains the serology (e.g., drug-induced or autoimmune hemolysis).
- Assuming the reverse type is always the confirmatory test.
- Forgetting that cold autoagglutinins cause false-positive reverse typing and antibody screens.