Immunohematology (Blood Bank)
What this domain tests
Immunohematology covers pretransfusion compatibility testing, ABO/Rh typing and discrepancy resolution, antibody detection and identification, component storage, and transfusion reaction workup.
Core rules
- ABO typing requires forward typing (patient cells plus anti-A and anti-B) and reverse typing (patient serum plus A1 and B cells). Reverse typing is invalid in newborns under about 4 months because anti-A and anti-B are not yet produced.
- Rh typing uses anti-D; weak D testing policy differs between donors (test) and patients (often not required, but needed in pregnancy).
- Antibody screen: indirect antiglobulin test at 37 °C, tube or gel; a positive screen triggers an identification panel and selection of antigen-negative units.
- Crossmatch: immediate spin for ABO compatibility, AHG for patients with clinically significant antibodies. An electronic crossmatch is permitted only when two ABO typings agree, there is no history of antibodies, and the computer system is validated.
- Direct antiglobulin test detects antibody or complement on the patient's own red cells - hemolytic disease of the newborn, autoimmune hemolysis, drug-induced hemolysis.
Components
- Red cells 1–6 °C for up to 42 days with additive solution; platelets 20–24 °C with continuous agitation for 5 days; plasma at or below −18 °C, used within 24 hours of thawing at 1–6 °C.
Worked example
Forward typing: anti-A 4+, anti-B 0, anti-D 3+. Reverse typing: A1 cells 0, B cells 0. Forward typing says group A, but the reverse shows no anti-B, an apparent group AB. Reasoning: missing reverse reactivity suggests rouleaux, a cold autoantibody, hypogammaglobulinemia, or a newborn. Saline replacement resolves rouleaux; prewarming at 37 °C addresses a cold autoantibody. Until resolved, give group A red cells, or group O if transfusion cannot wait - never group AB.
Traps that cost marks
- Ignoring a mixed-field reaction.
- A cold autoantibody masking an underlying alloantibody.
- Applying donor weak D rules to a patient.
- Accepting a compatible crossmatch when the antibody screen is positive.
- Storing platelets without agitation or beyond 5 days.