Transfusion Science: Grouping, Antibody Investigation and Components
Immunohaematology questions test the logic of pretransfusion testing: establish the ABO type beyond doubt, detect unexpected antibodies, select antigen-negative units, and recognise a transfusion reaction early.
Core rules
- ABO typing requires concordance between forward (cells) and reverse (serum) grouping. Anti-A and anti-B are expected in the reverse group; a mismatch is an ABO discrepancy and must be resolved before any unit is issued.
- Antibody screening uses group O reagent cells and detects clinically significant IgG antibodies at the indirect antiglobulin (IAT) phase, with reactions graded from 1+ to 4+. A positive screen triggers a panel and an autocontrol.
- Antibody identification logic: reactive cells share an antigen, non-reactive cells lack it, reactivity has dosage (homozygous cells react more strongly for Rh, MNS, Kidd), phase and enzyme sensitivity. Ficin or papain destroys Kell, Duffy and M/N antigens while enhancing Rh and Kidd reactivity.
- Components: red cells 1-6 degrees C for 42 days in additive solution; platelets 20-24 degrees C with agitation for 5 days; frozen plasma at -18 degrees C or colder for 12 months.
Worked example
Serum from a patient reacts 3+ at AHG with three panel cells and is negative with two cells. The autocontrol and direct antiglobulin test are both negative. Both non-reactive cells are K-negative, and all reactive cells are K-positive; reactivity disappears when panel cells are treated with ficin. Reasoning: reactivity at IAT with a negative autocontrol indicates an alloantibody; correlation with K, combined with loss of reactivity after ficin treatment, identifies anti-K. Antibodies to Kell antigens are characteristically destroyed by proteolytic enzymes, while Rh antibodies are enhanced.
Traps that cost marks
- Reporting an ABO group from forward typing alone, or ignoring an extra reaction in the reverse group that signals an alloantibody directed against A1 or B reagent cells.
- Missing a masking antibody that only reacts with enzyme-treated or homozygous (dosage) cells.
- Failing to recognise delayed haemolytic transfusion reactions caused by anamnestic alloantibody responses; they appear 3-14 days post-transfusion.
- Rouleaux in the reverse group mimics agglutination; saline replacement resolves it.
- Transfusion reactions: stop the transfusion, keep the line open with saline, return the bag and tubing, and send post-transfusion samples with a clerical check.