Blood Banking: ABO/Rh, Antibody Detection, and Compatibility Testing
The blood banking portion probes the ABO/Rh system, antibody detection and identification, compatibility testing, and the reasoning that resolves discrepant or unexpected results - not just memorized tube reactions.
ABO and D fundamentals
Forward (cell) and reverse (serum) grouping must agree before a type is reported. Discrepancies have a short differential: weak or missing reverse reactivity in A2/A2B (anti-A1), cold autoagglutinins, rouleaux, acquired B, or a mislabeled specimen. Anti-A1 is usually IgM, nonreactive at 37 °C, and not clinically significant unless reactive at that temperature, in which case A2 or O red cells are selected. The Bombay phenotype (Oh) lacks H antigen and produces anti-H, incompatible with every unit except other Oh units.
Antibody detection
A two- or three-cell screen uses group O reagent cells homozygous for clinically significant antigens (Rh, Kell, Duffy, Kidd, MNS, Lewis, P1) so that dosage is expressed. Identification panels follow: rule out a specificity only with cells homozygous for the antigen in question - a single heterozygous reactive cell can never exclude an antibody. Enzyme treatment (ficin, papain) enhances Rh, Kidd, Lewis, and P1 reactivity and destroys Duffy and MNS.
Compatibility testing
The immediate spin crossmatch confirms ABO compatibility; the AHG crossmatch detects clinically significant IgG. An electronic crossmatch is permitted only with two concordant ABO determinations, no history of a clinically significant antibody, and a validated computer system.
Worked example
Screen cells I and III are negative; cell II is 2+ at AHG. The patient types Jk(a−b+). The panel reacts only with Jk(a+b+) cells and is negative with Jk(a−b+) cells, so anti-Jka is identified. Select Jk(a−) units and perform AHG crossmatches, because the electronic crossmatch is now prohibited.
Traps that cost marks
Calling rouleaux agglutination - saline replacement disperses rouleaux but not true agglutination. Missing mixed-field reactions in recently transfused patients. Assuming a negative screen excludes antibody when the patient was transfused within three months and the antibody is absorbed onto donor cells.