Blood Banking and Transfusion Medicine
Pretransfusion testing rests on three pillars: unambiguous patient identification, accurate ABO/RhD typing confirmed by a reverse (serum) grouping, and detection of clinically significant alloantibodies by antibody screen, identification, and antigen-negative crossmatch. Understand the immunogenicity hierarchy (D > K > c > E > Jka > Fya), which predicts what a newly formed antibody is likely to be, and the concept of dosage, in which homozygous cells express more antigen and heterozygous cells can lead to an antibody being falsely ruled out.
Know component characteristics: leukoreduction reduces febrile nonhemolytic reactions and CMV transmission; irradiation prevents transfusion-associated graft-versus-host disease in at-risk recipients; washed products remove plasma IgA and reduce hyperkalemia risk in large-volume neonatal transfusion.
Traps
- Treating a positive antibody screen as an alloantibody without excluding an autoantibody; distinguish using the autocontrol, direct antiglobulin test, and adsorption studies (allogeneic adsorption when recently transfused, autologous otherwise).
- Forgetting that weak or absent reverse grouping in a newborn or elderly patient reflects age-related antibody production, not an ABO subtype.
- Attributing fever alone to a hemolytic reaction; fever with hemoglobinuria, hypotension, or oozing at the IV site demands stopping the transfusion, clerical recheck, DAT, plasma free hemoglobin, and repeat ABO typing.
- Missing delayed hemolytic reactions appearing 3-14 days later with a falling hematocrit and newly positive DAT.
Worked example. Quantify fetomaternal hemorrhage with the Kleihauer-Betke stain: fetal bleed (mL) = percent fetal cells × 5000 mL maternal blood volume. Divide by 30 mL per RhIG vial and add one vial for safety, so a 1.5% result (~75 mL) implies three vials rather than the single standard antenatal dose.