ASCP Transfusion Medicine 1 — Questions and Answers

Question 1: Which ABO blood group is considered the 'universal donor' for red blood cell transfusions?

Group A
Group B
Group O (Correct answer)
Group AB

Correct answer: Group O

Group O red cells lack A and B antigens, so they will not trigger an ABO incompatibility reaction in A, B, or AB recipients. Group O negative is the universal donor for red cells as it also lacks the D antigen.

ABO antigens are carbohydrate structures on red cell membranes encoded by glycosyltransferases. Group O has H antigen only. Group A has the A antigen (GalNAc added to H). Group B has the B antigen (Gal added to H). Group AB has both. ABO antibodies (IgM, complement-activating) are naturally occurring (formed against gut bacterial antigens). Isohemagglutinins: Group A has anti-B; Group B has anti-A; Group O has anti-A and anti-B; Group AB has neither. Group O negative red cells are limited and reserved for emergencies and unknown blood type situations. Group AB plasma is universal donor plasma as it has no anti-A or anti-B.

Question 2: The indirect antiglobulin test (IAT or indirect Coombs test) detects:

Antibodies already coating red cells in vivo
Unexpected antibodies in patient serum that react with reagent red cells in vitro (Correct answer)
ABO forward grouping
Platelet antibodies

Correct answer: Unexpected antibodies in patient serum that react with reagent red cells in vitro

IAT detects unexpected alloantibodies or autoantibodies in patient serum or plasma by incubating patient serum with reagent red cells, washing to remove unbound antibody, then adding anti-human globulin (AHG) reagent that agglutinates IgG-coated cells.

IAT procedure: (1) Incubate patient serum with reagent or donor red cells at 37 degrees C for sensitization, typically 30–60 minutes. (2) Wash cells 3–4 times with saline to remove unbound immunoglobulin. (3) Add AHG (anti-IgG plus or minus anti-C3). (4) Centrifuge and read agglutination. Uses include antibody screen to detect unexpected antibodies, antibody identification via panel testing, compatibility testing (crossmatch), phenotyping to confirm D or other antigens, and titration. A positive IAT indicates a clinically significant antibody is present.

Question 3: Hemolytic disease of the fetus and newborn (HDFN) caused by anti-D requires all of the following EXCEPT:

D-negative mother
D-positive fetus
Maternal IgG anti-D crossing the placenta
ABO incompatibility between mother and fetus (Correct answer)

Correct answer: ABO incompatibility between mother and fetus

ABO incompatibility between mother and fetus does NOT cause Rh HDFN — it actually may reduce the risk by rapidly clearing fetal cells from maternal circulation before alloimmunization can occur. HDFN requires D-negative mother, D-positive fetus, and maternal IgG anti-D.

Anti-D HDFN mechanism: D-negative mother exposed to D-positive fetal cells during pregnancy or delivery leads to a primary immune response (IgM anti-D), then subsequent pregnancy with D-positive fetus triggers secondary IgG anti-D response. IgG anti-D crosses the placenta, binds fetal D-positive RBCs, and causes extravascular hemolysis leading to anemia, hyperbilirubinemia, and potentially hydrops fetalis. Prevention: Rh immunoglobulin (RhIG, Rhogam) given at 28 weeks plus within 72 hours postpartum to D-negative mothers. ABO incompatibility effect: fetal ABO-incompatible cells are cleared rapidly from maternal circulation, reducing D alloimmunization risk.

Question 4: Which blood product is most appropriate for a patient with isolated thrombocytopenia (platelet count 15,000/µL) and active bleeding?

Fresh frozen plasma (FFP)
Cryoprecipitate
Apheresis platelets (single donor) (Correct answer)
Packed red blood cells

Correct answer: Apheresis platelets (single donor)

Apheresis (single-donor) platelets are the product of choice for thrombocytopenic bleeding. One apheresis unit contains at least 3.0 times 10 to the 11th platelets (equivalent to 4–6 random donor units) and raises platelet count by approximately 30,000–50,000/µL.

Blood product selection: FFP contains all coagulation factors and is used for factor deficiencies, warfarin reversal, and TTP. Cryoprecipitate is enriched for fibrinogen, Factor VIII, vWF, Factor XIII, and fibronectin, and is used for fibrinogen deficiency and DIC. Platelets are supplied as apheresis (1 unit) or whole blood-derived random donor pools (4–6 units). Platelet transfusion triggers include active bleeding below 50,000/µL, surgical prophylaxis below 50,000, neurosurgery below 100,000, and ITP with active bleeding regardless of count. Contraindications include TTP and HIT where platelet transfusion may be harmful.

Question 5: A positive direct antiglobulin test (DAT) combined with a positive elution study suggests:

Warm autoimmune hemolytic anemia (WAIHA) or alloantibody causing hemolysis (Correct answer)
ABO incompatibility in typing only
Factor deficiency
Normal newborn screening result

Correct answer: Warm autoimmune hemolytic anemia (WAIHA) or alloantibody causing hemolysis

Positive DAT means IgG and/or complement is coating the patient's red cells in vivo. Elution removes and identifies the antibody. If the eluate is reactive with antigen-positive cells, it confirms an antibody causing in vivo sensitization consistent with WAIHA or alloantibody from hemolytic transfusion reaction.

DAT (direct Coombs): red cells are washed and AHG is added directly to detect IgG and/or C3d on the RBC surface in vivo. Causes of positive DAT include WAIHA (pan-reactive IgG; eluate reactive against all cells including autologous), drug-induced immune hemolytic anemia (from cephalosporins, penicillin, methyldopa, or fludarabine), alloantibody from hemolytic transfusion reaction or HDFN, and cold agglutinin disease (IgM leading to C3d deposition). Elution techniques include heat at 56 degrees C, acid (glycine pH 3.0), freeze-thaw, and Lui strip methods.

Question 6: TACO (transfusion-associated circulatory overload) is best distinguished from TRALI (transfusion-related acute lung injury) by:

Time of onset after transfusion
Response to diuretics and elevated BNP in TACO versus normal filling pressures in TRALI (Correct answer)
Presence of fever
Blood product type implicated

Correct answer: Response to diuretics and elevated BNP in TACO versus normal filling pressures in TRALI

TACO is hydrostatic pulmonary edema (elevated filling pressures, elevated BNP, responds to diuretics). TRALI is non-cardiogenic pulmonary edema (bilateral, normal or low filling pressures, caused by donor anti-HLA or anti-HNA antibodies activating recipient neutrophils). Both present within 6 hours.

TACO criteria: new or worsening pulmonary edema within 6 hours of transfusion, elevated BNP or NT-proBNP, response to diuresis, hypertension, positive fluid balance, cardiomegaly on CXR, and elevated PCWP above 18 mmHg. Risk factors include elderly, CHF, renal failure, rapid infusion, and large volume. TRALI criteria: acute hypoxia (PaO2/FiO2 below 300), bilateral infiltrates, no pre-existing ALI, within 6 hours of transfusion, and not TACO. Mechanism: donor anti-HLA class I or II or anti-HNA antibodies activate recipient PMNs causing capillary leak. TRALI is the most common cause of transfusion-related death. Prevention includes male-predominant plasma donors and quarantine of implicated donors.

ASCP Transfusion Medicine 1

Which ABO blood group is considered the 'universal donor' for red blood cell transfusions?