6.4 Compatibility Testing, Crossmatching, and Hemolytic Disease of the Fetus and Newborn
Key Takeaways
- The major crossmatch tests patient serum against donor red cells to prevent acute hemolytic transfusion reactions.
- Computer crossmatching requires two separate ABO determinations and a completely negative antibody screen.
- Rh HDFN is caused by maternal IgG Anti-D crossing the placenta; it usually affects subsequent pregnancies after initial sensitization.
- One standard 300 µg dose of RhIG (RhoGAM) protects against 30 mL of fetal whole blood; dosage is calculated via the Kleihauer-Betke test.
6.4 Compatibility Testing, Crossmatching, and Hemolytic Disease of the Fetus and Newborn
Pretransfusion Compatibility Testing
Compatibility testing represents the final safeguard before blood components enter a patient's vein. The primary objective is to prevent acute hemolytic transfusion reactions (AHTRs) and ensure maximum survival of the transfused red cells. The capstone of this process is the crossmatch.
The Major Crossmatch
The major crossmatch physically tests donor red blood cells against recipient (patient) serum/plasma. It confirms ABO compatibility and detects any antibodies in the patient's serum that target antigens on the specific donor unit.
- Immediate Spin (IS) Crossmatch: Performed when the patient has a negative antibody screen and no historical record of clinically significant antibodies. The technologist mixes patient serum and donor RBCs, centrifuges immediately, and checks for agglutination. The IS crossmatch exclusively verifies ABO compatibility. It does not guarantee survival of the RBCs against low-titer IgG antibodies.
- Full (Antiglobulin) Crossmatch: Mandatory if the patient currently has a positive antibody screen or a historical record of an antibody (even if currently undetectable). This protocol includes a 37°C incubation phase and the AHG phase to conclusively prove the donor unit lacks the corresponding antigen and is fully compatible.
The Computer (Electronic) Crossmatch
Modern blood banks often replace the serologic IS crossmatch with an electronic crossmatch, utilizing a validated laboratory information system (LIS) to verify ABO compatibility computationally. Rigorous Requirements for Electronic Crossmatch:
- The computer system must be validated on-site and programmed to halt any incompatible issue.
- The patient must have a negative antibody screen and no history of unexpected antibodies.
- The blood bank must have two distinct, concordant ABO/Rh determinations on record for the patient (e.g., a current specimen and a historical record, or two independently drawn current specimens).
Hemolytic Disease of the Fetus and Newborn (HDFN)
HDFN is a severe condition where maternal IgG alloantibodies cross the placental barrier and bind to fetal red blood cells possessing the corresponding antigen inherited from the father. This leads to profound fetal RBC destruction by macrophages in the fetal spleen, resulting in anemia, hyperbilirubinemia, extramedullary hematopoiesis, and potentially hydrops fetalis (severe edema and heart failure).
ABO vs. Rh(D) HDFN
HDFN presents primarily in two forms:
| Feature | ABO HDFN | Rh(D) HDFN |
|---|---|---|
| Etiology | Group O mother produces IgG Anti-A,B against A or B fetus. | Rh-negative mother produces IgG Anti-D against Rh-positive fetus. |
| Severity | Typically mild to moderate. | Often severe to fatal without intervention. |
| Onset | Can occur in the first pregnancy (Anti-A,B is naturally occurring). | Usually occurs in subsequent pregnancies (requires sensitizing event). |
| DAT Result on Cord Blood | Weakly positive or negative. | Strongly positive (3+ to 4+). |
| Postnatal Treatment | Phototherapy for bilirubin reduction. | Exchange transfusion. |
Prevention of Rh HDFN: RhIG (RhoGAM)
To prevent Rh-negative mothers from forming Anti-D, Rh Immune Globulin (RhIG) is administered. RhIG is an extremely concentrated form of passive anti-D IgG. It binds to any fetal Rh-positive cells that have entered the maternal circulation, sequestering them in the mother's spleen before her immune system can recognize them and undergo primary sensitization.
- Routine Administration: RhIG is given prophylactically at 28 weeks gestation, and a subsequent dose is administered within 72 hours postpartum if the delivered infant types as Rh-positive (or unknown).
Fetal-Maternal Hemorrhage Quantification and Dosing
To determine the correct postpartum dose of RhIG, the laboratory must calculate the volume of fetal blood that bled into the maternal circulation during delivery.
- The Rosette Test: A qualitative screening test. Maternal blood is incubated with Anti-D, then Rh-positive indicator cells are added. If fetal Rh-positive cells are present, rosettes form. If positive, a quantitative test must be ordered.
- The Kleihauer-Betke Test (Acid Elution): A quantitative test based on the principle that fetal hemoglobin (HbF) is highly resistant to acid elution, while adult hemoglobin (HbA) is not. A maternal blood smear is treated with acid, then stained. Maternal cells appear as pale "ghost cells," while fetal cells retain their bright pink hue. Technologists count the percentage of fetal cells.
RhIG Dosing Calculation:
- One standard 300 µg vial of RhIG protects against 30 mL of fetal whole blood (which is equivalent to 15 mL of packed fetal RBCs).
- Step-by-Step Calculation:
- Multiply the percentage of fetal cells from the Kleihauer-Betke test by 50 to estimate the mL of fetal whole blood. (e.g., 1.2% × 50 = 60 mL).
- Divide the mL of fetal whole blood by 30 to determine the base number of vials. (e.g., 60 / 30 = 2.0).
- The Standard Rounding Rule: If the decimal is ≥0.5, round up to the next whole number and add 1 vial. If the decimal is <0.5, round down to the whole number and add 1 vial. (e.g., 2.0 rounds to 2, plus 1 = 3 vials needed).
Exam Trap: Always remember the final step of adding 1 vial to the rounded calculation to ensure a safety margin for the mother. Proper calculation is critical for exam success.
A single 300 µg dose of Rh Immune Globulin (RhIG) is designed to protect a mother from sensitizing against a maximum of what volume of fetal whole blood?
In a major crossmatch procedure, which of the following components are physically mixed to ensure compatibility before transfusion?
Which specific condition must be met for a blood bank to utilize an electronic (computer) crossmatch in lieu of a serologic immediate spin crossmatch?