11.1 Blood Grouping Tests
Key Takeaways
- ABO discrepancies fall into four classes: weak/missing antibodies (Group I), weak/missing antigens (Group II), rouleaux/protein abnormalities (Group III), and unexpected alloantibodies/autoantibodies (Group IV).
- Partial D patients can form alloanti-D and are managed as D-negative, while weak D types 1-3 essentially never form anti-D and are managed as D-positive.
- Bombay phenotype (Oh) individuals produce anti-A, anti-B, and anti-H reactive through the AHG phase, making them incompatible with red cells from every ABO group, including group O.
- Acidifying anti-B reagent to about pH 6 distinguishes true B antigen from acquired B caused by bacterial deacetylation of the A antigen.
- Saline replacement disperses rouleaux while leaving true antibody-mediated agglutination intact, resolving Group III discrepancies.
11.1 Blood Grouping Tests
SBB Focus: At the specialist level, blood grouping is not "read the reactions and report the type." You are the person the bench technologist calls when the forward and reverse groups do not match, when a newborn's group looks impossible, or when a patient's D typing is ambiguous before an emergency release. This section builds the discrepancy-resolution logic ASCP expects under Immunohematology domain IV.A-B.
Forward and Reverse Grouping Must Agree
Forward (cell) typing mixes patient red blood cells with commercial anti-A and anti-B reagents. Reverse (serum) typing mixes patient serum or plasma with reagent A1 and B red blood cells. In a normal adult, the two results must be mirror images: if forward typing shows A antigen, reverse typing must show anti-B and no anti-A. Any mismatch is an ABO discrepancy and must be resolved before a unit is released — never report or transfuse based on forward typing alone.
The Four Classes of ABO Discrepancy
| Group | Defect | Common Causes | Resolution Strategy |
|---|---|---|---|
| Group I | Weak or missing antibodies (reverse group weaker than expected) | Newborns under 4-6 months, elderly patients, hypogammaglobulinemia, leukemia/lymphoma, ABO subgroups (A2, A3) | Room-temperature or 4°C incubation to enhance weak antibody; repeat with a fresh sample; AABB does not require reverse grouping for infants under 4 months |
| Group II | Weak or missing antigens (forward group weaker than reverse) | Subgroups (Ax, Am, Bx, B3), acquired B, leukemia-related antigen loss, post-transplant chimerism | Extended incubation, adsorption-elution, anti-A1 lectin (Dolichos biflorus) to separate A1 from A2, molecular ABO genotyping |
| Group III | Rouleaux or unexpected protein abnormalities | Multiple myeloma, Waldenström macroglobulinemia, elevated fibrinogen, Wharton's jelly in cord blood | Saline replacement technique — wash and resuspend cells in saline; true agglutination persists, rouleaux disperses |
| Group IV | Unexpected alloantibodies or autoantibodies reacting with reagent cells | Cold alloantibodies (anti-M, anti-Lea), cold autoantibodies reacting with A1/B reagent cells | Prewarm technique (37°C throughout), autoadsorption or allogeneic adsorption, repeat testing across a temperature gradient |
Acquired B deserves special attention: bacterial deacetylation of the A antigen, seen with gram-negative sepsis or colon cancer, creates a structure that mimics B antigen and produces a false forward-type B reaction in a true group A patient. Acidifying the anti-B reagent to approximately pH 6 abolishes reactivity with acquired B but not with true B antigen — a classic SBB resolution technique.
Rh Typing Pitfalls
Weak D (formerly "Du") describes red cells with a quantitatively reduced but structurally normal D antigen. These individuals are managed as D-positive for transfusion and pregnancy once confirmed by molecular RHD genotyping, since weak D types 1, 2, and 3 essentially never form anti-D. Partial D, by contrast, is a qualitative change: one or more D epitopes are missing entirely. A partial D individual can be exposed to a transfused or fetal epitope they lack and form alloanti-D, so partial D patients are managed as D-negative for transfusion and pregnancy despite typing D-positive by routine serology.
| Feature | Weak D | Partial D |
|---|---|---|
| Antigen density | Reduced | Normal or reduced |
| Epitopes present | All, in reduced amount | One or more missing |
| Can form anti-D? | Essentially no (types 1-3) | Yes |
| Transfusion/pregnancy management | D-positive | D-negative |
An Rh control (a reagent lacking anti-D but otherwise matched to the test reagent) is required whenever spontaneous agglutination, rouleaux, or a positive DAT is suspected — without it, a heavily protein-coated or polyagglutinable cell can falsely type as D-positive.
Bombay and Para-Bombay Phenotypes
The Bombay phenotype (Oh) results from homozygous inactivation of the FUT1 gene, so the red cells lack H antigen — and without H, neither A nor B antigen can be built regardless of the person's underlying ABO genotype. Bombay serum contains potent anti-A, anti-B, and anti-H, all reactive through the AHG phase. This means a Bombay individual is incompatible with every ABO group, including group O, since group O cells are H-antigen-rich. Testing with Ulex europaeus lectin (anti-H) shows no reaction, distinguishing Bombay from a routine group O sample. Para-Bombay individuals retain a secretor (FUT2) pathway that deposits trace ABH substance detectable by adsorption-elution, even though routine red cell typing looks Bombay-like.
Reagent Red Blood Cells
Reverse grouping and antibody detection both depend on reagent red blood cells — group O cells of known, QC-verified phenotype from selected or pooled donors. A1 and B reagent cells must be confirmed for A1 versus A2 subtype reactivity, and lot-to-lot antigen strength must be verified, since weakly expressed antigens on an aging reagent lot can produce a false discrepancy that has nothing to do with the patient.
Worked SBB Scenario
A 68-year-old patient's forward type shows 4+ reactions with anti-A and anti-B, but the reverse type shows no reaction with either A1 or B reagent cells. This pattern — strong forward reactions with a "missing" reverse reaction — does not fit a routine Group I or Group II pattern and should immediately raise suspicion for passively acquired antibody from massive transfusion, or a mixed-field picture from a recent group-incompatible marrow or organ transplant. The specialist's job is to pull the transfusion and transplant history and check specimen timing before treating this as a routine subgroup problem.
Molecular ABO Genotyping
When serologic discrepancies cannot be resolved with adsorption-elution, extended incubation, or lectin studies, molecular ABO genotyping sequences the ABO gene to identify weak subgroup alleles (such as A2, Ael, or Bel) that serology alone cannot reliably distinguish from a true O allele. This has become an increasingly important tie-breaker for the subgroup discrepancies that dominate SBB-level exam scenarios.
A patient's reverse ABO typing shows unexpected agglutination with both A1 and B reagent cells, and the antibody screen also shows weak reactivity with all screening cells. Which technique best distinguishes true antibody-mediated agglutination from rouleaux caused by an elevated globulin or fibrinogen level?
A patient types D-positive by routine tube testing, but molecular RHD genotyping reveals a partial D phenotype with several missing D epitopes. How should this patient be managed for transfusion and pregnancy purposes?
A patient's red cells fail to react with anti-A, anti-B, and Ulex europaeus (anti-H) lectin, and the serum reacts strongly with A1 cells, B cells, and group O screening cells through the AHG phase. What is the transfusion implication of this finding?