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PA21.1-6 | Blood Grouping, Crossmatch & Autologous Transfusion — Part 2

Grouping Discrepancies — When Forward and Reverse Disagree

A discrepancy means the forward and reverse results do not match the expected pattern. Transfusion must stop until the cause is found.

Common causes:

Category	Example
Technical errors	Wrong ratio of cells to serum, centrifugation error, clerical mix-up
Rouleaux	Elevated fibrinogen/immunoglobulins cause RBCs to stack like coins — false positive in reverse grouping; saline replacement technique distinguishes rouleaux from true agglutination
Cold agglutinins	IgM autoantibodies reactive at room temperature — can mask true reactions in reverse grouping; warm sample to 37°C and repeat
Missing antibody	Neonates (<4 months) lack reverse-grouping antibodies (maternal IgG present, own ABO antibodies not yet formed) — forward typing only in newborns
Extra antigen/antibody	Chimerism, acquired B phenomenon, recent transfusion with non-group-identical blood
Subgroups of A	A2 individuals have weaker forward reaction — may appear negative with some Anti-A lots

Resolution steps: Repeat with fresh sample → check reagents and technique → warm to 37°C → saline replacement for rouleaux → perform autocontrol → escalate to reference laboratory if unresolved.

Clinical rule: Never transfuse with an unresolved discrepancy. Use O Rh-negative blood in a life-threatening emergency while investigating.

Crossmatching — Purpose and Overview

A three-panel diagram explains crossmatching as the final compatibility test and compares major crossmatch with minor crossmatch in transfusion medicine. — **Panel A:** Patient sample, donor unit, ABO/Rh typing, patient antibody screen, final in-vitro compatibility test, ABO incompatibility, complement activation, intravascular haemolysis, clinically significant alloantibodies. **Panel B:** Major crossmatch, donor RBCs, recipient serum, recipient antibodies, donor RBC antigens, agglutination, CRITICAL, incompatible major crossmatch contraindicates transfusion. **Panel C:** Minor crossmatch, donor serum/plasma, recipient RBCs, donor antibodies, recipient RBC antigens, smaller agglutination reaction, LESS CRITICAL, relevance to whole blood and packed RBCs.

Crossmatching is the final in-vitro compatibility test between a specific donor unit and a specific recipient. It detects:
1. ABO incompatibility (most dangerous — complement-activating, intravascular haemolysis)
2. Clinically significant alloantibodies in the recipient that react with donor antigens not detected by antibody screen

Crossmatch is performed after:
• ABO/Rh typing of both patient and donor
• Antibody screen of patient serum (detects unexpected antibodies against panel cells)

Two crossmatches exist, and you must know both:

Major crossmatch (the critical one)
• Donor RBCs + Recipient serum
• Detects antibodies in the recipient that will destroy donor cells
• This is the safety-critical test — an incompatible major crossmatch is an absolute contraindication to transfusion

Minor crossmatch
• Donor serum/plasma + Recipient RBCs
• Detects antibodies in the donor plasma that might react against recipient cells
• Less critical in whole-blood era (packed red cells have little plasma); still done in some centres and for fresh whole blood

Side-by-side comparison diagram showing major crossmatch (donor RBCs + recipient serum) versus minor crossmatch (donor serum + recipient RBCs) with color-coded critical importance levels. — **Panel A:** Major crossmatch showing donor RBCs, recipient serum, agglutination reaction, arrow labeled 'CRITICAL', detection of recipient antibodies against donor antigens. **Panel B:** Minor crossmatch showing donor serum, recipient RBCs, reaction, arrow labeled 'LESS CRITICAL', detection of donor antibodies against recipient antigens.

Crossmatch Phases — Immediate-Spin and AHG Phase

A four-panel diagram compares immediate-spin and AHG crossmatch phases, showing their workflows, antibody targets, interpretation, check-cell validation, and electronic crossmatch criteria. — **Panel A:** Full workflow comparing Phase 1 immediate-spin crossmatch and Phase 2 AHG / indirect Coombs phase, including donor RBCs, recipient serum, centrifugation, incubation, washing, AHG reagent, and readout.. **Panel B:** Immediate-spin phase showing donor RBCs, recipient serum, IgM alloantibody, ABO incompatibility, immediate agglutination, emergency use, and limitation for IgG antibodies.. **Panel C:** AHG phase showing 37°C incubation, IgG-coated RBCs, washing step, anti-human globulin reagent bridging bound IgG, agglutination, clinically significant IgG alloantibodies, and incompatible-unit warning.. **Panel D:** Negative AHG validation with check cells / Coombs control cells, invalid negative result if check cells fail to agglutinate, plus electronic crossmatch criteria using two concordant ABO/Rh typings and negative antibody screen..

A full serological crossmatch has two phases:

Phase 1 — Immediate-spin (IS) crossmatch
• Mix donor RBCs + recipient serum in a tube; centrifuge immediately at room temperature; read.
• Detects: ABO incompatibility (IgM, activates complement immediately), most clinically significant IgM alloantibodies.
• Time: ~5 minutes. Used in emergencies when a full crossmatch is not feasible.
• Limitation: misses IgG antibodies (warm-reactive — clinically significant Rh, Kidd, Duffy, Kell, etc.).

Phase 2 — Antiglobulin (AHG) / Indirect Coombs phase
• After IS phase: incubate at 37°C → wash red cells (removes unbound immunoglobulin) → add anti-human globulin (AHG) reagent → centrifuge → read.
• Anti-human globulin (Coombs reagent) bridges IgG antibodies bound to RBC surface, causing agglutination — this is the indirect antiglobulin test (IAT).
• Detects: IgG alloantibodies (warm-reactive, clinically significant), complement components bound in vivo.
• Mandatory when the antibody screen is positive or when a full crossmatch is required.
• Positive AHG crossmatch = incompatible; unit must not be transfused.

Add check cells (Coombs control cells) at the end of every negative AHG test to confirm the AHG reagent was active — no agglutination with check cells invalidates the entire test.

Electronic crossmatch: Computer-based matching of ABO/Rh types without serological mixing — permitted only when:
• Two ABO/Rh typings on separate samples agree
• Antibody screen is negative
• Laboratory has validated the system

Crossmatch workflow diagram showing sequential steps from tube mixing to final AHG read with timing and detection annotations. — **Panel A:** Complete crossmatch workflow showing: tube mixing (0 min), immediate-spin read (1 min, detects ABO/high-titer antibodies), 37°C incubation (15-60 min, enhances IgG binding), washing (2-3 min, removes unbound proteins), AHG addition (1 min, detects IgG antibodies), final centrifuge read (1 min, final compatibility assessment). **Panel B:** Agglutination reaction patterns showing negative reaction (smooth cell suspension) versus positive reactions (1+ to 4+ agglutination grades) as seen in test tubes. **Panel C:** Summary table listing each phase, primary antibodies detected, typical reaction strength, and clinical significance for crossmatch interpretation.

SELF-CHECK

A crossmatch is set up between donor unit and recipient. The immediate-spin phase is negative (compatible). After incubation at 37°C, washing, and addition of AHG reagent, 2+ agglutination is seen. What does this indicate?

A. ABO incompatibility — the unit must not be transfused

B. Rouleaux formation due to the recipient's elevated plasma proteins

C. The AHG reagent was inactive — add check cells and repeat

D. A warm-reactive IgG alloantibody in the recipient is incompatible with donor RBC antigens

Reveal Answer

Answer: D. A warm-reactive IgG alloantibody in the recipient is incompatible with donor RBC antigens

A negative immediate-spin but positive AHG phase indicates a warm-reactive IgG antibody in the recipient's serum that reacts with an antigen on donor RBCs — an incompatible crossmatch; the unit must not be transfused. ABO incompatibility (option A) would almost always appear at the immediate-spin phase, not exclusively at the AHG phase. Rouleaux (option B) is washed away during the cell-wash step before AHG is added. Option C (inactive AHG) applies only when the AHG result is negative AND check cells also fail to agglutinate.

CLINICAL PEARL

The 'ABC' of transfusion safety errors: Almost all fatal haemolytic transfusion reactions follow one of three root causes — ABO incompatibility from a clerical mix-up (patient or sample mislabelling), Bedside failure (right blood, wrong patient), or Crossmatching bypassed under time pressure. The laboratory crossmatch is the last gate before the clinical team takes over. Both must hold.

SELF-CHECK

You perform a crossmatch. The AHG phase is negative. You add Coombs check cells and observe NO agglutination. What is the correct interpretation?

A. The crossmatch result is invalid — the AHG reagent may have been inactivated; repeat the test

B. The check cells confirm the AHG reagent was active — the compatible result stands

C. The crossmatch is compatible — proceed with transfusion

D. The recipient has no alloantibodies — electronic crossmatch is now authorised for this patient

Reveal Answer

Answer: A. The crossmatch result is invalid — the AHG reagent may have been inactivated; repeat the test

Coombs check cells (IgG-sensitised red cells) MUST agglutinate when added to a negative AHG test — their agglutination proves the AHG reagent was functional. Failure of check cells to agglutinate means the AHG reagent was inactivated, not added, or washed away; the entire test is invalid and must be repeated from the wash step. Options B and C falsely accept a compatible result from a technically invalid test. Option D confuses an invalid AHG result with a criterion for electronic crossmatch (which requires a negative antibody screen on a separately validated system, not merely a negative AHG crossmatch).