Crossmatch (immediate spin and AHG)

Principle

• Patient plasma is tested against red cells from the donor unit (from an attached segment) to detect incompatibility before transfusion.

Specimen

• Patient EDTA plasma (or serum) within 3 days if transfused or pregnant in the last 3 months; donor red cells from segments as 2–5% saline suspension.

Procedure

• Immediate spin (IS) crossmatch: 2 drops patient plasma + 1 drop donor cell suspension; centrifuge immediately and read for agglutination or haemolysis. Mainly detects ABO incompatibility.
• AHG (full) crossmatch: continue after IS by adding enhancement, incubating at 37 °C (e.g. 10–15 min LISS), washing 3–4 times, adding AHG, reading and confirming negatives with check cells.
• Electronic (computer) crossmatch: allowed where a validated system holds two concordant ABO/D results and the antibody screen is negative with no history of antibodies.

Results

• Compatible: no agglutination or haemolysis at any phase.
• Incompatible: agglutination or haemolysis; do not issue the unit until resolved.

Interpretation

• IS crossmatch is acceptable when the current antibody screen is negative and there is no history of clinically significant antibodies.
• AHG crossmatch with antigen-negative units is required when clinically significant antibodies are present or have been detected previously.
• Causes of incompatibility: ABO error, alloantibody against a donor antigen, donor cells with positive DAT, autoantibodies, rouleaux (use saline replacement), cold antibodies at IS.

Quality control and pitfalls

• Check patient identification and sample labelling; most fatal ABO reactions come from clerical errors.
• A compatible crossmatch does not detect D typing errors (if no anti-D present), does not prevent alloimmunisation and does not guarantee normal red cell survival.
• Antibodies to low-frequency antigens may be detected only by crossmatch.

Clinical use

• Final compatibility check before red cell transfusion.