• Cold autoantibodies usually cause a positive DAT due to complement (commonly C3d) coating the red cell surface.
  • Strategies to determine if a cold antibody is clinically-significant:
    • Incubate patient serum with normal red cells suspended in saline at room temperature for 30-60 minutes. A negative test practically rules out a clinically-significant cold antibody.
    • If the screening test is positive, further tests are necessary:
    • Antibody titration (clinically-significant cold agglutinins usually have a titre of 1:500 – 1:1000 at 4°C).
    • Thermal amplitude: antibodies active at >30°C (detected by tube NISS direct agglutination) are usually clinically-significant.
    • Samples should be transported to the lab at 37°C. If this is not possible, then EDTA samples should be warmed to 37°C for one hour prior to testing.
  • Cold antibodies may agglutinate all cells within the reverse ABO group, leading to discrepant ABO typing.
    • Because the antibody is a cold-reacting IgM, patient red cells can be washed at 37°C to remove the IgM autoantibody.
  • For antibody screening, separately pre-warming patient plasma and screening red cells to 37°C before the IAT can avoid the non-specific reactivity of the cold autoantibody.
  • For potent, cold-reacting antibodies active at 37°C:
    • The patient’s plasma can be treated with DTT, which disrupts the J chain of the IgM antibody. Post-DTT-treated samples should then be tested at 37°C by IAT technique using a monospecific IgG antiglobulin rather than a polyspecific one.
    • Alloadsorptions at 4°C may be performed.
  • Blood should be transfused using a blood warmer.

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