Investigation - Red Cell Alloimmunization

Transfusion services should have clear policies describing the required investigations for transfusion complications.

Laboratory investigation of RBC alloimmunization is initiated when a patient is tested subsequent to transfusion or pregnancy, and found to have a positive antibody screen and/or positive DAT.

Depending on the results of the initial investigation, the transfusion service will perform follow-up serologic testing according to its own policies and procedures. Some examples are outlined below.

NOTE: The expected results include the appearance of a "new" positive antibody screen and negative direct antiglobulin test (DAT) unless the patient has been recently transfused in which case the DAT may also be positive. In patients with known red cell antibodies a change in strength or pattern of the antibody screen is suggestive of additional antibody formation.

Antibody identification

If a new or unexpected antibody is found in the antibody screen, it must be identified prior to any subsequent RBC transfusion. If the DAT is positive and an alloantibody (as opposed to an autoantibody) is suspected, an elution should also be performed.

Antigen type

If a specific antibody is identified, the patient's pre-transfusion specimen should be phenotyped for the antigen(s) corresponding to the antibody(ies) identified.

NOTE: If the patient was transfused with a product such as intravenous immune globulin (IVIG) or a large dose of Rh immune globulin (RhIG), e.g., to treat Immune Thrombocytopenia Purpura (ITP), suspect passive antibodies.