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The page below is a sample from the LabCE course Transfusion Reactions. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Delayed Transfusion Reactions

The appearance of an antibody post-transfusion is a sign of a delayed transfusion reaction. Only when there is evidence of increased red cell destruction is the reaction termed a delayed hemolytic transfusion reaction (DHTR). Appearance of an antibody without cell destruction is a delayed serologic reaction.
Delayed reactions occur more than 24 hours post cessation of transfusion. Usually, the blood appears serologically compatible at initial testing. Delayed reactions are common in patients who have been immunized to a foreign antigen from a previous transfusion or pregnancy, but the antibody titers decrease over time and the antibody is not detectable during pre-transfusion testing.
The transfusion leads to a secondary (anamnestic) response, causing increased antibody production that sensitizes antigen-positive donor red cells. Hemolysis is extravascular with a DHTR. Sensitized cells are removed from circulation by the reticuloendothelial system, also called the monocyte-macrophage system. Because there is a delay in the presentation of symptoms, DHTR is not usually considered as a cause of the clinical presentation. The transfusion service usually initiates investigation of a DHTR because of serologic findings in a post-transfusion specimen.

DHTRs occur more frequently than acute hemolytic reactions.