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

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ABO Typing Discrepancies

Forward and reverse ABO typing are both performed in order to validate the ABO interpretation. Blood groups (with the exception of the AB blood group) possess naturally occurring ABO antibodies. These antibodies are present in the plasma when the corresponding antigen is absent from the red cells.
An ABO typing discrepancy occurs when forward and reverse typing reactions do not correlate or when test results are interpreted as unexpected reactions. ABO discrepancies MUST be resolved PRIOR to the administration of blood components. A transfusion reaction could occur if cellular or non-cellular blood components are administered to a patient who demonstrates an ABO discrepancy.

Patient Blood Groups and Corresponding Naturally Occurring Plasma Antibodies

Blood Group
RBC Antigen
Naturally Occurring Antibody
A A only
Anti-B (IgM)
B B only
Anti-A (IgM)
AB A and B
O NoneAnti-A (IgM), Anti-B (IgM) & Anti-A, B (IgG)
Note: Anti-A,B produced by a Group O individual is a separate antibody (not a combination of Anti-A and Anti-B) and has the capacity to react with both A and B antigens. It is usually IgG in nature.

Discrepant reactions are reactions that result in outcomes that do not fit into the above table.