MLAB 2461 CLINICAL II
Immunohematology

Resolution of ABO Discrepancies

ABO discrepancies are recognized when the reactions obtained in the forward type do not "match" the reactions obtained in the reverse type.  For example,

• Forward type is group O and reverse type is group B.
• If an individual is group B they MUST have anti-A present in their serum or plasma.  Failure to agglutinate the reagent A1 cells will cause additional investigation to be performed.   

When a discrepancy is observed one MUST determine if the problem is associated with the forward type, reverse type or both.

FALSE POSITIVE means the reaction should have been NEGATIVE but agglutination occurred. FALSE NEGATIVE means the reactions should have been POSITIVE but no agglutination occurred. The key is to recognize which type of problem is occurring so that appropriate testing can be performed. 

When evaluating an ABO discrepancy it needs to be determined whether there is a problem with the FORWARD type (antigens on cells), the REVERSE type (antibodies in serum/plasma) or BOTH.

Problems With the Forward Type

The forward type detects antigens present on the RED BLOOD CELLS. 

False negative reactions in the forward type may occur due to:

• Weak subgroups of A or B.
• A, B or AB individual transfused with massive quantities of group O blood.
• ABO non-identical bone marrow or stem cell transplant.
• Inhibitor substances which neutralize anti-A or anti-B.

False positive reactions may occur due to:

• heavy protein coating of the red blood cells. 
• coating of cord blood cells with Wharton's jelly.
• antibodies to dyes used to color anti-A or anti-B

Problems with the Reverse Type

The reverse type detects antibodies present in the SERUM or PLASMA. 

False negative reactions in the reverse type may be due to weak or missing antibodies due to a variety of factors which influence the production of antibodies. 

• Patient is an infant.
• Patient is elderly.
• Seriously immunocompromised due to disease, therapy or depressed immunoglobulin levels.
• Large amounts of IV fluids present due to treatment or drawing blood above an IV.
• Antibodies passively transfused in non-ABO identical products containing excessive plasma, usually platelets.

False positive reactions may be due to "unexpected antibodies", those antibodies other than anti-A or anti-B. 

• False positive reactions occur in situations such as a group A individual reacting with BOTH the A₁ and B reagent cells. In this situation one must think of the most frequent cause of the problem.  Anti-A₁ is not uncommon in A₂ or A₂B individuals.
• Cold agglutinins are another cause of an unexpected positive in the reverse type.  It is important that red blood cell reagents be allowed to warm to room temperature prior to use to prevent false positives due to cold agglutinins.

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