GlossaryA List of Commonly Used Terms in Blood Banking
Weak D Phenotype
The main Rh antigen (the one that determines whether a person is “Rh-positive” or “Rh-negative”) is known to blood bankers as the “D” antigen. Most D-positive (“Rh-positive”) people express the antigen quite well, and the person is obviously D-positive when anti-D is mixed with their red cells. In test tubes, the reaction occurs immediately after centrifugation (in other words, at the “immediate spin” phase). Uncommonly, D-positive people have significantly decreased quantities of the D antigen, so they do NOT test as D-positive with routine immediate spin testing (instead, positive reaction is only seen at the “AHG phase“). Such persons are defined as “weak D” (formerly “Du“).
Those with weak D were traditionally thought to have completely normal but significantly decreased amounts of D antigen on their RBCs (note that this is in contrast to classic partial D, in which the person is assumed to have abnormal D antigen in normal amounts on their RBCs). Weak D and partial D are in fact not as separate as we would like to think from the above “classic” definitions. There are overlap categories with decreased quantities of D having qualitative abnormalities (which some call “weak partial D” or “partial weak D”). As a result, the AABB recently recommended that blood recipients and pregnant females with weak D be tested with Rh genotyping to define their D variant in a more precise manner (potentially conserving the Rh negative blood supply and reducing the number of unnecessary RhIg injections).
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