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).

For more information, please listen to the BBGuy Essentials Podcast, Episode 005, and watch my “Weak in the D’s” video from 2012.

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