Weak D
In serologic testing, D positive blood is easily identified. Units which are D negative are often retested to rule out a weaker reaction. This was previously referred to as Du, which has been replaced. By definition, weak D phenotype is characterized by negative reaction with anti-D reagent at immediate spin (IS), negative reaction after 37C incubation, and positive reaction at anti-human globulin (AHG) phase. Weak D phenotype can occur in several ways. In some cases, this phenotype occurs because of an altered surface protein that is more common in people of European descent. An inheritable form also occurs, most often in African-Americans, as a result of a weakened form of the R0 gene. Weak D may also occur as "C in trans," whereby a C gene is present on the opposite chromosome to a D gene (as in the combination R0r’, or "Dce/dCe"). The testing is difficult, since using different anti-D reagents, especially the older polyclonal reagents, may give different results.
The practical implication of this is that people with this sub-phenotype will have a product labeled as "D positive" when donating blood. When receiving blood, they are sometimes typed as a "D negative", though this is the subject of some debate. Most "Weak D" patients can receive "D positive" blood without complications. However, it is important to correctly identify the ones that have to be considered D+ or D-. This is important, since most blood banks have a limited supply of "D negative" blood and the correct transfusion is clinically relevant. In this respect, genotyping of blood groups has much simplified this detection of the various variants in the Rh blood group system.
Read more about this topic: Rh Blood Group System
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