As noted, policies for administering RhIg to mothers with a variant of D vary among countries and within some countries.
An Rh(D) red blood cell phenotype with a weak or variant expression of the D antigen occurs in 0.2% to 1% of whites and is slightly more common in African Americans. The phenotype is routinely called weak D, although several variants exist.
A simple model includes these D variants:
1. Weak D
Multiple weak D variants exist. Red cells have fewer D antigens/red cell (quantitative difference) and only minor variations in D antigen proteins.
Some, but not all, weak D phenotypes are detected by today's Rh typing sera and may be classified as Rh positive or Rh negative by routine testing but will be positive when a weak D test (IAT with anti-D) is done.
An extreme form of weak D is the Del phenotype, in which the D antigen is so weakly expressed that it may be demonstrated only by adsorption and elution of anti-D.
Weak D individuals do NOT produce anti-D and can be considered to be Rh positive for transfusion and RhIg purposes.
2. Partial D
Partial D variants have altered Rh(D) proteins that differ sufficiently from normal D antigens (qualitative difference) to allow anti-D production. Partial D red cells may react with some but not all anti-D typing reagents. There are many categories of partial D antigens (e.g., DIIIa, DVI, DAR), each with a unique genetic basis.
Some persons with partial D have weakly expressed D epitopes and are designated "partial weak D."
In practice, partial D and weak partial D can be considered similarly, i.e., ideally they should be transfused with Rh negative RBC and are candidates to receive perinatal Rh immune globulin depending on the policy in their location.