Some individuals have a variant RHD gene that encodes a reduced concentration of D antigen (weak D) or a D antigen with missing D epitopes (partial D). Various anti-D reagents react differently with these red cells and interpreting Rh(D) type can vary with the method used (i.e., tubes, solid phase, or gel).
Differentiating between weak D and partial D is important in pregnant patients. Those with partial D, but not usually weak D, may make anti-D and should be considered D negative for transfusion and as RhIg candidates. Currently, serologic reagents cannot distinguish the two D variants, but RHD genotyping can.