The Weak D (Du) Phenotype

Definition: The weak D phenotype (Du) is a weakened form of the D antigen that in routine D typing will react with some anti-D but not with others (when an immediate spin or 37° incubation is done). Weak D red cells have the D antigen, but have fewer D antigens per cell than normal Rh positive cells. The current preferred term for Du is "weak D." The frequency of the weak D phenotype in Caucasians is approximately 0.3 percent (3 in 1000). However, the frequency varies with the method used, the reagent used, and the racial mix tested, since the frequency of weak D among Blacks is higher than in Whites.


Historically, the weak D phenotype was believed to arise from three mechanisms, reviewed here as they help to explain how blood bankers came to think of weak D and transfusion:

  1. Gene Interaction (Position Type)
  2. Mosaic Type (partial D)
  3. Hereditary

Today the molecular basis of weak D has largely been resolved. See these excellent resources:

Significance in Blood Banking:

Method of Weak D Phenotyping (Du Testing)

As defined earlier, weak D red cells react with some anti-D but not with others in routine typing tests. The way to detect weak D cells reliably is to do a test for weak D (usually called a Du test). The Du test is an indirect antiglobulin test using the patient's red cells and an IgG anti-D. An IgG anti-D must be used because antiglobulin serum contains anti-IgG. The Du test, like other antigen typing done using an antiglobulin test, is controlled by doing a DAT on the test cells. The DAT serves as an autocontrol which reveals whether the test cells are sensitized with antibody in vivo . If the DAT is positive, the Du test is invalid, as it will be positive whether or not the patient is a weak D. Any IgG anti-D can be used for Du typing; usually this means that all typing sera except saline anti-D are suitable.

Weak D is discussed further in the articles associated with Assignment 2.

The Weak D (Du) Phenotype