The weak D (formerly Du) phenotype describes an individual with a variant RHD allele, leading to low expression of complete D antigen on the surface of their red blood cells. The partial D phenotype describes a variant RHD allele that results in modification of the surface D antigen and can result in the loss of D epitope. The prevalence of these phenotypes is thought to occur in 0.2 to 1% of Caucasians. Typically, alloimmunization with anti-D is more likely to occur in partial D individuals who are exposed to the Rh(D) antigen than in weak D individuals. However, there are instances when weak D individuals may develop an anti-D alloantibody after exposure to Rh(D) positive blood. Since the most common methods of immunohematology testing in the Blood Bank cannot reliably discern between weak D and partial D expression, a standard practice is treat both weak D and partial D individuals as Rh(D) negative when they are recipients of blood products.
The AABB Standards for Blood Banks and Transfusion Services does not require that weak D testing be performed on Rh(D)-negative recipients of blood products. In line with this, our transfusion service does not routinely perform serologic weak D testing on transfusion recipients and our testing algorithm is designed to consider weak D and partial D individuals as Rh(D) negative for transfusion purposes. We believe our testing strategy helps prevent anti-D alloimmunization in a vulnerable population, especially women of childbearing potential, and helps streamline test utilization in the Blood Bank. That said, we continue to perform weak D testing on potential red blood cell donors (i.e. fetus and newborn of Rh(D) negative mother, stem cell and solid organ donors).
Rh typing discrepancies may occur in the following situations:
- Obstetric patients: A patient typed as weak Rh(D) positive during her prior pregnancy and did not receive Rh immune globulin prophylaxis (RhIg). However, during the current pregnancy, this patient is now typed as Rh(D) negative due to our updated procedure. If the newborn is Rh(D) positive, a fetal screen and Kleihauer-Betke test will be performed as needed, and an appropriate RhIg dose is recommended.
- Previous blood donors, organ donors, and cord blood from neonates: It is important to identify the weak D phenotype in blood donors (including cord blood from neonates) since very low levels of D antigen are sufficient to elicit the formation of an anti-D alloantibody in Rh(D) negative transfusion recipients. A patient who was previously a blood donor would be typed by the blood collection center as Rh(D) positive due to the presence of weak D, but as a transfusion recipient would be typed as Rh(D) negative since weak D testing is not performed on transfusion recipients.
-Thomas S. Rogers, DO is a second-year resident at the University of Vermont Medical Center, a clinical instructor at the University of Vermont College of Medicine, and the assistant medical director of the Blood Bank and Transfusion Medicine service.