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RHD genotyping is a relatively new practice that has become an increasingly valuable tool in obstetrics. Not only does it improve the accuracy of RhD test results, but it also reduces inappropriate use of Rh immune globulin (RhIG) prophylaxis.

Over time, there has been inconsistency in both the determination of RhD status of pregnant women, and administration of RhIG. Let’s look at this evolving history and discover how RHD genotyping can ensure more accurate and consistent management of obstetric patients.

The Discovery of the Rh Factor

In 1901, Austrian scientist and physician Karl Landsteiner first described the ABO blood group system. He suggested that transfusions between individuals with the same blood type do not result in the destruction of red blood cells (RBCs), and that the previously reported serious reactions to transfusion could be avoided by transfusing blood of the same blood type.

Together, Landsteiner and his American colleague Alexander Wiener described the Rhesus (Rh) factor in 1940, which we now identify as the D antigen (RhD). The expression of RhD on RBCs is extremely important in obstetrics, as it is associated with the development of hemolytic disease of the fetus/newborn (HDFN).

The Testing of D Variant Antigens

In 1946, F Stratton reported the first D variant antigen, eventually referred to as “weak D.” The serologic weak D phenotype is characterized by RBCs that initially are nonreactive or weakly reactive (<=2+) with anti-D sera, but agglutinate moderately or strongly following the addition of antihuman globulin. Since its discovery, there has been considerable confusion and lack of standardization regarding testing for the serologic weak D phenotype.

The first edition of AABB Standards, published in 1958, required weak D testing of donor RBCs that initially typed as RhD negative; this was meant to ensure that donor RBCs with the weak D phenotype wouldn’t inadvertently be transfused as RhD negative.  For patients, weak D testing was not required. Generally, this practice continues until today, with the current edition of Standards (31st) requiring weak D testing for blood donors and stating that the weak D test is optional for transfusion recipients, including pregnant women. As such, donors with a serologic weak D phenotypes are considered RhD positive, whereas patients with a serologic weak D phenotype are likely to be treated as RhD negative.

The introduction of monoclonal anti-D reagents in the 1980s further changed RhD typing practice in the United States. Because of their increased sensitivity, monoclonal antibodies may detect RBCs with weak D expression as RhD positive on initial testing.

Recommendations for RhIG Administration Over Time

Because pregnant women who are RhD negative may develop harmful antibodies when carrying an RhD positive fetus, RhIG prophylaxis is administered to RhD negative mothers to prevent alloimunization. Thus, determining the RhD status of pregnant women is necessary to ensure proper treatment.

The 10th edition of Standards in 1981 first addressed RhD typing for the administration of RhIG, recommending the same method of RhD typing of blood donors for determining a pregnant patient’s candidacy for RhIG treatment. According to this recommendation, a woman with a serologic weak D phenotype would be interpreted as RhD-positive and would not be a candidate for RhIG.

In contrast, the American College of Obstetricians and Gynecologists (ACOG) addressed the issue of administration of RhIG in women with a serologic weak D phenotype that same year, recommending that patients with a serologic weak D phenotype receive RhIG prophylaxis; however, they reversed this recommendation within a few months, stating that administration of RhIG for patients with a serologic weak D phenotype is unnecessary.

Over time, confusion regarding the treatment of pregnant women with a serologic weak D phenotype continued. Depending on laboratory policy, testing methodologies and reagents used, the RhD status of patients with a serologic weak D phenotype could be interpreted as either RhD positive or RhD negative, resulting in inconsistent administration of RhIG for these patients.  This inconsistency could lead to both unnecessary injections of RhIG for some women and failure to prophylactically treat other women with RhIG appropriately.

The Role of RHD Genotyping

Following a 2014 survey conducted by the College of American Pathologists (CAP) Transfusion Medicine Resource Committee (TMRC) confirming inconsistencies in testing policies for serologic weak D phenotypes and administration of RhIG, AABB and CAP convened a Work Group on RHD Genotyping to develop recommendations regarding patients with serologic weak D phenotypes. The Work Group recommended RHD genotyping for patients, including pregnant women, with discrepant or weak RhD typing results in order to accurately define the specific RhD variant expressed on the patient’s cells.

The Work Group estimated that RHD genotyping would determine that a majority of these patients would be weak D types 1, 2 or 3, and that these RhD variants can be managed as RhD positive. Therefore, pregnant patients with weak D types 1, 2 or 3 would not be candidates for RhIG prophylaxis. However, if RHD genotyping determined that a pregnant patient’s RBCs expressed an RhD variant other than weak D types 1, 2, or 3, she should be treated as RhD negative, and would be a candidate for RhIG treatment.

The implementation of RHD genotyping for patients with serologic weak D phenotypes or discrepant RhD typings ensures accuracy in the determination of RhD status. Furthermore, the evidence-based recommendations of managing patients with weak D types 1, 2 or 3 as RhD positive prevents unnecessary injections of RhIG, while identifying true candidates for RhIG.

For more information on RHD genotyping as well as an in-depth analysis of its potential benefits, download It’s time to phase in RHD genotyping for patients with a serologic weak D phenotype by Sandler et al., 2015 here.