How RHD Genotyping Provides a Solution for Pregnant Women with a Serologic Weak D Phenotype

DOWNLOAD AND READ HASPEL AND WESTHOFF, 2015

Obstetric patients with an RhD-negative serologic phenotype are at risk for developing anti-D antibodies when carrying an RhD-positive fetus.  These antibodies can cross the placenta and cause harm to the fetus during subsequent pregnancies, a condition known as hemolytic disease of the fetus/newborn (HDFN).  Pregnant women who test RhD-negative serologically are routinely treated with Rh immune globulin (RhIG) to prevent alloimmunization from exposure to RhD-positive fetal red blood cells (RBCs).

However, pregnant women with a serologic weak D phenotype may have discrepant serologic RhD test results, which can complicate a physician’s decision to treat with RhIG. Below, we explore serologic weak D phenotypes and how they affect pregnant patients. 

The Basics of RHD Variants

First, it may help to understand what we mean by “serologic weak D phenotype.”

A small percentage of individuals (0.2% to 1.0 % of Caucasians) have a serologic weak D phenotype, characterized by RBCs that are initially nonreactive or weakly reactive (<=2+) with anti-D antibodies, but react moderately or strongly with antihuman globulin. For individuals with serologic weak D phenotypes, RHD genotyping can provide important information regarding RhD status.

Genomic testing can distinguish up to 80 RHD gene variants using PCR and DNA sequence-specific hybridization methodologies. Among Caucasians, the most common variant genotypes are weak D types 1, 2 and 3.  Individuals with weak D types 1, 2 and 3 in the homozygous and hemizygous state are not at risk of forming alloanti-D when exposed to conventional RhD-positive RBCs, whereas individuals with other weak D types may, in some cases, form alloanti-D when exposed to RhD-positive RBCs.

Inconsistencies in Reporting Serologic Results

A recent survey conducted by the College of American Pathologists (CAP) Transfusion Medicine Resource Committee (TMRC) demonstrated that the treatment of patients with serologic weak D phenotypes is not consistent across different institutions. In some laboratories, obstetric patients with serologic weak D phenotypes are considered RhD-negative, out of caution, for the purpose of RhIG prophylaxis and transfusion. Other laboratories have considered these patients RhD-positive. 

Current practice to minimize risk of alloimmunization in RhD-negative obstetric patients with RhIG prophylaxis has led to 98.4% to 99% success in the prevention of RhD alloimmunization and HDFN due to anti-D.  However, inconsistency in the treatment of patients with serologic weak D phenotypes causes many patients to be treated with RhIG or transfused RhD-negative RBCs unnecessarily, leading to higher healthcare costs and inefficient use of a limited resource. 

Inconsistency in the treatment of patients with serologic weak D phenotypes also causes  confusion among clinicians.  A patient with a serologic weak D phenotype may be reported as RhD-positive initially and RhD-negative in subsequent tests.  For obstetric patients, the decision of whether to treat with RhIG in these cases can be difficult.

The Value of RHD Genotyping

In order to address this inconsistency in managing obstetric patients with serologic weak D phenotypes, the AABB and CAP convened a Work Group on RHD Genotyping in 2014 to develop a better way to classify and treat these patients (these findings are summarized in Sandler et al., 2015). The Work Group recommends RHD genotyping for pregnant patients with serologic weak D phenotypes or discordant RhD serologic testing results. 

Genotyping identifies specific gene variants through molecular techniques. It is estimated that a majority of individuals with serologic weak D phenotypes will be weak D types 1, 2 or 3, and can be safely treated as RhD-positive. Thus, the Work Group recommends that patients with weak D types 1, 2 or 3 not receive RhIG and, if necessary, receive RhD-positive RBCs for transfusion. Patients with a gene variant other than weak D types 1, 2 or 3 should be treated as RhD-negative for the purpose of RhIG prophylaxis and for transfusion.

Simply put, RHD genotyping of selected patients minimizes unnecessary RhIG treatment and conserves RhD-negative RBCs, a limited resource, for patients requiring this blood type.  It also ensures proper RhIG treatment for patients at risk of RhD alloimmunization.  Overall, RHD genotyping leads to consistent and appropriate patient management and conserves valuable healthcare resources.

For more on RHD genotyping and standards for patient management, download and read Haspel and Westhoff, 2015 here.