If an Rh-negative mother has a baby with an Rh-positive father, the result for future pregnancies could be hemolytic disease of the fetus and newborn (HDFN). Therefore, identifying an obstetrics patient’s RhD-status is a critical care factor. 

Pregnant women with a serologic weak D phenotype or partial D may have discrepant serologic RhD test results, which can lead to inconsistent patient management. RHD genotyping standardizes the process, while also reducing the unnecessary and excessive use of RhIG.

But, what is RHD genotyping? And how often should OB-GYNs run this essential test on their patients? We explore the answers to these and other related questions below. 

What Is Hemolytic Disease of the Fetus & Newborn?

Expectant mothers with the RhD-negative serologic phenotype may carry a fetus with the RhD-positive serologic phenotype of the father. The Rh-positive blood cells may cross to the mother when the placenta detaches at delivery, during miscarriage or abortion resulting in Rh sensitization. In response, the mother’s immune system creates anti-D antibodies (alloimmunization).

With hemolytic disease, the anti-D antibodies may cross the placenta in future pregnancies and harm the developing fetus. The immune system attempts to destroy what it perceives as a foreign invader, specifically the baby’s red blood cells.

The destruction of the red blood cells makes the baby anemic. As a result, its body produces more red blood cells, critical organs enlarge and excess bilirubin accumulates causing jaundice with yellowing of the skin. Determining the mother’s Rh-status during their first prenatal visit provides a chance for therapeutic interventions that prevent the immune system response.

The Problem

Traditional Rh typing can result in indeterminate or weak reactivity. RHD genotyping is needed to accurately identify Rh-status.  This will prevent excess use of Rh-negative blood and inconsistent recommendations for and unnecessary administration of RhIG. It can also reduce confusion for both OB-GYNs and their patients.   

What Treatments Are Available for Rh-Negative Mothers?

The American College of Obstetricians and Gynecologists recommends Rh-negative women be given Rh immunoprophylaxis—typically Rh immune globulin (RhIG)—at 28 weeks’ gestation to stop antepartum Rh immunization or a transfusion with Rh-positive RBCs. RHD genotyping could result in significantly fewer injections of RhIG annually.

With RHD genotyping, mothers who appear to be RhD-negative may actually be RhD-positive.

How Often Should an OB-GYN Test for Serologic Weak D Phenotype?

The College of American Pathologists (CAP) and the American Association of Blood Banks (AABB) convened in a Work Groups to review the current state of RHD publications and standardized testing approaches. They proposed that RHD genotyping be performed on pregnant women and females of child bearing potential who need transfusions, who present with a weaker than expected RhD typing test or who have variable or discordant RhD typing results or whenever the RhD type is in question. Those with results indicating Weak D Types 1, 2 or 3 can be managed as RhD-positive. Those with RHD variants called “partial D,” on the other hand, should be treated as RhD-negative, meaning they are candidates for RhIG prophylaxis and should receive RhD-negative blood if transfused.

Creating a standard that involves testing at-risk OB patients will lead to consistency within the industry. As is, patients with serologic weak D phenotype may be seen as Rh-positive in one facility and Rh-negative in another.

Standardized testing for serologic weak D phenotypes and other RHD variants takes the guesswork out of obstetrics care. For more on RHD genotyping and recommendations for patient management, download a free copy of “How do I manage Rh typing in obstetric patients?” by myself and Richard Haspel here