Understanding Rh Status and RHD Gene Detection, Fetal® Noninvasive cfDNA screening

RhD is a protein expressed on the red blood cells that can lead to life-threatening complications during pregnancy if the maternal and fetal types are incompatible. RhD-immune globulin medications (e.g. RhoGAM) have drastically reduced these complications and are routinely administered proactively to RhD-negative pregnant patients. Shortages of these medications in recent years, as well as the expansion of cfDNA screening technology, have led clinicians to adapt this workflow.

Testing for fetal Rh-status is clinically indicated when a pregnant person is confirmed to be Rh-negative to determine risk of Rh-incompatibility or alloimmunization. Noninvasive screening can now help assess the fetal RhD status by looking for the absence or presence of the RHD gene in cell-free DNA isolated from maternal plasma.

RHD Gene Detection, Fetal (Test Code: 16592) is a screen and is not intended to be used for diagnostic purposes. Fetal RHD status can be confirmed through amniocentesis or chorionic villus sampling during pregnancy or newborn blood testing after birth.

Early determination of fetal RhD status may provide a more accurate assessment of the risk for hemolytic disease of the fetus and newborn (HDFN) and could reduce the use of prenatal RhIG prophylaxis. Approximately 40% of RhD-negative pregnant patients carry an RhD-negative fetus who is not at risk.¹ The American College of Obstetrics and Gynecology (ACOG) supports the use of cfDNA screening for fetal RhD status, especially in the case of an RhD-immune globulin shortage.²

Genotyping of cell-free DNA (cfDNA) isolated from maternal plasma is a noninvasive methodology and can be performed as early as 10 weeks’ gestation (sensitivity >99%; specificity >98%).^1,2 Detection of the RHD gene is performed by real-time PCR analysis. This method yields 1% false-positive and false-negative rates for RHD determination.²

In rare cases, RHD genotypes associated with absence or partial expression of the RhD antigen may cause discrepancies between cfDNA screening results and serological phenotypes. A fetus with a screen-positive result may have an Rh D–negative phenotype because of variants not detectable by this assay (eg. RHD pseudogene).

1. ACOG Practice Bulletin No. 181: Prevention of Rh D Alloimmunization. Obstetrics & Gynecology. 130(2):p e57-e70, August 2017;130(2):e57-e70. DOI:10.1097/AOG.0000000000002232
2. ACOG Clinical Practice Update: Paternal and Fetal Genotyping in the Management of Alloimmunization in Pregnancy. Obstetrics & Gynecology 144(2):p e47-e49, August 2024. DOI: 10.1097/AOG.0000000000005630