Fetal RHD Positive, Yet RH Blood Type Negative at Birth? YEP.

Dr. Chapa’s Clinical Pearls.

We have made SIGNIFICANT progress in our ability to screen for fetal RH factor using cell free DNA from maternal blood. Cell-free DNA to determine the fetal RHD genotype from the maternal circulation was first described in 1993. We have come so far since them. In March 2024, the ACOG released a Practice Advisory stating, “the use of NIPT to prioritize use of RhIg and conserve RhIg supply is a reasonable consideration”. Two U.S. companies have introduced cell-free DNA assays for RHD as part of their noninvasive prenatal testing assays. These assays use next generation sequencing to determine the presence of fetal RHD DNA. These tests are NEAR perfect in accuracy (we will review the latest data here). So, how can it be possible to detect the RHD gene (when truly present), yet the fetus ultimately be found to have RH negative blood? In other words, how do we explain the occurrence of genotype/phenotype discrepancy? The science is clear. In this episode, we will review this unusual phenomenon and summarize a recent (November 2024) clinical validation study on the use of cell free DNA test testing to look for this “genomic variance”. This article was also on the Green Journal’s “Spotlight on Fetal RHD” on 12/16/2024. Is this common? And which patient population is more likely to have this? Listen in for details.