RHD Genotyping from Maternal Plasma: Guidelines and Technical Challenges

Rhesus D (RhD) blood group incompatibility between mother and fetus can occasionally result in maternal alloimmunization where the resultant anti-D can cross the placenta and attack the fetal red cells, which in worse case scenarios can cause fetal anemia and ultimately death. Fetal RHD genotyping was introduced in the mid-1990s after the molecular characterization of the RH genes as an aid to the clinical management of these cases. Initially, these tests used fetal DNA extracted invasively from chorionic villus and amniocyte samples. RHD genotyping of fetuses carried by RhD-negative women has become the first large-scale application of noninvasive prenatal diagnosis (NIPD). Initially the real-time polymerase chain reaction (PCR)-based tests were devised to characterize free fetal DNA in maternal plasma and serum, and RHD genotyping was a convenient assay to develop this exciting new technology, because the accuracy of tests could easily be confirmed after the simple RhD phenotyping of fetal cord blood cells after birth. "First generation" RHD genotyping tests were based on the incorrect concept that all D-negative phenotypes were caused by a complete RHD gene deletion. Thus, it was a relatively simple task to develop diagnostic PCR strategies based on the detection of RHD where D-negative genomes will completely lack RHD. Subsequent research into the molecular basis of D-negative phenotypes revealed that a significant number of D-negative genomes possess fragments of, or mutated RHD genes, the most notable of which is the RHD pseudogene found in Africans. Thus, more comprehensive RHD genotyping tests have evolved to differentiate these alleles, and are more appropriate in the diagnosis of multi-ethnic population groups such as those found in Europe and North America. Many European Union countries have suggested the mass application of RHD NIPD for all fetuses carried by D-negative women. This is of clear benefit, because most RhD prophylaxis programs have switched to antenatal administration. This will help conserve anti-D stocks and it will prevent unnecessary administration of a human-derived blood product to a vulnerable patient group. Although anti-D stocks are inherently safe, there is a moral obligation to eliminate unnecessary administration of it because there have been instances of hepatitis C infection due to contamination. Furthermore, as yet undescribed viruses may be contaminants of blood products. Mass-scale RHD NIPD will shortly be implemented in several countries in the European Community as a consequence.