RHD genotyping

The D (RH1) antigen is the most immunogenic of all the erythrocyte antigens. It is the cause of the majority of severe cases of maternal foetal alloimmunity.

Anti-D alloimmunization mainly affects patients who are rhesus negative (RhD negative), but patients with partial D phenotype, characterized by the absence of one or several epitopes of antigen D, can also develop immunity against the missing epitopes following a transfusion or pregnancy.

It is not possible with currently available phenotyping techniques to distinguish weak D variants without the risk of immunization of the partial D variants, which are susceptible to developing immunity to antigen D.

Molecular biological studies of the RHD gene are indicated in cases of weak expression of antigen D as evidenced by RH-KEL1 phenotyping in any woman of childbearing age.

The value of RHD genotyping lies in the possibility of determining whether this weak antigen D expression is also of partial nature, as carriers of partial D antigen are at risk of anti-D immunization. In the event of a partial D variant, anti-D immunization can be prevented by injection of anti-D gamma globulins (Rhophylac®) during pregnancy and by the administration of rhesus negative red blood cells by transfusion.

If the polymorphism detected is the cause of weak antigen D expression without partial character, these preventative measures are superfluous, as there is no risk of anti-D immunization.

The technology used allows the qualitative detection of 35 polymorphisms of the RHD gene in less than 5 hours, employing chip-based analysis of genomic DNA. These 35 polymorphisms correspond to 68 phenotype variants of the D antigen, including the partial D variants.

Please note: this test must not be confused with foetal RHD genotyping in maternal blood; the latter test is indicated for the determination of the RHD phenotype of the foetus in a rhesus negative pregnant woman whose partner is rhesus positive.