Blood group molecular genotyping

DNA-based typing methods of red cell antigens are applied in several fields. Genotyping is used to clarify problems in patient serology. It is also increasingly applied for routine mass-scale typing of blood donors for minor red cell antigens and screening for donors with rare blood types. An advance in this field is the non-invasive fetal RHD diagnostics in pregnancies of D-negative women to determine the fetal RHD. Reference centres for immunohaematology commonly use molecular methods to clarify problems, discrepancies or unusual results in patient serology. It is often applied in individuals with variant RhD expression to detect the contributing weak D or partial D types. Genotyping is also used to identify the individual ABO and RhD blood group in patients after mismatched transfusion. In some patients with positive direct antiglobulin test (DAT) it is necessary to type for antigens by DNA techniques. Genotyping is also helpful when reagents are not available or only weakly reactive, or to confirm weakly expressed antigens. Blood establishments are constantly challenged with the blood supply for patients carrying irregular red cell antibodies. A high number of donors have to be typed to find compatible blood units. Increasingly, DNA-based methods are used alongside standard serological typing. The advantages of molecular methods are the wider range of different antigens available for typing and that some DNA methods are less expensive than phenotyping. Extensive donor antigen typing leads to short response times, from request to issuing blood units. Genotyping is also used for identifying donors with extremely weak RhD expression (DEL) to prevent mistyping as RhD-negative. Non-invasive fetal typing of RHD from maternal blood in pregnancies of D-negative women is implemented in many Caucasian blood establishments. It allows that D-negative fetuses can be identified at early stage. In these cases, no anti-D immunoglobulin is required, which provides a more efficient use of this human blood derivate. Furthermore, in anti-D alloimmunized women, the assay allows to accurately identify fetuses at risk for haemolytic disease.