Once a significant cause of perinatal loss, alloimmunization to red cell antigens is infrequently encountered in obstetrical practice today. Although maternal alloimmunization to the rhesus blood group D (RhD) antigen remains the leading cause of fetal anemia, more than 50 different red cell antigens have been associated with hemolytic disease of the fetus and newborn (HDFN). Sensitization to any one of these antigens occurs in approximately 1% of pregnancies.¹ Most of these anti-red cell antibodies result in the need for only phototherapy or simple transfusions after birth. However, after anti-RhD, only a few red cell antibodies are associated with profound HDFN necessitating fetal therapy. A review of one large series of intrauterine transfusions (IUTs) from a national referral center indicated that 10% of IUTs were secondary to Kell disease, and 3.5% of IUTs were secondary to anti-c alloimmunization.² In another large series of IUTs, antiE, anti-e, and anti-Fya were reported to require IUT in single cases.³ Significant advances in diagnostic tools for the management of red cell alloimmunization have occurred with the addition of genetic testing of the parents and fetus and the use of Doppler ultrasonography for the detection of fetal anemia.

Pregnant women routinely undergo red cell typing and an antibody screen at their first prenatal visit. Identification of an antibody associated with HDFN should result in a reflex determination of the maternal titer. Until recently, titers were performed manually by laboratory technicians using an indirect Coombs test on serial dilutions of the patient’s serum combined with indicator red cells. Gel technology now allows for automation of the process of performing antibody screens and determining antibody titers. Gel titers typically provide results 2 to 3 dilutions higher than conventional tube titers (eg, 1:8 is found to be 1:64 with gel), making the threshold for a critical titer more difficult to determine.⁴ Obstetricians and maternalfetal medicine specialists should discuss the results of a titer with their local blood bank to better understand the methodology and to decide if further fetal surveillance is warranted.

Paternal antigen and subsequent zygosity testing is an important next step in the evaluation process. An RhD-negative paternal blood type indicates the fetus is not at risk for HDFN as long as paternity is assured. DNA technology is now the preferred approach to determine if 1 or 2 copies of the paternal RhD gene are present.⁵ A heterozygous result (only one RhD gene present) indicates a 50% chance of an affected fetus. For other cases of red cell alloimmunization, paternal testing through serologic methods can still be used to determine zygosity. For example, a Kell-positive partner can have his red cells tested with anti-K and anti-k reagents. A reaction to anti-K indicates the individual is Kell positive. If the anti-k serology test is also positive, the individual is heterozygous for Kell, which is true for the majority of individuals. If the anti-k reaction is negative, the individual is homozygous for Kell, which is a rare phenotype. Direct DNA testing can also determine paternal zygosity in these cases.

Another advance in the field is the ability to determine the fetal antigen status through free fetal DNA. Often employed in clinical practice for determining the risk for chromosomal abnormalities, free fetal DNA after approximately 12 weeks’ gestation can be used to determine the fetal RhD status when the patient’s partner is heterozygous or paternity is unknown.⁶ European reference labs have expanded this technology to determine the fetal antigen status for other important red cell antigens associated with severe HDFN, namely Kell, RhC, and RhE⁷; however, these tests are currently unavailable in the United States.