Should Transfusion Services Provide KEL1 Negative Blood to Obstetric Patients?
The Kell blood group is the third most polymorphic blood group system and its antigens are considered to be highly immunogenic. Antibodies to the KEL1 antigen (anti-K) are the most common red cell antibodies after ant-D. Anti-K is clinically significant because it can cause severe transfusion reactions and hemolytic disease of the fetus and newborn (HDFN).
Only 9% of the Caucasian population is positive for KEL1 antigen. The rate of KEL1 alloimmunization for KEL1 negative individuals after exposure to 1 unit of KEL1 positive RBCs is 5%. The frequency of anti-Kell antibody is one per 1000 pregnant females and the incidence of KEL1-related HDFN is 1 in 40,000 births. Unlike anti-D, Kell antibody titers may not correlate with the extent of fetal disease. Severe disease may occur in patients with low titers, most likely because anti-Kell antibodies cause fetal anemia by suppression of erythropoiesis.
Most anti-Kell antibodies identified during pregnancy are the result of prior transfusion and not transplacental hemorrhage from a fetus. Some experts have recommended providing KEL1 negative units of RBCs for women of childbearing age to avoid alloimmunization and HDFN. In Europe, many countries provide KEL1 negative RBC to women not more than 45 to 50 years of age (Solheim BG, Transfusion 2015;55:468-469). This policy is based on the following considerations: more than 80% of anti-K in pregnant women has been caused by prior blood transfusion; that ant-K is the second most important cause of HDFN after anti-D; and that treatment of severe HDFN due to anti-K can be challenging and expensive.
Recently, O’Brien etal published the results of a 3-year trial of providing KEL1 negative blood to obstetric patients at Beth Israel Deaconess Hospital in Boston, MA (O'Brien KL etal. Transfusion 2015;55:599-604). They calculated that the number needed to treat (NNT) is 500, meaning that 500 women have to be transfused with 1 unit of KEL1 antigen negative RBC to prevent one case of anti-K alloimmunization. This is largely because of the low prevalence of KEL1 positive RBC donors in the United States. Another reason is that the chance of a woman becoming alloimmunized to KEL1 positive RBCs during a transfusion and then becoming pregnant by a male who is at least heterozygous for KEL1 antigen is extremely low. The authors concluded that provision of KEL1 negative units to women of childbearing age yielded no clinical benefit and recommended against adoption of this transfusion practice in the United States.
