Decreased fetal erythropoiesis and hemolysis in Kell hemolytic anemia.

Objective: Lower changes in optical density (450 nm) measurements have been reported in fetuses with anti-Kell anemia compared with those anti-D anemia. The purpose of this investigation was to determine if hemolysis and erythropoiesis differ between anti-Kell and anti-D hemolytic disease.
Study Design: Ninety-three pregnancies complicated by either anti-D or anti-Kell alloimmunization were evaluated. Fetal blood samples obtained at the first cordocentesis were tested for the red blood cell antigen type, hemoglobin, hematocrit, reticulocyte count, nucleated red blood cells, total serum bilirubin concentration, umbilical venous respiratory blood gases, serum erythropoietin level, and strength of the direct Coombs test. To determine the evolution of hemolytic anemia in the two antigen groups, these laboratory parameters were repeated on the fetal blood samples triggering the decision to perform a fetal intravascular transfusion (hematocrit <30%).
Results: A total of 65 of 93 fetuses were antigen positive (11 for Kell and 54 for RhD). The mean gestational age and laboratory measurements of antigen- positive, nonanemic fetuses at first blood sampling did not differ significantly between groups. There was a strong inverse relationship observed between the hemoglobin concentration and reticulocyte count independent of gestational age in the anti-D group but not in the anti-Kell group. Eight (73%) fetuses with anti-Kell antibodies and 37 (69%) with anti-D antibodies underwent intravascular transfusion. At the cordocentesis when the decision for transfusion was made, anti-Kell anemic fetuses had lower reticulocyte counts and total bilirubin concentrations. The strong inverse relationship between the hemoglobin and reticulocyte count was again seen only in the anti-D group. In both groups, fetal erythropoietin increased significantly between the first and last blood samplings and in each group were negatively correlated with hemoglobin independent of gestational age.
Conclusion: Anti-Kell anemic fetuses have lower reticulocyte counts and total serum bilirubin levels than do comparable anti-D anemic fetuses. This finding argues in favor of fetal blood sampling rather than amniotic fluid analyses for the management of fetal hemolytic disease resulting from Kell antibodies. Unlike RhD alloimmunized fetuses, these fetuses do not manifest an inverse relationship between hemoglobin concentration and reticulocyte count. We speculate that compared to anti-D fetal anemia, anti-Kell anemia is associated with increased hemolysis of nonhemoglobinized or incompletely hemoglobinized erythroid precursors.