Serologic evidence that factor IX inhibitor in the plasma of hemophilia B patients detects factor IX on normal red cells

Background: Patients with hemophilia B lack factor IX (F IX). These patients may become alloimmunized after the transfusion of F IX concentrates and may develop F IX inhibitors, which have been characterized as polyclonal IgG4 alloantibodies. Two cases in which F IX inhibitors caused difficulty in compatibility testing and antibody identification were encountered. It was hypothesized that, because F IX is present in normal plasma, it might be adsorbed by red cells in vivo and then be detected during antibody screening tests with serum containing F IX inhibitors. Case Report: Sera from two African American half-brothers with hemophilia B were incompatible with all common and rare red cell phenotypes tested in the anti-human globulin test, but did not react with each other's red cells. The brothers' red cell antibodies were neutralized with both normal plasma and a commercially available F IX concentrate, which indicated that the red cell incompatibility was most probably caused by their F IX inhibitors. Red cells from an unrelated patient with hemophilia B and a very low titer of F IX inhibitor were tested against the half-brothers' sere and did not react. The compatible red cells from one of the half-brothers and the unrelated patient with hemophilia B adsorbed F IX from normal plasma or F IX concentrate after 37°C incubation; this rendered them incompatible with the plasma containing F IX inhibitor from the other half-brother. Conclusion: F IX appears to be present on normal red cells and may be detected during compatibility and antibody identification procedures when serum or plasma containing F IX inhibitors is tested.