A factor IX inhibitor may be encountered:
(1) in a patient with hereditary Factor IX deficiency (hemophilia B)
(2) as an acquired disorder in a person with normal production of Factor IX
A patient with hemophilia B can develop antibodies to Factor IX:
(1) following repeated treatment with Factor IX concentrates
(2) if the patient has a large gene deletion, resulting in little antigenic Factor IX
Findings suggesting an inhibitor in a patient with hemophilia B:
(1) increasing doses of Factor IX replacement required to achieve a given effect
(2) shorter duration of effect following replacement
Conditions associated with acquired Factor IX inhibitors in a nonhemophilic patient:
(1) autoimmune disease, especially SLE
(2) female post-partum
Laboratory features of the acquired inhibitor:
(1) normal PT with a prolonged aPTT (new onset if not hemophilia B)
(2) failure of normal plasma to correct the aPTT
(3) markedly decreased Factor IX antigen and activity
(4) demonstration of an antibody targeting Factor IX
