Incidence of factor IX inhibitor development in severe haemophilia B patients treated with only one brand of high purity plasma derived factor IX concentrate

Fifteen previously untreated patients (Pups) with severe haemophilia B (fac tor IX activity less than or equal to 2 U/dl) only treated with one brand o f plasma-derived high purity factor IX concentrate (FIX LFB) were studied. Age at first injection varied from 1 to 137 months and follow-up since this first injection from 21 to 86 months (median: 35). Cumulative exposure day s (CED) were from 4 to over 100 (median: 26). Among these 15 Pups only one developed an inhibitor. Mutation analysis performed in all patients showed total gene deletion in the patient with inhibitor, partial gene deletion in another one, and missense mutations in 9 families. Mutation was not found in one patient. Actually, according to the data already published, only two patients were at high risk for inhibitor development in our population. Ou r study, although rather small, confirms the previously reported low incide nce of inhibitors in haemophilia B. Large studies on incidence of FIX inhib itors are indeed difficult tio perform, due to both the overall small numbe r of severe haemophilia B patients and the low incidence of FIX inhibitors. Consequently, the impact of bias, such as prevalence of different types of gene defects in a given population, is major. Therefore, any study, dealin g with incidence of FIX inhibitors in severe haemophilia B should report, f or each patient, the type of gene defect.