Prevalence of factor IX inhibitors among patients with haemophilia B: results of a large-scale North American survey.

This survey provides new information on the severity of factor IX deficiencies among patients being treated for haemophilia B and on the prevalence of factor IX inhibitors in this population. A questionnaire was sent to 150 haemophilia treatment centres in the United States and Canada. 82 centres responded and provided data on 1967 patients with haemophilia B. 37% of these patients had severe haemophilia B (<1% of the normal level of factor IX), 33% had moderate haemophilia B (1-5% of the normal level of factor IX), and 30% had mild haemophilia B (>5% of the normal level of factor IX). Only 29 (1.5%) of the patients had factor IX inhibitors; 28 of these patients (96.6%) had severe haemophilia B, and one of these patients (3.4%) had moderate haemophilia B. Factor IX inhibitor titres were 0.6-1 Bethesda unit (BU) in seven patients, > 1-5 BU in four patients, > 5-10 BU in one patient, and > 10 BU in 17 patients. Factor IX inhibitors are much less common in patients with haemophilia B than in patients with haemophilia A.