The pivotal clinical data support prolonged dosing intervals of up to 14 days for routine prophylaxis in hemophilia B patients. The majority of adult and pediatric patients using Idelvion for routine prophylaxis had a median annualized spontaneous bleeding rate (AsBR) of 0.00. The majority (99%) of bleeding episodes were managed successfully with one or two infusions, and 94% of bleeds were controlled with only one infusion regardless of the cause or location.

The rationale for this study is to collect the effectiveness and safety data of severe hemophilia B patients treated with Idelvion following institutional standard of care. It is the aim of this study to extend the results of the clinical trial program to a broader hemophilia B population, and to compare with current alternative factor IX treatments in Taiwan.

Inclusion Criteria:

• Male patients with age ≥ 20 years old
• Endogenous FIX activity ≤1 IU/dL,
• At least 50 exposure days (EDs) with FIX products,
• No detectable inhibitor to FIX or inhibitor history,
• Had a minimum of 2 nontrauma-induced bleeding episodes (any type or location) treated in the 6 months preceding study entry,
• The patient or patient's parents or legal authorized representative, as applicable, are capable to understand the study objectives and procedure, and sign the written consent,
• Accept that the supply of Idelvion might be stopped once the study is completed,
• Able to complete a diary during 12 months or 50 EDs, whichever comes first.

Exclusion Criteria:

• Currently participating in an interventional clinical trial,
• Known hypersensitivity to any FIX product or hamster protein,
• Known inhibitor to FIX or inhibitor history,
• With other comorbidities which are not suitable for this study, at investigator's discretion,
• Not able to compliant with the prophylactic treatment,
• Lacking previous treatment and bleeding records.