What's hemophilia B?
Hemophilia B is an inherited bleeding disorder caused by deficiency of coagulation factor IX, the result of variants in the F9 gene. It is X-linked recessive, so it mainly affects males, while females are usually carriers. Disease severity tracks the residual factor IX level (mild, moderate, severe). It is distinct from hemophilia A, which is factor VIII deficiency from the F8 gene; treatments specific to hemophilia A, such as factor VIII concentrate and emicizumab, do not apply to hemophilia B.
| Also indexed as | ORPHA:98879, MONDO:0010604 |
|---|---|
| Features mapped | 10 |
| Treatments mapped | 4 |
| Published sources | 17 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
X-linked recessive inheritance
Hemophilia B is an X-linked recessive bleeding disorder, caused by a deficiency of clotting factor IX. Because the F9 gene sits on the X chromosome, it most often affects boys and men.
Reduced factor IX activity
Hemophilia B is caused by a deficiency of clotting factor IX, a protein the blood needs to form clots. With too little working factor IX, the blood cannot clot normally.
Bruising susceptibility
Easy bruising and bleeding into the skin are typical of hemophilia B because clot formation is impaired.
Persistent bleeding after trauma
Bleeding that continues longer than expected after an injury is a common early presentation of hemophilia B, reflecting the missing clotting factor.
Epistaxis
Nosebleeds (epistaxis) are among the bleeding symptoms of hemophilia B and can be a presenting sign in childhood.
Joint hemorrhage
Bleeding into the joints is a hallmark of severe hemophilia, and repeated joint bleeds can lead to progressive joint damage over time.
Intracranial hemorrhage
Bleeding inside the skull (intracranial hemorrhage) is one of the life-threatening bleeds that hemophilia care aims to prevent.
Hemarthrosis
Bleeding into the joints (hemarthrosis) is the hallmark bleeding problem of hemophilia B and, if it recurs, is the main cause of long-term joint damage, pain, and reduced mobility.
Hemophilic arthropathy
In severe hemophilia, repeated bleeding into the joints leads to hemophilic arthropathy: long-term joint damage with ongoing inflammation of the joint lining and destruction of cartilage.
Factor IX inhibitor
In hemophilia B, a small number of people (about 1 to 3% of those severely affected) develop antibodies, called inhibitors, against the factor IX used to treat them. In roughly half of those who do, an infusion of factor IX can trigger anaphylaxis or a severe allergic reaction.
How it is diagnosed
Hemophilia B
Diagnosed using: coagulation factor IX activity assay.
“Isolated elevated PT or APTT were followed by mixing studies to confirm factor deficiency and inhibitor screen testing to rule out inhibitors.”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
factor IX replacement
Replacing the missing clotting factor remains the cornerstone of hemophilia treatment. The target factor level and how long it is given depend on where the bleeding is and the type of procedure.
Used to help with: Hemophilia B.
“…received factor (F)IX…”
etranacogene dezaparvovec
Etranacogene dezaparvovec is an approved AAV-based gene therapy for hemophilia B given as a single infusion. It delivers a working copy of the factor IX gene so the body makes its own factor IX, markedly reducing bleeding and the need for factor infusions in suitable patients; long-term durability and safety continue to be studied.
Used to help with: Hemophilia B.
“Etranacogene dezaparvovec proved to be highly effective, superior to FIX prophylaxis for bleeding protection, and safe for 4 years postinfusion in NAb- persons with severe or moderately severe hemophilia B.”
concizumab
Concizumab is a non-factor, subcutaneous rebalancing therapy that works by blocking tissue factor pathway inhibitor to restore clotting balance. It is an option in hemophilia B, including for people who have developed inhibitors, and needs individualized monitoring for thrombotic risk.
Used to help with: Hemophilia B.
“…concizumab prophylaxis was effective, well tolerated, and manageable in routine practice, including during surgical procedures.”
extended half-life factor IX (rFIX-FP)
Extended half-life factor IX products (such as rFIX-FP) can simplify the timing of treatment around surgery, though how much each person needs can vary.
Used to help with: Hemophilia B.
“Extended half-life rFIX-FP simplifies perioperative management but exhibits substantial pharmacokinetic variability.”
What changes how it shows up
Carrying the genetic change is not the whole story. The factors below are described in the research mapped here as changing whether, or how strongly, the condition appears. They modulate how the genotype is expressed; they do not, on their own, cause or cure it.
F9 gene mutation
Hemophilia B is caused by mutations in the gene responsible for producing clotting factor IX (the F9 gene).
Described as modulating: Hemophilia B.
“Hemophilia is an inherited disorder characterized by impaired blood clotting caused by mutations in the genes responsible for producing coagulation factor (F) VIII (hemophilia A, HA) or FIX (hemophilia B, HB).”
baseline factor IX level
Hemophilia B severity is graded by the baseline factor IX level; a factor IX level at or below about 2 percent marks moderately severe to severe disease.
Described as modulating: Hemophilia B.
“…people with moderately severe/severe hemophilia B (factor [F]IX ≤…”
factor IX inhibitor development
Replacement therapy carries the risk of the body developing neutralizing antibodies, called inhibitors, against the infused clotting factor.
Described as modulating: Hemophilia B.
“…these treatments come with the risk of developing neutralizing antibodies (inhibitors) against the infused factor.”
How to read the evidence labels
Where this comes from
This guide is built from 17 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.