Haemophilia B is an X-linked recessive bleeding disorder caused by deficiency or dysfunction of clotting factor IX, leading to impaired intrinsic coagulation and prolonged or spontaneous hemorrhage.

Exam domain	One-glance essentials
Definition / diagnosis	X-linked Factor IX deficiency (Christmas disease) causing defective intrinsic-pathway thrombin generation and deep-tissue bleeding ^[1]^[2] Severity follows baseline FIX: severe < 1%, moderate 1–5%, mild > 5–40%
Epidemiology / risks	Usually affects males with a carrier mother; ask about maternal male relatives, but de novo variants mean family history may be absent ^[1]^[2] Large F9 deletions confer higher risk of FIX inhibitors and infusion-related anaphylaxis
Core mechanism	FIXa normally combines with FVIIIa to activate FX; deficiency reduces thrombin burst and formation of a stable fibrin clot ^[1] Primary haemostasis remains intact → delayed haemarthrosis and muscle bleeding, not a petechial pattern
Clinical picture	Haemarthroses, muscle haematomas, prolonged post-traumatic/postoperative bleeding and potentially fatal intracranial bleeding ^[1]^[2] *A painful swollen joint may be haemarthrosis; congenital FVIII and FIX deficiency are key causes* ^[3]
Investigations	*Male + isolated prolonged APTT: repeat the sample, exclude heparin contamination, then consider haemophilia A/B* ^[4] Normal PT and platelets; APTT corrects on mixing unless an inhibitor is present; confirm with low FIX assay ^[1]^[2] If mixing fails or replacement response is poor, test a FIX inhibitor with a Bethesda assay
Management	Acute major bleed: give recombinant FIX promptly—do not await imaging; add local measures and tranexamic acid for mucosal bleeding ^[1]^[2] Severe disease: regular extended-half-life FIX prophylaxis; inhibitors require specialist bypassing therapy DDAVP is ineffective because it releases FVIII/vWF, not FIX ^[2]
Complications	Recurrent haemarthrosis → haemophilic arthropathy, deformity and disability ^[2] FIX inhibitors are uncommon but may cause anaphylaxis and nephrotic syndrome; other threats include compartment syndrome and ICH
Prevention / follow-up	Early FIX prophylaxis, physiotherapy, dental care and a written peri-procedural factor plan prevent joint and surgical bleeding ^[2] Avoid aspirin/NSAIDs, IM injections and unnecessary trauma; vaccinate against hepatitis A/B and offer genetic counselling
Exam traps	Haemophilia A and B are clinically indistinguishable—factor assay, not phenotype, separates them ^[1]^[2] A normal APTT can occur in mild disease; test FIX when suspicion remains Never use DDAVP as haemostatic treatment for haemophilia B; monitor early FIX exposures in severe disease for allergy/inhibitors

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Haemophilia A/B section ^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B ^[3] Lecture slides: GC 075. Pain red joint.pdf, p.60 (haemarthrosis) ^[4] Lecture slides: GC 027. Abnormal bleeding after tooth extraction.pdf (isolated prolonged APTT and haemophilia A/B; factor-concentrate treatment)

Haemophilia B (Christmas Disease)

Haemophilia B, also known as Christmas disease (named after Stephen Christmas, the first patient described), is an X-linked recessive (XLR) inherited bleeding disorder caused by deficiency or dysfunction of coagulation Factor IX (FIX) ^[1]^[2]^[3].

Breaking down the terminology:

"Haemo-" (Greek haima) = blood
"-philia" (Greek philia) = tendency/affinity for
So "haemophilia" literally means "a tendency to bleed" — the blood has an affinity for flowing out, i.e., it cannot clot properly.

Factor IX (also called Christmas factor) is a vitamin K-dependent serine protease that participates in the intrinsic pathway of the coagulation cascade. When Factor IX is deficient or dysfunctional, the intrinsic pathway is impaired, leading to inadequate thrombin generation and therefore poor fibrin clot formation — resulting in a deep-seated, coagulation-type bleeding tendency.

Haemophilia B accounts for approximately 15% of all haemophilia cases (whereas Haemophilia A accounts for ~85%) ^[1]^[2]. Despite this difference in prevalence, Haemophilia A and B are clinically indistinguishable — you cannot tell them apart at the bedside. The only way to differentiate is by specific factor assays.

Key Distinction

Haemophilia A = Factor VIII deficiency. Haemophilia B = Factor IX deficiency. Haemophilia C (Rosenthal syndrome) = Factor XI deficiency (autosomal recessive, especially common in Ashkenazi Jews). "Acquired haemophilia" = acquired autoantibody against a clotting factor (most commonly Factor VIII) — this is NOT the same as inherited haemophilia ^[1]^[2].

2. Epidemiology

Haemophilia B incidence: approximately 1 in 15,000–30,000 live male births ^[2].
Compare with Haemophilia A: 1 in 4,000–5,000 live male births — so Haemophilia B is roughly 4–6 times less common than Haemophilia A.
Prevalence is estimated at ~3.4 per 100,000 males worldwide (WFH Global Survey 2022–2023 data).
There is no racial or ethnic predilection — it occurs across all populations.

Affects almost exclusively males because it is X-linked recessive.
Females are typically carriers (heterozygous, one normal X and one affected X). Carriers may have reduced Factor IX levels (typically 30–70% of normal) and are usually asymptomatic, though some carriers with skewed X-inactivation (lyonization) can have factor levels low enough to cause mild bleeding symptoms — these are called symptomatic carriers.
Rarely, females can be affected if:
- Homozygous (father with haemophilia × carrier mother)
- Turner syndrome (45,X with the affected X)
- Extreme skewed X-inactivation

Approximately 40% of severe haemophilia B cases and ~30% of mild/moderate cases are sporadic (de novo mutations with no prior family history) ^[2].
This is clinically important: absence of family history does NOT exclude haemophilia.

In Hong Kong, all forms of haemophilia are managed through the Hong Kong Haemophilia Centre at Queen Mary Hospital and the Hong Kong Red Cross Blood Transfusion Service.
Local prevalence follows global patterns. Factor IX concentrate (both plasma-derived and recombinant) is available through the Hospital Authority.
Factor XII deficiency is found at a rate of ~20% in the local (Hong Kong Chinese) population and is the most common cause of an isolated prolonged APTT locally — this does NOT cause bleeding and must be distinguished from haemophilia ^[4].

Since Haemophilia B is a genetic disorder, the primary "risk factor" is inheritance:

Risk Factor	Explanation
Family history of haemophilia B	X-linked recessive inheritance; affected father → all daughters are carriers; carrier mother → 50% chance each son is affected, 50% chance each daughter is a carrier
Male sex	Males have only one X chromosome; a single defective F9 gene causes disease
Spontaneous mutation	~30–40% of cases arise de novo — no family history required
Consanguinity	Increases chance of homozygous females (very rare)

There are no modifiable lifestyle risk factors for developing Haemophilia B. However, certain factors increase bleeding risk in an affected individual: trauma, surgery, dental procedures, use of antiplatelet or anticoagulant drugs, and contact sports.

4. Anatomy and Function: Factor IX in the Coagulation Cascade

The F9 gene is located on the long arm of the X chromosome at Xq26 ^[2].
It encodes a 461-amino acid precursor protein that undergoes extensive post-translational modification (gamma-carboxylation of glutamic acid residues — this is vitamin K-dependent).
Over 1,100 unique mutations have been described in F9, including point mutations, deletions, insertions, and splice-site variants.

Factor IX Leiden Variant

A specific F9 promoter mutation causes Haemophilia B Leyden (not to be confused with Factor V Leiden). In this variant, Factor IX levels are very low in childhood (causing bleeding), but rise spontaneously at puberty due to testosterone-responsive elements in the promoter. By adulthood, factor levels may normalise and patients "grow out" of their haemophilia. This is unique to Haemophilia B and does not occur in Haemophilia A.

To understand why Factor IX deficiency causes bleeding, you need to understand the coagulation cascade:

Key Points:

Extrinsic pathway (measured by PT/INR): Tissue Factor → Factor VII → Factor X.
Intrinsic pathway (measured by APTT): Factor XII → XI → IX → VIII → X.
Common pathway: Factor X → II (prothrombin) → I (fibrinogen) → fibrin.

Factor IX, once activated to Factor IXa, forms the intrinsic tenase complex with its cofactor Factor VIIIa on a phospholipid surface (platelet membrane) in the presence of calcium. This complex activates Factor X → Xa, which is the rate-limiting step for large-scale thrombin generation.

Why does Factor IX deficiency cause such significant bleeding?

The initial "spark" of coagulation comes from the extrinsic pathway (Tissue Factor + VIIa → small amount of Xa → small amount of thrombin). But this initial burst is quickly shut down by Tissue Factor Pathway Inhibitor (TFPI). To sustain and amplify thrombin generation, the body relies on the intrinsic pathway amplification loop:

The small amount of thrombin generated activates Factor XI → XIa
Factor XIa activates Factor IX → IXa
Factor IXa + VIIIa (tenase complex) generates large amounts of Factor Xa
This drives the thrombin burst needed for a stable fibrin clot

Without Factor IX, this amplification loop is broken. The patient can form a small initial clot (from the extrinsic pathway), but it is weak and unstable — hence the classic pattern of delayed rebleeding after initial haemostasis. This is why coagulation-type bleeding characteristically shows rebleeding after initial cessation, unlike platelet-type bleeding which is continuous oozing.

In Haemophilia B, the PT is normal (extrinsic pathway intact) and the APTT is prolonged (intrinsic pathway disrupted) ^[1]^[4]^[5].

Factor IX is one of the vitamin K-dependent clotting factors (Factors II, VII, IX, X — mnemonic: "1972" or "2, 7, 9, 10"). Its synthesis in the liver requires vitamin K for gamma-carboxylation of glutamic acid residues, which allows Factor IX to bind calcium and phospholipid surfaces.

This means:

Warfarin (a vitamin K antagonist) reduces Factor IX levels (among others) — relevant for acquired causes of Factor IX reduction.
Liver disease also reduces Factor IX synthesis.
But in inherited Haemophilia B, the problem is a genetic defect in the F9 gene itself, not a deficiency of vitamin K or liver dysfunction.

5. Aetiology (with Focus on Hong Kong)

Aetiology	Details
X-linked recessive inheritance	Defective F9 gene at Xq26; > 1,100 mutations described
Mutation types	Point mutations (missense/nonsense) — most common; large deletions (~5%); insertions; splice-site mutations
De novo mutations	~30–40% of cases; no family history
Haemophilia B Leyden	Specific promoter mutations; Factor IX levels rise at puberty due to androgen-responsive elements

Pathophysiology of each mutation type:

Missense mutations → produce a Factor IX protein with altered structure/function → may have some residual activity → often associated with mild/moderate disease.
Nonsense mutations/large deletions → produce no functional Factor IX (or none at all) → usually cause severe disease.
Large gene deletions carry a higher risk of developing inhibitor antibodies (alloantibodies against infused Factor IX), because the immune system has never "seen" Factor IX and recognizes it as completely foreign.

Cause	Mechanism
Liver disease	Factor IX is synthesised in the liver; hepatocellular failure reduces production of ALL liver-derived factors
Vitamin K deficiency	Factor IX requires vitamin K for gamma-carboxylation; deficiency impairs function (obstructive jaundice, malabsorption, prolonged antibiotics, neonates — haemorrhagic disease of the newborn)
Warfarin	Inhibits vitamin K epoxide reductase → prevents recycling of vitamin K → reduces functional Factor IX (and II, VII, X)
Acquired inhibitors	Autoantibodies against Factor IX (much rarer than acquired anti-Factor VIII inhibitors); associated with autoimmune disease, malignancy, postpartum
DIC	Consumptive coagulopathy → consumption of multiple clotting factors including Factor IX

The Hong Kong Haemophilia Registry tracks all haemophilia patients. Data from Hong Kong shows distribution consistent with global patterns (~85% Haemophilia A, ~15% Haemophilia B).
Factor XII deficiency is extremely common in Hong Kong Chinese (~20% prevalence) and is the most common cause of an isolated prolonged APTT locally ^[4]. It does NOT cause bleeding. This is a crucial differential to consider when you encounter a prolonged APTT in a Hong Kong patient.
Genetic counselling and prenatal diagnosis are offered through the Clinical Genetics Service at Queen Mary Hospital.

6. Classification

This classification is universal across Haemophilia A, B, and C, and directly correlates with clinical phenotype:

Severity	Factor Level	Bleeding Pattern	Frequency
Severe	< 1% (< 0.01 IU/mL)	Spontaneous bleeding into joints (haemarthrosis) and muscles; may have ICH	1–2 episodes per week
Moderate	1–5% (0.01–0.05 IU/mL)	Bleeding with minor trauma; may have joint bleeding	~1 episode per month
Mild	> 5% and < 40% (0.05–0.40 IU/mL)	Bleeding only with major trauma or surgery; rarely has joint bleeding	May never have a bleeding problem

^[1]^[2]

Key points:

Approximately 1/3 to 1/2 of Haemophilia B patients have severe disease ^[2] — this is proportionally less than Haemophilia A, where 1/2 to 2/3 are severe.
Factor level ≥ 50% is considered normal and is not expected to cause clinical bleeding ^[1].
Individuals may also be classified as having mild haemophilia despite having factor level ≥ 40% if they share a genetic variant in the relevant factor with a family member who has haemophilia ^[1].
The severity classification is based on baseline (untreated) factor levels, not levels during treatment.

Non-inhibitor Haemophilia B: Standard replacement therapy is effective.
Inhibitor Haemophilia B: Alloantibodies against Factor IX develop (occurs in ~3–5% of Haemophilia B patients overall, but up to ~20% in those with large gene deletions). This makes standard Factor IX replacement ineffective and requires bypassing agents.
- Note: inhibitor development is much less common in Haemophilia B (~3–5%) compared to Haemophilia A (~25–30%).
- Uniquely in Haemophilia B, inhibitor development can be associated with anaphylaxis to Factor IX concentrates and nephrotic syndrome — this does NOT occur in Haemophilia A inhibitors.

Inhibitor-related Anaphylaxis in Haemophilia B

When Haemophilia B patients with large gene deletions develop inhibitors, they can experience severe anaphylactic reactions to Factor IX infusion. This is because their immune system has NEVER seen Factor IX protein and mounts a vigorous IgE-mediated response. Always be vigilant when starting Factor IX replacement in newly diagnosed severe Haemophilia B patients — initial doses should be given in a monitored setting.

7. Clinical Features

Haemophilia A and B are clinically indistinguishable — they both present as a coagulation-type bleeding disorder with deep-seated bleeding. The pattern of bleeding is determined by severity, not by which factor is deficient.

Key concept: Coagulation-type bleeding vs. Platelet-type bleeding ^[5]^[6]

Feature	Platelet-type (Primary Haemostasis)	Coagulation-type (Secondary Haemostasis)
Site of bleeding	Mucocutaneous: petechiae, purpura, epistaxis, gum bleeding, menorrhagia, GI bleeding	Deep-seated: haemarthrosis, muscle haematoma, retroperitoneal haematoma
Onset after injury	Immediate	Delayed (hours)
Duration	Continuous oozing	Rebleeding after initial cessation
Petechiae	Yes	No
Haemarthrosis	Rare	Characteristic
Response to local pressure	Effective	Often ineffective
Gender predominance	Equal (or female for platelet disorders)	Male (X-linked)

7.2 Symptoms (with Pathophysiological Basis)

Severe haemophilia B typically presents in infancy or early childhood:
- May present at birth with prolonged bleeding from circumcision or cephalhaematoma (subperiosteal bleeding of the skull after birth trauma).
- Often presents when the child starts crawling/walking (~6–12 months) — increased minor trauma triggers bleeding.
- Unusual to present at birth with bleeding from cord stump (this is more suggestive of Factor XIII deficiency).
Moderate haemophilia B typically presents in early childhood with bleeding after minor trauma.
Mild haemophilia B may not present until adulthood, often discovered incidentally during:
- Pre-operative screening (prolonged APTT found)
- Prolonged bleeding after tooth extraction ^[6]^[7]
- Post-surgical bleeding
- Family screening after a relative is diagnosed

Haemarthrosis (Joint Bleeding) — the hallmark of haemophilia
- Occurs in ~70–80% of all bleeding episodes in severe haemophilia.
- Most commonly affects large, weight-bearing, hinge joints: knees > elbows > ankles > hips > shoulders > wrists.
- Why joints? Joints are enclosed spaces with a rich synovial blood supply. The synovial membrane has abundant small blood vessels that are subject to shear stress during movement. In the absence of adequate secondary haemostasis (Factor IX-dependent), even minor mechanical stress causes bleeding into the joint space.
- Symptoms: acute onset of warm, painful, swollen joint with reduced range of motion. The patient often describes an "aura" or tingling sensation before the joint visibly swells.
- Recurrent haemarthroses lead to haemophilic arthropathy (see Complications — to be discussed later).
Muscle Haematoma
- Second most common bleeding site.
- Can occur in any muscle group but is clinically most significant in:
  - Iliopsoas muscle (retroperitoneal) → can mimic appendicitis or hip pathology; may compress the femoral nerve → hip flexion contracture + anterior thigh numbness.
  - Forearm/calf compartments → risk of compartment syndrome.
- Why muscles? Muscles are vascular, well-perfused tissues that undergo constant contraction. Without adequate clot stabilisation, bleeding propagates along fascial planes.
Prolonged Bleeding After Surgery/Dental Procedures
- Prolonged bleeding after wisdom tooth extraction is a classic presentation of mild haemophilia ^[6]^[7].
- Characteristically, there is initial haemostasis (primary platelet plug forms normally) but delayed rebleeding hours later (because the fibrin clot is inadequate without Factor IXa-dependent thrombin amplification).
Easy Bruising / Ecchymoses
- Deep, often palpable ecchymoses (not petechiae — petechiae are a platelet-type feature).
- Old bruise over the shin is a classic exam finding ^[7].
- Bruises may be disproportionately large for the degree of trauma.
Intracranial Haemorrhage (ICH)
- The most feared complication and the leading cause of death from bleeding in haemophilia.
- Can be spontaneous (in severe disease) or post-traumatic.
- Both platelet and coagulation disorders can cause ICH — cannot say one is more deadly than the other ^[5].
- Any haemophilia patient with headache, altered consciousness, or neurological deficit must be treated empirically with factor replacement BEFORE imaging.
Mucosal Bleeding
- Epistaxis, oral mucosal bleeding, and GI bleeding can occur but are less prominent than in platelet disorders.
- Haematuria (renal bleeding) can occur, especially spontaneously in severe haemophilia.
Neonatal Bleeding
- Cephalhaematoma (subperiosteal haemorrhage in the newborn skull)
- Prolonged bleeding from heel prick, venepuncture sites, or circumcision
- Intracranial haemorrhage (rare but life-threatening)

Why No Petechiae in Haemophilia?

Petechiae result from failure of primary haemostasis (platelet plug formation). In haemophilia, platelets are normal in number and function → the primary platelet plug forms normally → so petechiae do NOT occur. Instead, the problem is failure of secondary haemostasis (fibrin clot stabilisation), which manifests as deep-seated bleeding and rebleeding.

7.3 Signs (with Pathophysiological Basis)

Sign	Location	Pathophysiology
Swollen, warm, tender joint	Knees, elbows, ankles	Blood accumulates in joint space → distension of capsule → inflammatory response to blood → warmth, tenderness, and loss of range of motion
Tense muscle swelling	Iliopsoas, forearm, calf	Blood dissects along fascial planes → muscle compartment swelling → may cause nerve compression
Large ecchymoses	Shins, thighs, trunk	Bleeding into subcutaneous tissue and deeper layers; no petechiae (platelets are normal)
Oral mucosal haematoma	Tongue, buccal mucosa	Trauma to oral mucosa → failure of secondary haemostasis → expanding haematoma (can compromise airway if large)

Target joint: a joint that has had ≥3 bleeds within a 6-month period.
Chronic synovitis: thickened, boggy synovium from repeated iron deposition (haemosiderin) → chronic inflammatory response.
Joint contractures and deformity: fixed flexion contracture of the knee or elbow from chronic inflammation and fibrosis.
Muscle wasting: disuse atrophy around chronically affected joints.
Leg length discrepancy (in children): chronic knee haemarthrosis → hyperaemia of growth plates → overgrowth of the affected limb.
Chronic pain: from arthropathy, analogous to osteoarthritis.
Pseudotumour: encapsulated haematoma that erodes into bone — a rare but classic finding in severe haemophilia. Appears as an expanding cystic mass, typically in long bones or pelvis.

Hepatomegaly: may be present due to chronic hepatitis B/C (historically, haemophilia patients were at high risk from contaminated blood products — this is less of a concern with current recombinant products and viral inactivation, but many older patients in Hong Kong may carry hepatitis B or C).
Absence of splenomegaly: haemophilia itself does not cause splenomegaly.
Fundoscopy: retinal haemorrhage may indicate risk of CNS bleeding ^[6].

High Yield: Approach to a Young Male with Deep-Seated Bleeding and Isolated Prolonged APTT

The classic exam scenario:

A 20-year-old man presents with prolonged bleeding after wisdom tooth extraction. There is an old bruise over the shin. Otherwise, examination shows no physical abnormalities.

Hb normal, WCC normal, Platelet count normal
PT normal
APTT prolonged

This pattern = isolated prolonged APTT with coagulation-type bleeding = Think intrinsic pathway factor deficiency:

Factor VIII → Haemophilia A
Factor IX → Haemophilia B
Factor XI → Haemophilia C
vWF deficiency → von Willebrand disease (vWF carries and stabilises Factor VIII → low vWF → low Factor VIII → prolonged APTT)

To distinguish: specific factor assays and vWF antigen/activity.

Do NOT forget: In Hong Kong, the most common cause of an isolated prolonged APTT is Factor XII deficiency (~20% local prevalence) — this does NOT cause bleeding ^[4].

Other causes of isolated prolonged APTT that do NOT cause bleeding: Factor XII deficiency, prekallikrein deficiency, HMWK deficiency, lupus anticoagulant (actually causes thrombosis, not bleeding).

Use a mixing study to differentiate factor deficiency from an inhibitor: Mix 50% patient plasma + 50% normal plasma → if APTT corrects → factor deficiency; if APTT does NOT correct → inhibitor present ^[4].

When evaluating a patient with suspected Haemophilia B, obtain:

Duration and pattern of bleeding tendency
- Long-standing → inherited (haemophilia, vWD)
- Acute onset → acquired (drugs, liver disease, acquired inhibitors)
Pattern of bleeding
- Mucocutaneous → platelet disorder / vWD
- Deep-seated → coagulation factor deficiency (haemophilia)
Previous bleeding challenges
- Dental procedures, surgeries, circumcision, trauma
- Prolonged bleeding after tooth extraction is a classic trigger ^[6]^[7]
Family history
- X-linked pattern: affected males on the maternal side (maternal grandfather, maternal uncles, maternal cousins)
- Absence of family history does NOT exclude haemophilia (30–40% sporadic)
Age of onset
- Since birth → severe haemophilia
- Childhood → moderate haemophilia
- Adulthood / discovered incidentally → mild haemophilia
Drug history
- Aspirin, NSAIDs, anticoagulants, antiplatelet agents
- These can unmask or worsen mild haemophilia
Other bleeding symptoms (head-to-toe survey) ^[6]
- ICH, epistaxis, gum bleeding, easy bruising, joint/muscle haematoma, GI bleeding, haematuria, menstrual history (if female carrier)

Pathophysiology	Clinical Manifestation
Factor IX deficiency → impaired intrinsic tenase complex → inadequate thrombin amplification → weak fibrin clot	Delayed rebleeding after initial haemostasis
Weak fibrin clot in enclosed joint space → blood cannot escape	Haemarthrosis (hallmark)
Haemosiderin deposition in synovium → chronic inflammation → cartilage destruction	Haemophilic arthropathy, target joints, contractures
Bleeding into muscle compartments → pressure on nerves/vessels	Compartment syndrome, nerve palsy (e.g., femoral nerve in iliopsoas bleed)
Intrinsic pathway disrupted but extrinsic pathway intact	Isolated prolonged APTT with normal PT
Primary haemostasis (platelet plug) intact	No petechiae; initial haemostasis is normal
X-linked recessive inheritance	Males affected; females are carriers

High Yield Summary

Haemophilia B = X-linked recessive deficiency of Factor IX (Christmas disease), accounting for ~15% of haemophilia cases ^[1]^[2].
Incidence: 1 in 15,000–30,000 live male births; ~1/3–1/2 have severe disease ^[2].
Classified by residual factor activity: Severe (< 1%), Moderate (1–5%), Mild (> 5% to < 40%) — severity determines bleeding pattern ^[1]^[2].
Gene: F9 at Xq26; > 1,100 mutations; ~30–40% sporadic (no family history) ^[2].
Factor IX is part of the intrinsic tenase complex (IXa + VIIIa); deficiency impairs thrombin amplification → weak fibrin clot — explains why bleeding is delayed and deep-seated.
Coagulation-type bleeding: haemarthrosis (hallmark), muscle haematoma, prolonged surgical/dental bleeding, ICH (most feared), rebleeding after initial cessation ^[5]^[6].
Laboratory: Normal PT + Prolonged APTT + Normal platelet count. Specific Factor IX assay confirms diagnosis ^[4]^[5].
In Hong Kong, Factor XII deficiency (~20% prevalence) is the most common cause of isolated prolonged APTT and does NOT cause bleeding — must distinguish from haemophilia ^[4].
Mixing study: corrects in factor deficiency; does NOT correct if inhibitor present ^[4].
Inhibitors develop in ~3–5% of Haemophilia B patients (lower than the ~25–30% in Haemophilia A) but can be associated with anaphylaxis to Factor IX concentrates — unique to Haemophilia B.
Haemophilia B Leyden: specific promoter mutations → Factor IX levels rise at puberty → patients can "outgrow" the disease.

Active Recall - Haemophilia B (Definition, Epidemiology, Aetiology, Classification, Clinical Features)

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Haemophilia A/B section ^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B ^[3] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — Haemophilia A/B section ^[4] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf — Coagulation interpretation section ^[5] Senior notes: Block A - Abnormal bleeding after tooth extraction_ bleeding tendency; thrombocytopenia.pdf — Patterns of bleeding section ^[6] Senior notes: Ryan Ho Fundamentals.pdf — Section 3.6.5.2 Approach to Bleeding Disorders ^[7] Senior notes: Block A - Hematology Interactive Tutorial.pdf — Case 2

Differential Diagnosis of Haemophilia B

Before listing differentials, let's be clear about what we are actually differentiating. A patient with Haemophilia B typically presents in one of these clinical scenarios:

A young male with deep-seated (coagulation-type) bleeding — haemarthrosis, muscle haematoma, prolonged post-surgical/dental bleeding.
An isolated prolonged APTT with normal PT and normal platelet count found on screening or during investigation of bleeding.
Haemarthrosis — a swollen, painful joint with blood in it.
A neonate with excessive bleeding — post-circumcision, cephalhaematoma, ICH.

The differential diagnosis must address each of these overlapping presentations. I will organise by the most clinically useful approach: the clotting profile pattern, then by bleeding pattern, then specifically for haemarthrosis.

This is the single most powerful differentiating tool. When you get a clotting profile, the pattern of PT and APTT derangement immediately narrows your differential.

The table below is a classic exam framework for interpreting PT/APTT results ^[3]^[5]^[8]:

PT	APTT	Inherited Causes	Acquired Causes
↑	Normal	Factor VII deficiency	Warfarin; Vitamin K deficiency (mild — Factor VII has shortest half-life so falls first); Mild liver disease; Lupus anticoagulant; Inhibitors of Factor VII
Normal	↑	Factor VIII deficiency (Haemophilia A); Factor IX deficiency (Haemophilia B); Factor XI deficiency (Haemophilia C); Factor XII deficiency	Heparin (UFH); Von Willebrand disease; Lupus anticoagulant; Inhibitors of Factor XII, XI, IX, VIII
↑	↑	Factor V, X, II (prothrombin), or I (fibrinogen) deficiency; Combined factor deficiency	Combined warfarin + heparin; Direct thrombin inhibitors (e.g., dabigatran); Direct Factor Xa inhibitors (e.g., rivaroxaban — though often only prolongs PT); Supratherapeutic anticoagulation; Severe vitamin K deficiency (2, 7, 9, 10 all affected); Liver failure; DIC; Inhibitors of common pathway factors
Normal	Normal	Platelet disorders; Factor XIII deficiency; Mild vWD; α2-antiplasmin deficiency	Platelet disorders (ITP, drug-induced); Mild coagulopathy

Haemophilia B sits in the "Normal PT, Prolonged APTT" box. So the key differential is among conditions that cause an isolated prolonged APTT.

2. Differential Diagnosis of Isolated Prolonged APTT (Normal PT)

This is the most important differential for Haemophilia B in exams. The APTT measures the intrinsic pathway (Factors XII → XI → IX → VIII) plus the common pathway. A normal PT tells you the extrinsic pathway (Factor VII, and common pathway factors V, X, II, I) is intact.

I split these into two categories: those that cause bleeding and those that do NOT cause bleeding — because the clinical implication is completely different.

Condition	Deficient Factor	Key Distinguishing Features
Haemophilia A	Factor VIII	X-linked recessive; clinically identical to Haemophilia B; ~85% of haemophilia; differentiated ONLY by specific factor assay showing low Factor VIII with normal Factor IX
Haemophilia B (Christmas disease)	Factor IX	X-linked recessive; ~15% of haemophilia; specific Factor IX assay low
Haemophilia C (Rosenthal syndrome)	Factor XI	Autosomal recessive (affects both sexes); especially common in Ashkenazi Jews; bleeding is unpredictable — ~50% will bleed, ~50% will not ^[4]; typically milder than A/B
Von Willebrand Disease (vWD)	vWF (± secondary Factor VIII reduction)	Most common inherited bleeding disorder (~1% prevalence); vWF carries and stabilises Factor VIII → low vWF → low Factor VIII → prolonged APTT ^[4]; typically causes mixed mucocutaneous + coagulation-type bleeding; differentiated by vWF:Ag, vWF:RCo (ristocetin cofactor activity), and Factor VIII level
Acquired Factor VIII inhibitors (Acquired Haemophilia A)	Autoantibody against Factor VIII	Classically postpartum or associated with autoimmune disease (SLE, RA), malignancy, or elderly poorly mobilized patients ^[2]; presents with sudden-onset severe bleeding (often retroperitoneal haematoma → back pain + profound anaemia) ^[2]; mixing study does NOT correct (inhibitor present); Bethesda assay positive
Acquired Factor IX inhibitors	Autoantibody against Factor IX	Very rare; similar principle to acquired Haemophilia A but much less common
Heparin (unfractionated)	Potentiates antithrombin → inactivates thrombin (IIa) and Factor Xa; also inactivates thrombin-induced activation of Factors V, VIII, and XI ^[5]	Drug history!; No change in PT, increase in APTT ^[5]; typically in hospitalised patients on anticoagulation

Condition	Mechanism	Key Points
Factor XII deficiency	Factor XII (Hageman factor) is the first step of the in-vitro intrinsic pathway but is NOT required for in-vivo haemostasis	Most common cause of isolated prolonged APTT in Hong Kong (~20% local prevalence) ^[4]; does NOT cause bleeding; discovered incidentally; no treatment needed
Prekallikrein deficiency	Contact pathway factor; not required for in-vivo clotting	Very rare; no bleeding
High molecular weight kininogen (HMWK) deficiency	Contact pathway factor	Very rare; no bleeding
Lupus anticoagulant (LA)	Antiphospholipid antibody that interferes with phospholipid-dependent in-vitro clotting tests → prolongs APTT	Paradoxically causes thrombosis, NOT bleeding (antiphospholipid syndrome); mixing study does NOT correct (inhibitor); differentiated from factor inhibitors by specific lupus anticoagulant testing (dRVVT, silica clotting time)

High Yield GC Exam Point: Factor XII Deficiency in Hong Kong

In Hong Kong, the most common cause of an isolated prolonged APTT is Factor XII deficiency (~20% local prevalence). It does NOT cause a bleeding tendency ^[4]. When you see a prolonged APTT in a Hong Kong patient, always interpret it in clinical context — if the patient has NO bleeding symptoms, think Factor XII deficiency first. Only pursue haemophilia workup if there is a genuine bleeding history.

The mixing study (50:50 mix) is how you determine whether a prolonged APTT is due to a factor deficiency (which will correct) or an inhibitor (which will NOT correct) ^[4]:

Mix 50% patient plasma + 50% normal plasma
If the APTT corrects → the patient was simply missing a factor; the normal plasma supplied it → factor deficiency (Haemophilia A, B, C, vWD, Factor XII deficiency)
If the APTT does NOT correct → there is something in the patient's plasma that is actively destroying/inhibiting the clotting factors in the normal plasma → inhibitor (acquired haemophilia, lupus anticoagulant)

Special note on lupus anticoagulant vs. acquired haemophilia: Both cause a mixing study that does not correct. But:

Lupus anticoagulant acts immediately — the APTT fails to correct on immediate mixing ^[4].
Acquired Factor VIII inhibitors (especially Type II) may show partial correction initially, but after incubation at 37°C for 1–2 hours, the APTT rises again (time-dependent inhibition). However, some strong (Type I) inhibitors also fail to correct immediately.
Further differentiation: Bethesda assay (for factor inhibitor titre) and specific lupus anticoagulant tests (dRVVT, silica clotting time).

4. Differentiating Haemophilia B from the Closest Mimics

Feature	Haemophilia A	Haemophilia B
Deficient factor	Factor VIII	Factor IX
Gene / locus	F8 at Xq28	F9 at Xq26
Frequency	~85% of haemophilia; 1 in 4,000–5,000	~15% of haemophilia; 1 in 15,000–30,000
Inheritance	X-linked recessive	X-linked recessive
Clinical features	Identical	Identical
PT / APTT	Normal PT, ↑ APTT	Normal PT, ↑ APTT
DDAVP response	Effective in mild Haemophilia A (releases Factor VIII from endothelial stores)	NOT effective (DDAVP releases vWF and Factor VIII, but NOT Factor IX)
Inhibitor rate	~25–30% in severe disease	~3–5% in severe disease
Inhibitor complications	No anaphylaxis	Anaphylaxis to Factor IX concentrates; nephrotic syndrome
Haemophilia Leyden variant	Does not exist	Exists (Factor IX levels rise at puberty)

You CANNOT distinguish Haemophilia A from B clinically. The only way is by specific factor assays (Factor VIII level vs. Factor IX level).

Feature	Haemophilia B	Von Willebrand Disease
Inheritance	X-linked recessive (males)	Autosomal dominant (Types 1, 2) or recessive (Type 3) — affects both sexes equally
Bleeding pattern	Deep-seated (coagulation-type)	Mixed: mucocutaneous + coagulation-type (especially menorrhagia in females)
Factor IX level	Low	Normal
Factor VIII level	Normal	Low (because vWF stabilises Factor VIII → low vWF → accelerated Factor VIII degradation)
vWF:Ag	Normal	Low
vWF:RCo	Normal	Low
Ristocetin-induced platelet aggregation (RIPA)	Normal	Reduced (except Type 2B where it is increased)
DDAVP response	Not effective	Effective (for Type 1 vWD — releases vWF from endothelial Weibel-Palade bodies)

The interactive tutorial Case 2 ^[7] provides an excellent example: Factor VIII level 0.13 u/mL, vWF:Ag 1.11 u/mL, vWF:RCo 1.22 u/mL → this is Haemophilia A, not vWD (because vWF is normal but Factor VIII is low). If it were vWD, both vWF:Ag and Factor VIII would be low.

For Haemophilia B, the Factor IX assay would be low, with normal Factor VIII AND normal vWF.

Feature	Haemophilia B	Liver Disease
PT	Normal	Prolonged (Factor VII has shortest half-life → falls first) ^[5]
APTT	Prolonged	Prolonged (multiple factors affected)
Other clotting factors	Only Factor IX low	Multiple factors low (all liver-synthesised: I, II, V, VII, IX, X, XI, XII, XIII)
Platelet count	Normal	Often low (hypersplenism from portal hypertension; decreased thrombopoietin)
Clinical context	Young male with lifelong bleeding history	Older patient with stigmata of chronic liver disease (jaundice, spider naevi, ascites, etc.)
Response to Vitamin K	No improvement	Improvement if cholestatic (not if hepatocellular failure)

Vitamin K is required for gamma-carboxylation of Factors II, VII, IX, X (mnemonic: "1972"). In vitamin K deficiency:

Factor VII falls first (shortest half-life ~6 hours) → PT prolonged first
As deficiency worsens, Factors IX, X, and II also fall → APTT also prolonged
In mild vitamin K deficiency, you may see isolated prolonged PT with normal APTT ^[5]^[8] — this is opposite to Haemophilia B
In severe vitamin K deficiency, both PT and APTT are prolonged
Corrects with vitamin K administration (Haemophilia B does not)

Haemarthrosis (blood in the joint) is the hallmark of haemophilia, but it is not exclusive to haemophilia. The GC lecture slides on joint disease provide the key differential ^[9]:

Haemarthrosis — Common causes: Trauma; Haemophilia (Congenital VIII and IX deficiency); Acquired haemophilia (autoantibodies); Drugs (warfarin or heparin) ^[9]

Cause	Key Features
Trauma	History of injury; may have intra-articular fracture, ligamentous tear, meniscal injury; occurs seconds to minutes after trauma ^[10]
Haemophilia A/B	Congenital Factor VIII or IX deficiency; recurrent joint bleeds; positive family history (or sporadic); male; onset in childhood ^[9]
Acquired haemophilia	Autoantibodies against clotting factors (usually Factor VIII); typically older patients or postpartum; sudden-onset severe bleeding with no prior bleeding history ^[2]^[9]
Anticoagulant drugs	Warfarin or heparin use ^[9]; always check drug history
Other coagulopathies	DIC, liver disease, severe vitamin K deficiency
Pigmented villonodular synovitis (PVNS)	Chronic monoarthritis with haemorrhagic synovial fluid; MRI shows characteristic low-signal nodular synovial proliferation; not a true coagulopathy

GC High Yield — Haemarthrosis Differential

From GC 075 (Pain Red Joint) lecture slides ^[9]:

Common causes of haemarthrosis: Trauma, Haemophilia (Congenital VIII and IX deficiency), Acquired haemophilia (autoantibodies), Drugs (warfarin or heparin)
Symptoms: Pain, swelling, stiffness
Diagnosis: Arthrocentesis
Treatment: Treat the "treatable" underlying causes

The following mermaid diagram summarises the systematic approach to differentiating Haemophilia B from its mimics:

Condition	Inheritance	Deficiency	PT	APTT	Mixing Study	Bleeding Pattern	Key Differentiator
Haemophilia A	XLR	Factor VIII	N	↑	Corrects	Deep-seated	Factor VIII assay low; ~85% of haemophilia
Haemophilia B	XLR	Factor IX	N	↑	Corrects	Deep-seated	Factor IX assay low; ~15% of haemophilia
Haemophilia C	AR	Factor XI	N	↑	Corrects	Variable / unpredictable	Common in Ashkenazi Jews; both sexes
vWD	AD/AR	vWF ± VIII	N	N or ↑	Corrects	Mixed mucocutaneous + coagulation	vWF:Ag and vWF:RCo low; RIPA abnormal
Factor XII deficiency	AR	Factor XII	N	↑	Corrects	None	Most common isolated ↑APTT in HK; no Rx needed
Lupus anticoagulant	Acquired	None (antibody)	N	↑	Does NOT correct	Thrombosis (not bleeding!)	Paradoxical; dRVVT positive
Acquired haemophilia	Acquired	Autoantibody vs. VIII (or rarely IX)	N	↑	Does NOT correct	Severe deep-seated, often retroperitoneal	Sudden onset; elderly/postpartum; Bethesda assay+
UFH therapy	Acquired (drug)	Antithrombin potentiated → ↓IIa, Xa	N	↑	—	Variable	Drug history; thrombin time markedly prolonged
Liver disease	Acquired	Multiple factors	↑	↑	—	Mixed	LFTs abnormal; low albumin; stigmata of CLD
Vitamin K deficiency	Acquired	II, VII, IX, X	↑ (first)	↑ (later)	—	Mixed	Corrects with Vitamin K; causes: malabsorption, cholestasis, antibiotics, warfarin
DIC	Acquired	Consumption of all factors + platelets	↑	↑	—	Mixed + microvascular thrombosis	Low fibrinogen; high D-dimer; low platelets; schistocytes on PBS

High Yield: Must-Know Differentials for Exams

Isolated prolonged APTT → Haemophilia A (VIII), Haemophilia B (IX), Haemophilia C (XI), Factor XII deficiency, vWD, Lupus anticoagulant, Heparin, Acquired haemophilia ^[4]^[5]^[8].
In HK, Factor XII deficiency is the most common cause of isolated prolonged APTT (~20% prevalence) and does NOT cause bleeding ^[4].
Mixing study corrects in factor deficiency; does NOT correct with inhibitors (acquired haemophilia, lupus anticoagulant) ^[4].
vWD is distinguished from haemophilia by vWF:Ag, vWF:RCo — low in vWD, normal in haemophilia ^[7].
Haemophilia A and B are clinically indistinguishable — only specific factor assays differentiate them ^[2].
Haemarthrosis DDx: Trauma, Haemophilia (VIII/IX deficiency), Acquired haemophilia (autoantibodies), Drugs (warfarin/heparin) ^[9].

Active Recall - Haemophilia B Differential Diagnosis

References

^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B ^[3] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — Haemophilia A/B, Etiology/Differential Diagnosis table ^[4] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf — Coagulation interpretation, mixing study ^[5] Senior notes: Block A - Abnormal bleeding after tooth extraction_ bleeding tendency; thrombocytopenia.pdf — Coagulation tests and patterns ^[7] Senior notes: Block A - Hematology Interactive Tutorial.pdf — Case 2 ^[8] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Haemophilia A/B section ^[9] Lecture slides: GC 075. Pain red joint.pdf — Haemarthrosis slide ^[10] Senior notes: Ryan Ho Rheumatology.pdf — Section 2.1 Approach to Acute Monoarthritis

Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Haemophilia B

Unlike many conditions in medicine, Haemophilia B does not have a formal "diagnostic criteria" checklist (like the Jones criteria for rheumatic fever or the ACR criteria for SLE). Instead, diagnosis rests on a combination of clinical features, laboratory findings, and genetic confirmation. The World Federation of Hemophilia (WFH) and International Society on Thrombosis and Haemostasis (ISTH) define the diagnosis as follows:

Haemophilia B is diagnosed when ALL of the following are present:

Component	Requirement
Clinical	History of coagulation-type bleeding (deep-seated: haemarthrosis, muscle haematoma, prolonged post-surgical/dental bleeding) OR positive family history OR suspected based on screening
Clotting profile	Normal PT, prolonged APTT ^[5]^[11]
Mixing study	APTT corrects when patient plasma is mixed 1:1 with normal plasma (excludes inhibitors) ^[4]^[11]
Specific Factor IX assay	Factor IX activity < 40% (< 0.40 IU/mL) ^[1]^[2]
Normal vWF and Factor VIII	vWF:Ag normal, vWF:RCo normal, Factor VIII normal (excludes vWD and Haemophilia A) ^[7]^[12]
Genetic confirmation (recommended but not strictly required for diagnosis)	Pathogenic variant identified in the F9 gene (Xq26) ^[2]

High Yield: What Makes the Diagnosis?

The sine qua non of Haemophilia B diagnosis is a low Factor IX activity level (< 40% or < 0.40 IU/mL) in the setting of a normal PT and prolonged APTT that corrects on mixing study ^[2]^[5]. Genetic testing confirms the molecular defect and is essential for family screening and prenatal counselling, but a low Factor IX assay with appropriate clinical context is sufficient for a working diagnosis.

Severity Classification (Revisited — Central to Diagnostic Reporting)

Once Haemophilia B is confirmed, severity is formally classified by the baseline Factor IX activity level, as this directly determines management strategy:

Severity	Factor IX Level	Clinical Correlation
Severe	< 1% (< 0.01 IU/mL)	Spontaneous bleeding; haemarthrosis; 1–2 episodes/week
Moderate	1–5% (0.01–0.05 IU/mL)	Bleeding with minor trauma; ~1/month
Mild	> 5% to < 40% (0.05–0.40 IU/mL)	Bleeding only with major trauma/surgery

^[1]^[2]

Note: APTT may be normal in some mild haemophilia patients ^[2]. This is a critical exam pitfall — a normal APTT does NOT exclude mild haemophilia if the clinical suspicion is high. In such cases, specific factor assays should still be requested.

Exam Pitfall

A normal APTT does NOT rule out mild haemophilia. The APTT may be normal when Factor IX levels are > 15–30% because the test is not sensitive enough to detect mild reductions. If clinical suspicion exists (e.g., positive family history, excessive post-surgical bleeding), always request specific factor assays regardless of APTT result ^[2].

The algorithm follows a systematic, stepwise approach from clinical suspicion through screening tests to confirmatory tests. This mirrors real clinical practice and exam-style reasoning.

3. Investigation Modalities: Detailed Breakdown

I will organise investigations into screening (first-line), confirmatory (second-line), supportive/ancillary, and monitoring investigations.

3.1 First-Line Screening Investigations

These are the tests you order when any patient presents with a bleeding tendency. They help you categorise the problem (platelet vs. coagulation) and guide further workup.

Parameter	Expected Finding in Haemophilia B	Interpretation
Haemoglobin	Usually normal (unless recent major bleed → anaemia from blood loss)	A low Hb suggests acute/chronic blood loss; does not point to haemophilia specifically
White cell count	Normal	Abnormal WCC suggests an alternative diagnosis (leukaemia, infection)
Platelet count	Normal	This is crucial — normal platelets confirm that primary haemostasis is intact. If platelets are low, you are looking at thrombocytopenia (ITP, TTP, DIC, etc.), not haemophilia ^[5]^[11]

In the classic GC Interactive Tutorial Case 2: Hb 14.5 g/dL, WCC 9.5 × 10⁹/L, Platelet count 260 × 10⁹/L — all normal ^[7]^[12]. A completely normal CBC in a patient with coagulation-type bleeding immediately shifts your focus to the clotting profile.

Usually normal in haemophilia B.
Useful to exclude other diagnoses: schistocytes (DIC, TTP/HUS), blasts (leukaemia), giant platelets (Bernard-Soulier, MYH9 disorders), platelet clumping (EDTA artefact) ^[11].

This is the single most important screening test for coagulation disorders.

Test	What It Measures	Expected in Haemophilia B	Why
Prothrombin Time (PT)	Extrinsic pathway (Factor VII) + common pathway (X, V, II, I)	Normal	Factor IX is NOT in the extrinsic pathway; the extrinsic cascade is completely intact
Activated Partial Thromboplastin Time (APTT)	Intrinsic pathway (Factors XII, XI, IX, VIII) + common pathway	Prolonged	Factor IX is in the intrinsic pathway; its deficiency impairs intrinsic tenase complex formation → less Factor Xa generated → less thrombin → delayed clotting ^[4]^[5]

From the GC Interactive Tutorial Case 2 ^[7]^[12]:

PT: 10.5 seconds (reference 11–13.5 seconds) — Normal
APTT: 60 seconds (reference 21–35 seconds) — Markedly Prolonged

Patient with hemophilia A / B: No change in PT, Increase in APTT ^[5]

Understanding why APTT is prolonged but PT is not:

The PT test adds tissue factor (extrinsic pathway activator) to patient plasma. This bypasses the intrinsic pathway entirely (Factors XII, XI, IX, VIII are not needed). Since Factor IX is irrelevant to this test, PT remains normal.
The APTT test uses a contact activator (e.g., kaolin, silica) + phospholipid + calcium to trigger the intrinsic pathway. This requires Factor XII → XI → IX → VIII in sequence. Missing Factor IX means the cascade stalls at the tenase complex step, and clot formation is delayed → prolonged APTT.

Test	What It Measures	Expected in Haemophilia B
Thrombin Time	Final step: Thrombin → Fibrinogen → Fibrin	Normal

Why order it? It helps exclude heparin contamination (heparin markedly prolongs TT) and fibrinogen disorders (afibrinogenaemia, dysfibrinogenaemia).
In haemophilia B, the problem is upstream of thrombin generation, not in the final fibrin-formation step, so TT is normal.

3.2 Second-Line Confirmatory Investigations

Once screening shows "normal PT + prolonged APTT," you proceed to confirmatory tests to identify which intrinsic pathway factor is deficient.

Test	Method	Expected in Haemophilia B	Interpretation
Mixing study	Mix 50% patient plasma with 50% normal pooled plasma, then repeat APTT	APTT corrects (normalises)	Correction = factor deficiency (the normal plasma supplied the missing factor). Non-correction = inhibitor ^[4]^[11]

Why does it correct? Normal plasma contains ~100% of all clotting factors. The patient's plasma contains 0% (or very little) Factor IX. When you mix 50:50, you get ~50% Factor IX activity, which is above the threshold for normal clotting (≥ 50% is haemostatic). So the APTT normalises.

Why does it NOT correct with an inhibitor? In acquired haemophilia or lupus anticoagulant, the patient's plasma contains antibodies that actively destroy/interfere with clotting factors or phospholipids in the normal plasma → the APTT remains prolonged even after mixing.

From the lecture notes: ^[4]

Von Willebrand disease is just a deficiency of the Factor 8 → after mixing the patient's plasma (which has zero% Factor 8) with the healthy plasma (which has 100% Factor 8), you get a 50% Factor 8, which is sufficient to correct the APTT
Lupus anticoagulant: antibodies in the patient's plasma begin immediately destroying the healthy plasma phospholipids → resulting in the inability to correct the APTT

The same principle applies to Haemophilia B: the patient lacks Factor IX → normal plasma provides Factor IX → APTT corrects.

Improved after mixing study (1:1 mix with normal plasma): haemophilia, vWD ^[11]

Test	Method	Expected in Haemophilia B	Key Values
Factor IX activity assay	One-stage clotting assay (APTT-based) or chromogenic assay	Low (< 0.40 IU/mL or < 40%)	Severe: < 1%; Moderate: 1–5%; Mild: > 5–40% ^[1]^[2]

This is THE diagnostic test for Haemophilia B. Once Factor IX activity is confirmed low, and Factor VIII + vWF are normal, the diagnosis is established.

Understanding the assay:

One-stage clotting assay: Patient plasma is mixed with Factor IX-deficient plasma (which lacks only Factor IX but has all other factors). The APTT of this mixture depends entirely on how much Factor IX the patient's plasma contributes. The shorter the APTT, the more Factor IX the patient has. Results are expressed as a percentage of normal pooled plasma (defined as 1.0 IU/mL = 100%).
Chromogenic assay: Uses a synthetic substrate that is cleaved by Factor Xa (generated by the tenase complex). The amount of colour produced is proportional to Factor IX activity. This is less affected by lupus anticoagulant or heparin interference.

One-Stage vs. Chromogenic Assay Discrepancy

Some F9 mutations (especially certain missense mutations) produce a Factor IX protein that behaves differently in the two assay systems. This is clinically relevant in the era of gene therapy — emicizumab and fitusiran monitoring require specific assay methods. For routine diagnosis, the one-stage clotting assay is standard.

Test	Expected in Haemophilia B	Purpose
Factor VIII activity	Normal	Excludes Haemophilia A
vWF:Ag (antigen)	Normal	Excludes vWD
vWF:RCo (ristocetin cofactor activity)	Normal	Excludes vWD

The GC Interactive Tutorial Case 2 demonstrates this beautifully ^[7]^[12]:

Factor VIII level: 0.13 u/mL — LOW
vWF:Ag: 1.11 u/mL — Normal
vWF:RCo: 1.22 u/mL — Normal

This pattern confirms Haemophilia A (not B, not vWD). For Haemophilia B, the pattern would be: Factor VIII normal, Factor IX low, vWF normal.

If it is an isolated decrease in Factor 8, can either be: Platelet function disorder → e.g Von Willebrand disease (VWD); or Factor 8 deficiency or inhibition ^[7]. For Haemophilia B, the isolated decrease is in Factor IX, not Factor VIII.

Test	When to Order	Purpose
Factor XI assay	If both Factor VIII and Factor IX are normal in a patient with isolated prolonged APTT and bleeding	Diagnoses Haemophilia C
Factor XII assay	If the patient has isolated prolonged APTT but no bleeding symptoms	Diagnoses Factor XII deficiency — the most common cause of isolated prolonged APTT in Hong Kong (~20% prevalence) ^[4]; no treatment needed

A. Bethesda Assay (Nijmegen-Bethesda Assay)

Test	Method	Expected in Uncomplicated Haemophilia B	Purpose
Bethesda assay	Patient plasma is incubated with normal plasma at 37°C for 2 hours, then residual Factor IX activity is measured. The titre is expressed in Bethesda Units (BU)	Negative (0 BU)	Detects and quantifies inhibitory alloantibodies against Factor IX

When to order:

At diagnosis (baseline)
Regularly during treatment (especially in the first 50 exposure days to Factor IX concentrate)
Whenever clinical response to Factor IX replacement is unexpectedly poor

Key thresholds:

Low-titre inhibitor: < 5 BU/mL — may still respond to higher doses of Factor IX
High-titre inhibitor: ≥ 5 BU/mL — requires bypassing agents (e.g., activated prothrombin complex concentrate, recombinant FVIIa)

Inhibitor development occurs in ~3–5% of severe Haemophilia B patients ^[2] — much less frequent than in Haemophilia A (~25–30%). However, unique to Haemophilia B: inhibitor development can be associated with anaphylaxis to Factor IX concentrates and nephrotic syndrome.

3.4 Genetic Testing

Test	Method	Purpose
F9 gene mutation analysis	Sanger sequencing or next-generation sequencing (NGS) of the F9 gene at Xq26	Identifies the causative mutation; essential for family screening, carrier detection, and prenatal diagnosis ^[2]

Why genetic testing matters:

Confirms diagnosis at the molecular level.
Predicts inhibitor risk: Large gene deletions → higher risk of inhibitors + anaphylaxis to Factor IX.
Identifies Haemophilia B Leyden: Specific promoter mutations → Factor IX rises at puberty.
Carrier detection: Essential for genetic counselling. A carrier female has one normal X and one affected X → her Factor IX level may be anywhere from ~30–70% depending on X-inactivation patterns. Genetic testing definitively identifies carriers even if their Factor IX level is in the normal range.
Prenatal diagnosis: Chorionic villus sampling (CVS) at 10–12 weeks or amniocentesis at 15–18 weeks can identify an affected male fetus. This information guides delivery planning (e.g., avoid instrumented delivery, avoid fetal scalp electrodes).

Prenatal diagnosis: often presumed based on sex on USG (invasive Ix does not change Mx) ^[2] — in practice, knowing the fetus is male alerts the obstetric team to plan for potential haemophilia, but invasive prenatal testing is offered mainly for families who want definitive diagnosis for decision-making.

Who to Test	Method	Clinical Implication
Mother of an affected male	Factor IX assay + F9 genetic testing	Obligate carrier if her son has haemophilia B and the mutation is confirmed in her
Sisters/daughters of affected males or known carriers	F9 genetic testing	50% chance of being a carrier
Female relatives with borderline low Factor IX	Factor IX assay + F9 genetic testing	Symptomatic carriers with skewed X-inactivation may have mildly reduced Factor IX and bleeding symptoms

Check APTT and factor levels in family members (e.g. siblings), genetic test ^[7] — this principle applies equally to Haemophilia B as to Haemophilia A.

3.5 Ancillary / Supportive Investigations

These are not diagnostic for haemophilia itself but are essential for managing the patient and assessing complications.

Modality	Indication	Key Findings
Joint ultrasound	Suspected haemarthrosis	Effusion within joint space; synovial hypertrophy in chronic cases
MRI of joints	Assessment of chronic haemophilic arthropathy	Synovial thickening (haemosiderin deposition → low T1/T2 signal), cartilage loss, subchondral cysts, erosions
CT head (non-contrast)	Any suspected intracranial haemorrhage	Hyperdense acute blood; treat first with factor replacement, then image — do NOT delay treatment for imaging
CT abdomen	Suspected retroperitoneal / iliopsoas haematoma	Hyperdense haematoma in psoas/retroperitoneal space; may show nerve compression
Ultrasound / CT of soft tissues	Suspected deep muscle haematoma or compartment syndrome	Haematoma extent; compartment pressure assessment

Severe bleeding disorders (severe haemophilia, DIC) are an absolute contraindication for bone marrow examination ^[2]^[13] — if marrow biopsy is needed for any reason, factor replacement must be given first.

Test	Purpose
Liver function tests	Baseline before factor therapy; monitor for transfusion-transmitted hepatitis (historical concern); screen for liver disease
Viral serology: HBV, HCV, HIV	Historically, haemophilia patients were at high risk for bloodborne infections from contaminated plasma-derived products ^[2]; all patients should be screened at baseline; reduced risk with current recombinant products and viral inactivation
Renal function	Baseline; monitor in patients with haematuria or nephrotic syndrome (if inhibitors develop)
Iron studies	Patients receiving frequent transfusions may develop iron overload (less relevant now with factor concentrates rather than whole blood/FFP)

Tool	Purpose
HEAD-US (Haemophilia Early Arthropathy Detection with Ultrasound)	Standardised point-of-care ultrasound scoring for early joint disease
Pettersson Score (X-ray based)	Classic radiographic scoring system for haemophilic arthropathy: assesses osteoporosis, epiphyseal enlargement, subchondral cyst formation, joint space narrowing, and joint contracture
HJHS (Haemophilia Joint Health Score)	Clinical examination-based score for joint function

Step	Investigation	Key Finding in Haemophilia B
1	CBC	Normal Hb, WCC, Platelets
2	PT	Normal
3	APTT	Prolonged (may be normal in mild disease)
4	Mixing study	APTT corrects
5	Factor IX activity assay	Low (< 40%) — severity graded by level
6	Factor VIII + vWF:Ag + vWF:RCo	All normal (excludes Haem A and vWD)
7	Bethesda assay	Negative at baseline (monitor for inhibitor development)
8	F9 gene sequencing	Pathogenic variant identified
9	Viral serology	Baseline screen for HBV, HCV, HIV
10	Joint imaging	Assess for haemophilic arthropathy

High Yield Summary: Investigations for Haemophilia B

Normal PT + Prolonged APTT + Normal platelets → isolated intrinsic pathway defect ^[5]^[11].
Mixing study corrects → factor deficiency (not inhibitor) ^[4]^[11].
Low Factor IX assay (< 40%) with normal Factor VIII and normal vWF confirms Haemophilia B ^[1]^[2].
APTT may be normal in mild haemophilia → always request specific factor assays if clinical suspicion is high ^[2].
Bethesda assay screens for inhibitory alloantibodies against Factor IX (present in ~3–5% of severe cases) ^[2].
Genetic testing (F9 at Xq26) confirms diagnosis, enables carrier detection, prenatal diagnosis, and predicts inhibitor/anaphylaxis risk ^[2].
Factor XII deficiency (~20% in HK) is the most common isolated prolonged APTT — does NOT cause bleeding ^[4].
Severe haemophilia and DIC are absolute contraindications for bone marrow biopsy ^[2]^[13].
Viral serology (HBV, HCV, HIV) should be performed at baseline — historical risk from plasma-derived products ^[2].

Active Recall - Haemophilia B Diagnostic Investigations

References

^[1] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — Haemophilia A/B section ^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B ^[4] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf — Coagulation interpretation, mixing study ^[5] Senior notes: Block A - Abnormal bleeding after tooth extraction_ bleeding tendency; thrombocytopenia.pdf — Coagulation tests section ^[7] Senior notes: Block A - Hematology Interactive Tutorial.pdf — Case 2 ^[11] Senior notes: Maksim Medicine Notes.pdf — Clotting cascade interpretation ^[12] Lecture slides: GC_Interactive tutorial (Haem case 2) student copy.pdf — Case scenario ^[13] Senior notes: Ryan Ho Fundamentals.pdf — Marrow examination contraindications

Management of Haemophilia B

Management of Haemophilia B rests on five pillars:

Prevention of bleeding (general measures + prophylactic factor replacement)
Treatment of acute bleeding episodes (on-demand factor replacement ± adjunctive therapies)
Management of inhibitors (the most challenging complication of treatment)
Management of complications (arthropathy, infections, psychosocial)
Genetic counselling and family planning

The overarching goal is to maintain Factor IX activity at a level sufficient to prevent spontaneous bleeding and enable a normal quality of life. In severe Haemophilia B (Factor IX < 1%), this means raising and sustaining Factor IX levels through regular prophylactic infusions or, increasingly, through newer non-factor therapies and gene therapy.

These apply to every patient with Haemophilia B, regardless of severity. The rationale is to reduce the frequency and severity of bleeding episodes through lifestyle modifications.

Management — Lifestyle changes (avoid trivial trauma) ^[7]

Measure	Rationale	Details
Avoid invasive procedures: smallest gauge for vaccination, minimize blood taking/IMI ^[2]^[14]	Larger needles cause more tissue trauma → more bleeding into muscle/subcutaneous tissue	Use subcutaneous rather than intramuscular injections where possible; apply firm pressure for ≥ 5 minutes after venepuncture
Dental hygiene to ↓ need for dental interventions ^[2]^[14]	Dental procedures (extractions, root canal) provoke bleeding in haemophilia patients	Regular brushing, fluoride, dental reviews every 6 months; pre-procedural factor cover for any dental work
Exercise: regular exercise, prefer non-contact sports, e.g. swimming, cycling, tennis ^[2]^[14]	Strong muscles protect joints from haemarthrosis; but contact sports (rugby, boxing, martial arts) → high trauma risk	Physiotherapy to strengthen periarticular muscles; avoid sports with high collision/impact risk
Avoid antiplatelets/anticoagulants ^[2]^[14]	Aspirin, NSAIDs (non-selective), warfarin, DOACs, heparin all impair haemostasis further	If analgesia needed, use paracetamol or COX-2 selective inhibitors (e.g., celecoxib) — these do not impair platelet function ^[2]
MedicAlert identification	In emergencies, clinicians need to know the patient has a bleeding disorder immediately	Bracelet or card stating "Haemophilia B — Factor IX deficiency" with severity and inhibitor status
Multidisciplinary team care	Complex chronic condition requiring coordinated care	Haematologist, physiotherapist, orthopaedic surgeon, dentist, nurse specialist, psychologist, geneticist
Hepatitis B vaccination	Historical risk of transfusion-transmitted HBV; ongoing need for blood product exposure	All haemophilia patients should be vaccinated against HBV

GC High Yield: Treatment of Haemostatic Disorders

Coagulation disorders as a cause of hemostatic disorder — how to treat: ^[5]

Vitamin K (jaundice, liver disease)
Specific factors / factor concentrates (haemophilia, vWD)
DDAVP (vWD)
Fresh frozen plasma

Note: DDAVP is NOT effective for Haemophilia B — it releases Factor VIII and vWF from endothelial stores, but has no effect on Factor IX.

4. Factor IX Replacement Therapy

This is the cornerstone of Haemophilia B management. The principle is simple: replace the missing Factor IX to achieve haemostatic levels.

Product Type	Examples	Half-Life	Key Features
Plasma-derived Factor IX concentrates	Mononine, AlphaNine	~18–24 hours	Derived from pooled human plasma; viral inactivation steps applied; risk of bloodborne infection (now very low); may contain small amounts of other vitamin K-dependent factors
Recombinant Factor IX (standard half-life, rFIX)	BeneFIX (nonacog alfa)	~18–24 hours	Recombinant = produced by genetically engineered cells (CHO cells); no human plasma → eliminates bloodborne infection risk; standard of care
Recombinant Factor IX — Extended Half-Life (EHL-rFIX)	Alprolix (eftrenonacog alfa — Fc fusion); Idelvion (albutrepenonacog alfa — albumin fusion); Rebinyn (nonacog beta pegol — PEGylated)	~80–104 hours (3–5× longer)	Engineered to have prolonged circulation time by fusing Factor IX to Fc fragment of IgG1, albumin, or PEG → recycled via FcRn pathway or reduced renal clearance; allows less frequent dosing (every 7–14 days instead of 2× per week)

Why does extending the half-life matter?

Standard rFIX has a half-life of ~18–24 hours. To maintain trough Factor IX levels > 1% (which prevents most spontaneous bleeds), patients need infusions 2–3 times per week.
EHL-rFIX products achieve trough levels > 1–3% with infusions only every 7–14 days, dramatically improving quality of life, adherence, and venous access burden (especially important in paediatric patients who may require central venous access devices for frequent infusions).

Key pharmacokinetic principle: 1 IU/kg of Factor IX raises plasma Factor IX level by approximately 0.8–1.0 IU/dL (0.8–1.0%) for standard rFIX. (Compare with Factor VIII in Haemophilia A: 1 IU/kg raises Factor VIII by ~2% — Factor IX has a larger volume of distribution because it distributes into the extravascular space more readily.)

Dose calculation:

$\text{Dose (IU)} = \text{Body weight (kg)} \times \text{Desired rise in FIX (%)} \times 1.0\text{ to }1.25$

For example, a 70 kg patient needing Factor IX to rise by 50%:

Dose = 70 × 50 × 1.0 = 3,500 IU (for standard rFIX)

Target Factor IX levels for different clinical scenarios:

Clinical Scenario	Target Factor IX Level	Duration of Treatment
Minor haemarthrosis	30–50%	1–2 days
Moderate muscle haematoma	40–60%	2–3 days
Major muscle haematoma / Iliopsoas bleed	60–80%	3–5 days
Intracranial haemorrhage	80–100%	7–14 days or until resolved
Major surgery	80–100% initially, then > 30–50% for 7–14 days	Until wound healing
Minor surgery / dental extraction	30–50%	1–3 days
Mucosal bleeding	30–50%	+ adjunctive tranexamic acid

Severe, life-threatening bleeding: immediate replacement (treat upon suspicion not dx) ^[2]. This cannot be overemphasised — in suspected ICH, give Factor IX BEFORE imaging.

4.3 Prophylactic vs. On-Demand Therapy

Recombinant factor replacement: can be administered in different strategies ^[2]^[14]:

Factor IX is infused reactively when a bleed occurs or is anticipated (e.g., pre-surgery).
Advantages: lower cost; fewer injections; fewer central venous catheter-related complications.
Disadvantage: does NOT prevent joint damage from subclinical bleeds; does NOT prevent spontaneous ICH.

This is the preferred strategy for severe Haemophilia B and increasingly for moderate disease. The goal is to convert a severe phenotype into a moderate/mild phenotype by maintaining trough Factor IX levels > 1–3%.

Types of prophylaxis ^[2]^[14]:

Type	Definition	Indication
Primary prophylaxis	Regular replacement started < 3 years of age, before clinically evident joint disease, and before or after the first joint bleed	Severe Haemophilia B — recommended standard of care
Secondary prophylaxis	Regular replacement started after ≥ 2 large joint bleeds but before onset of joint disease	Severe or moderate haemophilia with recurrent joint bleeds
Tertiary prophylaxis	Regular replacement started after established joint disease (arthropathy)	Any severity with documented arthropathy
Intermittent prophylaxis	Given for several weeks/months then discontinued; given to interrupt bleeding cycle in those with repeated bleeding especially into target joints	Target joint management

Prophylaxis regimens for Haemophilia B:

Product	Regimen
Standard rFIX (BeneFIX)	25–40 IU/kg every 2–3 days (or 2× per week)
EHL-rFIX (Alprolix)	50–100 IU/kg every 7–14 days
EHL-rFIX (Idelvion)	25–50 IU/kg every 7–14 days

Choice between prophylaxis vs on-demand therapy: largely dependent on local availability and institutional practice ^[2]^[14]

	Prophylactic Therapy	On-Demand Therapy
Advantages	↓ bleeding risk and ↑ overall QoL; ↓ risk of life-threatening bleeding, e.g. ICH; ↓ hospitalisation and absenteeism; ↓ long-term complications, especially chronic arthropathy	↓ cost (recombinant factor very expensive and not readily available); No need regular injections; No need for CVC placement; ↓ replacement-related complications, e.g. infection
Usual indications	Severe haemophilia (primary prophylaxis); Prior bleeding episodes (e.g. ≥ 2 large joint bleeds, i.e. secondary prophylaxis); Evidence of joint damage on PE or imaging (tertiary prophylaxis)	Resource-limited settings; Mild/moderate haemophilia with limited burden

^[2]^[14]

This is a critical distinction from Haemophilia A management.

DDAVP (desmopressin) binds to V2 receptor in endothelial cells and platelets → release of Factor VIII and vWF from storage pools ^[7]^[2]. This temporarily raises Factor VIII levels by ~3× baseline, making it useful for mild Haemophilia A (where boosting a mildly low Factor VIII can reach haemostatic levels) and for Type 1 vWD.

Why is DDAVP NOT useful in Haemophilia B?

Factor IX is synthesised in the liver (not stored in endothelial cells or platelet granules).
DDAVP releases Factor VIII and vWF from endothelial Weibel-Palade bodies, but has no effect on Factor IX release.
Therefore, DDAVP is contraindicated as monotherapy for Haemophilia B — it will NOT correct the underlying Factor IX deficiency.

Exam Pitfall: DDAVP in Haemophilia

DDAVP is effective for mild Haemophilia A and Type 1 vWD only. It is NOT effective for Haemophilia B because it does not affect Factor IX levels. A common exam trap is asking about DDAVP in the context of haemophilia without specifying which type — always clarify whether the patient has Haemophilia A (Factor VIII) or B (Factor IX) before considering DDAVP.

Treatment of haemophilia-related bleeding: factor replacement as above plus ^[2]:

Bleeding Scenario	Factor IX Target	Adjunctive Measures	Rationale
Haemarthrosis	30–50%	Analgesics (e.g. COX-2, Panadol → avoid NSAID); RICE as needed ^[2]	NSAIDs (non-selective) inhibit COX-1 → impair platelet aggregation → worsen bleeding; COX-2 inhibitors spare platelet function; RICE (Rest, Ice, Compression, Elevation) reduces swelling and pain
Muscle haematoma	40–80% (depending on severity/location)	Consider surgical decompression if failed medical therapy ^[2]; physiotherapy after resolution	Iliopsoas bleeds can compress the femoral nerve; compartment syndrome requires urgent fasciotomy
Haematuria	30–50%	Treat with forced diuresis if not severe ^[2]	Hydration promotes urine flow → prevents clot formation in the renal collecting system which can cause obstruction; avoid tranexamic acid in haematuria — it stabilises clots in the urinary tract → risk of ureteric obstruction
Mucosal bleeding	30–50%	Add antifibrinolytic agents, e.g. tranexamic acid, aminocaproic acid ^[2]	Mucosal surfaces are rich in fibrinolytic activity (tissue plasminogen activator) → clots on mucosal surfaces break down rapidly; antifibrinolytics inhibit plasmin → stabilise the clot
Intracranial haemorrhage	80–100%	Immediate Factor IX infusion on suspicion; urgent CT head; neurosurgical consultation; admit to ICU	Treat upon suspicion, not diagnosis ^[2] — mortality is high if treatment is delayed
Major surgery	80–100% preop, then > 30–50% for 7–14 days	Coordinate with haematology; check inhibitor status preop; monitor factor levels	Plan replacement schedule in advance; continuous infusion may be preferable to bolus for major procedures

Tranexamic Acid — When to Use and When to Avoid

Tranexamic acid (an antifibrinolytic) is an excellent adjunct for mucosal bleeding (oral, nasal, menstrual) because mucosal surfaces have high local fibrinolytic activity. However, it is contraindicated in haematuria because stabilising clots in the urinary tract can cause ureteric obstruction and renal injury. It is also generally avoided with prothrombin complex concentrates (PCC) due to theoretical thrombotic risk.

7. Management of Inhibitors

Inhibitors in haemophilia: usually presents as poor response to factor replacement ^[2]

This is the most challenging aspect of haemophilia management. When a patient with Haemophilia B develops alloantibodies against infused Factor IX, standard Factor IX replacement becomes ineffective.

Feature	Detail
Occurrence	1.5–5% in Haemophilia B (vs. 20–30% in Haemophilia A) ^[2]
Risk factors	Generally highest in those with severe disease during first 50 days of exposure to factor ^[2]; large gene deletions (which produce no endogenous Factor IX) carry the highest risk
Detection	Require regular screening, in pre-op setting and when response suboptimal (< 75%) ^[2]
Bethesda assay	Positive if ≥ 0.6 BU/mL; high titre if ≥ 5 BU/mL ^[2]
Unique to Haemophilia B	Inhibitor development can be associated with anaphylaxis to Factor IX concentrates and nephrotic syndrome — these do NOT occur in Haemophilia A inhibitors

Alternative product (or other strategies) in acute bleeding ^[2]:

Strategy	Product	Mechanism	Notes
Bypassing agents	Recombinant activated Factor VIIa (rFVIIa, NovoSeven)	Activates Factor X directly on the activated platelet surface, bypassing the need for Factor IXa/VIIIa (tenase complex) entirely	Works regardless of inhibitor titre; short half-life (~2.5 hours) → requires frequent dosing (every 2–3 hours)
Bypassing agents	Activated Prothrombin Complex Concentrate (aPCC, e.g. FEIBA)	Contains activated Factors II, VII, IX, X; bypasses the inhibitor by providing alternative coagulation pathways	Caution in Haemophilia B with inhibitors + anaphylaxis history — FEIBA contains Factor IX and can trigger anaphylaxis
High-dose factor infusion	High-dose Factor IX	Overwhelms low-titre inhibitor with excess Factor IX	Only for low-titre patients (< 5 BU/mL) ^[2]; ineffective for high-titre
Plasmapheresis	Plasma exchange + transient Factor IX infusion	Physically removes the inhibitor antibody from circulation; followed by immediate Factor IX replacement	↓ inhibitor titre if above not available ^[2]; temporary measure

Anaphylaxis Risk with Factor IX in Inhibitor Patients

Haemophilia B patients with inhibitors (especially those with large F9 gene deletions) can develop severe anaphylaxis to Factor IX-containing products. This means that FEIBA (aPCC), which contains Factor IX, may also trigger anaphylaxis in these patients. In such cases, recombinant FVIIa (NovoSeven) is the preferred bypassing agent as it does not contain Factor IX. Always check inhibitor and anaphylaxis history before administering any Factor IX-containing product.

Immune tolerance induction (ITI) → curative treatment to eradicate inhibitor ^[2]

Aspect	Detail
Method	Repeated doses of factor (± immunosuppression) to induce immune tolerance ^[2]
Principle	By exposing the immune system to repeated, regular doses of Factor IX, the goal is to "teach" the immune system to tolerate Factor IX (similar in concept to allergen desensitisation)
Efficacy in Haem B	ITI is less successful in Haemophilia B than in Haemophilia A (~30% success rate vs. ~70–80% in Haem A); risk of anaphylaxis and nephrotic syndrome during ITI in Haemophilia B
Protocol	Various protocols exist; may require concurrent immunosuppression (rituximab, cyclophosphamide, or mycophenolate)
Monitoring	Factor IX levels, inhibitor titre (Bethesda assay), renal function (for nephrotic syndrome), allergic reactions

8. Novel and Emerging Therapies

The haemophilia landscape has been transformed in recent years by therapies that go beyond simple factor replacement.

Feature	Detail
Mechanism	Subcutaneous monoclonal antibody targeting antithrombin III → by reducing antithrombin (a natural anticoagulant), fitusiran rebalances haemostasis by allowing more thrombin generation even without Factor IX
Route	Subcutaneous injection, once monthly
Advantage	Works for both Haemophilia A and B, regardless of inhibitor status (it does NOT target Factor VIII or IX)
Status	FDA-approved (2024, as Alhemo) for routine prophylaxis in Haemophilia A and B (with or without inhibitors)
Risk	Thrombotic events (because you are reducing a natural anticoagulant); must be used cautiously with concurrent bypassing agents

Feature	Detail
Mechanism	Adeno-associated virus serotype 5 (AAV5) vector delivers a functional copy of the F9 gene (specifically the hyperactive Padua variant, FIX-R338L) to hepatocytes → patient's liver cells begin producing their own Factor IX
Route	Single intravenous infusion
Efficacy	In the HOPE-B trial, mean Factor IX activity rose to ~39% (moderate/mild range) at 18 months; 96% of patients discontinued prophylaxis; annualized bleeding rate dropped by ~54%
Duration	Appears durable for > 3–5 years based on available data; long-term durability remains uncertain
Limitations	Expensive (~$3.5 million USD per infusion); requires negative AAV5 antibodies (pre-existing immunity to AAV5 from prior viral exposure can neutralise the vector); hepatotoxicity (transaminitis) common in first 3–6 months, often managed with corticosteroids; NOT suitable for patients with active hepatitis, significant liver fibrosis, or pre-existing AAV5 antibodies; NOT suitable for children (liver is still growing → transgene may be diluted with hepatocyte division)
Status	FDA-approved (2022) and EMA-approved (2023); first gene therapy approved for any haemophilia

Gene Therapy for Haemophilia B — A Paradigm Shift

Haemophilia B is arguably the ideal candidate for gene therapy because: (1) it is a single-gene disorder; (2) even modest increases in Factor IX (from < 1% to > 5%) convert severe disease to mild phenotype; (3) Factor IX is produced primarily by the liver, which is an excellent target for AAV vectors; (4) the Padua variant (FIX-R338L) is 8× more active than wild-type Factor IX, meaning lower protein expression is sufficient. Haemophilia B gene therapy is actually more advanced than Haemophilia A gene therapy, partly because the F9 cDNA (~1.4 kb) is much smaller than F8 cDNA (~7 kb) and fits easily into AAV vectors.

Emicizumab (Hemlibra) is a bispecific antibody that mimics Factor VIIIa by bridging Factor IXa and Factor X. It is revolutionary for Haemophilia A (with or without inhibitors). However, emicizumab is NOT effective for Haemophilia B because the problem in Haemophilia B is lack of Factor IX itself — emicizumab requires Factor IXa to work (it bridges IXa to X). If there is no Factor IX, there is no Factor IXa for emicizumab to bridge.

Emicizumab, DDAVP for prophylaxis in haemophilia A ^[2] — note this is specific to Haemophilia A, NOT Haemophilia B.

Any haemophilia B patient undergoing surgery requires careful planning with haematology.

Phase	Action
Preoperative	Confirm current Factor IX level; check inhibitor status (Bethesda assay); plan Factor IX replacement schedule; crossmatch blood products; inform anaesthesia team
Intraoperative	Infuse Factor IX to target 80–100% preop; continuous infusion or bolus dosing depending on procedure length; avoid regional anaesthesia (epidural, spinal) unless Factor IX > 50% and no inhibitors
Postoperative	Maintain Factor IX > 30–50% for 7–14 days (depending on surgery type); monitor Factor IX levels twice daily initially; thromboprophylaxis usually NOT required (bleeding disorder provides natural protection, but balance thrombotic risk in immobilised patients)

While specific Factor IX concentrates are the standard of care, other blood products contain Factor IX and may be used in emergencies or resource-limited settings:

Product	Factor IX Content	Indication	Key Points
Fresh Frozen Plasma (FFP)	Contains all clotting factors, including Factor IX; ~1 IU/mL Factor IX	PT/aPTT > 1.5× control with active bleeding PLUS single/multiple clotting factor deficiency other than haemophilia A/B ^[15] — note that FFP is NOT first-line for haemophilia A/B	Large volume needed → risk of volume overload; risk of transfusion reactions; risk of bloodborne infection
Prothrombin Complex Concentrate (PCC)	3-factor PCC: Factor II, IX, X; 4-factor PCC: Factor II, VII, IX, X ^[15]	Provides ↑ amount of factors with ↓ volume cf FFP → ↓ risk of volume overload; generally reserved for torrential bleeding or ICH ^[15]	PCC can be used as an alternative Factor IX source; risk of thrombosis; caution: aPCC (FEIBA) contains activated factors → thrombotic risk + anaphylaxis risk in Haemophilia B with inhibitors
Cryoprecipitate	Contains fibrinogen, Factor VIII, vWF — does NOT contain significant Factor IX	vWD if desmopressin or factor concentrate inappropriate; fibrinogen deficiency; Factor XIII deficiency ^[15]	NOT useful for Haemophilia B — cryoprecipitate does not contain Factor IX

FFP and Cryoprecipitate for Haemophilia B?

FFP contains Factor IX but at low concentration (~1 IU/mL). To raise Factor IX significantly, you would need large volumes → risk of TACO (transfusion-associated circulatory overload). It is a last resort, not first-line. Cryoprecipitate does NOT contain Factor IX (only fibrinogen, Factor VIII, vWF, Factor XIII) → it is useless for Haemophilia B. Always use specific Factor IX concentrates (recombinant or plasma-derived) as first-line treatment ^[15].

Treatment	Contraindication / Caution
DDAVP	Contraindicated in Haemophilia B (no effect on Factor IX); also avoid in children < 2 years (risk of hyponatraemia/seizures)
Factor IX concentrates	Caution in patients with known inhibitors (may be ineffective); contraindicated if history of anaphylaxis to Factor IX (use rFVIIa instead)
FEIBA (aPCC)	Contains Factor IX → risk of anaphylaxis in Haemophilia B inhibitor patients; thrombotic risk; avoid concurrent use with tranexamic acid (thrombotic risk)
Tranexamic acid	Contraindicated in haematuria (risk of ureteric clot obstruction); caution with concurrent aPCC/FEIBA
Non-selective NSAIDs (aspirin, ibuprofen, naproxen)	Avoid — inhibit COX-1 → impair platelet aggregation → compound the bleeding tendency; use paracetamol or COX-2 inhibitors instead ^[2]
Intramuscular injections	Avoid — risk of intramuscular haematoma; use subcutaneous or intravenous routes
Bone marrow biopsy	Absolute contraindication in severe haemophilia unless factor cover is given first ^[2]^[13]
Regional anaesthesia (epidural/spinal)	Avoid unless Factor IX level > 50% and sustained; risk of epidural/spinal haematoma
Gene therapy (etranacogene dezaparvovec)	Contraindicated in: pre-existing AAV5 antibodies; active hepatitis/significant liver fibrosis; children (growing liver); current inhibitors with anaphylaxis history

Prognosis: life expectancy 63 years for severe disease, 75 years for mild/moderate disease ^[2]

Cause of mortality: 33% liver failure, 15% (severe) / 11% (mild) haemorrhage ^[2]

The high proportion of liver failure-related deaths reflects the historical burden of transfusion-transmitted hepatitis B and C. With current recombinant products and viral inactivation, this is expected to decrease in younger cohorts.
Haemorrhage (especially ICH) remains the leading acute cause of death.
With prophylactic regimens and gene therapy, life expectancy is expected to approach that of the general population for newly diagnosed patients.

High Yield Summary: Management of Haemophilia B

General measures: avoid trauma, non-contact sports, dental hygiene, avoid antiplatelets/anticoagulants, smallest gauge needles, MedicAlert ^[2]^[14].
Factor IX replacement is the cornerstone — standard rFIX (BeneFIX) or extended half-life rFIX (Alprolix, Idelvion, Rebinyn) for both prophylaxis and on-demand treatment.
DDAVP is NOT effective for Haemophilia B — it releases Factor VIII/vWF, NOT Factor IX ^[7].
Emicizumab is NOT effective for Haemophilia B — it mimics Factor VIIIa and requires Factor IXa, which is absent ^[2].
Prophylaxis is preferred for severe disease: primary (< 3y), secondary (≥ 2 joint bleeds), tertiary (after arthropathy) ^[2]^[14].
Acute bleeding: treat on suspicion for life-threatening bleeds; haemarthrosis = FIX + COX-2/Panadol + RICE; mucosal = FIX + tranexamic acid; haematuria = FIX + forced diuresis (avoid tranexamic acid) ^[2].
Inhibitors (~3–5%): bypassing agents (rFVIIa preferred over FEIBA due to anaphylaxis risk); immune tolerance induction (less successful than in Haem A) ^[2].
Gene therapy (etranacogene dezaparvovec/Hemgenix): single IV infusion of AAV5-FIX Padua variant; FDA-approved 2022; raises FIX to ~39%; paradigm-shifting.
FFP contains Factor IX but large volumes needed; cryoprecipitate does NOT contain Factor IX ^[15].
Prognosis: 63y severe, 75y mild/moderate; leading cause of death: liver failure (33%), haemorrhage (11–15%) ^[2].

Active Recall - Haemophilia B Management

References

^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B, Management principles, Inhibitors, Prognosis ^[5] Senior notes: Block A - Abnormal bleeding after tooth extraction_ bleeding tendency; thrombocytopenia.pdf — Treatment options in haemostatic disorders ^[7] Senior notes: Block A - Hematology Interactive Tutorial.pdf — Case 2, Management section ^[13] Senior notes: Ryan Ho Fundamentals.pdf — Marrow examination contraindications ^[14] Senior notes: Adrian Lui Pediatrics Notes.pdf — Haemophilia management principles ^[15] Senior notes: Ryan Ho Haemtology.pdf — FFP, Cryoprecipitate, PCC sections

Complications of Haemophilia B

Complications of Haemophilia B can be divided into those caused by the disease itself (bleeding-related) and those caused by treatment (replacement-related). Understanding the pathophysiology behind each complication explains why it occurs and how to prevent it.

1.1 Haemophilic Arthropathy

Complications: hemophilic arthropathy ^[7]

This is the single most common chronic complication of haemophilia and the leading cause of long-term disability.

Haemophilic arthropathy: occurs in up to 50% in severe haemophilia ^[2]^[14]

The sequence from a single joint bleed to end-stage arthropathy is a self-perpetuating cycle:

Initial haemarthrosis → blood fills the joint space.
Haemoglobin degradation → iron released from red blood cells is deposited in the synovial membrane as haemosiderin.
Iron-induced synovial inflammation → haemosiderin is directly toxic to synoviocytes; it triggers an inflammatory cascade with macrophage activation, release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), and generation of reactive oxygen species.
Synovial hypertrophy → the synovium becomes thickened, hyperaemic, and highly vascular (neovascularisation). This thickened, fragile synovium is more susceptible to mechanical irritation during joint movement → bleeds more easily even with trivial trauma.
Cartilage destruction → inflammatory enzymes (matrix metalloproteinases) and direct iron toxicity degrade articular cartilage; additionally, the synovial hyperplasia physically encroaches on the cartilage surface.
Secondary osteoarthritis → loss of cartilage → bone-on-bone articulation → subchondral cysts, osteophytes, joint space narrowing.
Fibrosis and contracture → chronic inflammation leads to capsular fibrosis → fixed flexion contractures → loss of range of motion.
End-stage arthropathy → joint ankylosis, severe deformity, chronic pain, disability.

Pathology: multifactorial due to synovial hypertrophy, cartilage destruction, 2° OA ^[2]^[14]

Target joint: a prior bleed results in joint damage and inflammation → predisposes to further bleeding ^[2]^[14]

Defined as a joint with ≥ 3 spontaneous bleeds within a consecutive 6-month period.
Why does a prior bleed predispose to further bleeding? Because the hypertrophied, neovascularised synovium is fragile — the new blood vessels are thin-walled and lack normal structural support, so they rupture easily with normal joint movement. This creates a vicious cycle: bleed → synovial hypertrophy → more bleeding → more synovial damage.
The most commonly affected joints are knees (45%), elbows, and ankles ^[2]^[14].

Presentation: typically as joint pain, stiffness, contractures developing in adolescence ^[2]^[14]

Feature	Description
Joint pain	Chronic, dull, aching; worse with activity; may have superimposed acute episodes
Stiffness	Morning stiffness; progressive loss of range of motion
Fixed flexion contracture	Most commonly at the knee and elbow; the joint cannot fully extend
Joint swelling	May be persistent (chronic synovitis) or episodic (acute bleeds)
Muscle wasting	Disuse atrophy of the periarticular muscles (e.g., quadriceps wasting around a chronically affected knee)
Leg length discrepancy	In children, chronic knee haemarthrosis causes hyperaemia of the adjacent growth plate → overgrowth of the affected limb
Crepitus	From secondary osteoarthritis — cartilage loss → bone-on-bone grinding

Prevention is far more effective than treatment. This is the primary rationale for prophylactic Factor IX replacement in severe Haemophilia B:

Primary prophylaxis (started < 3 years of age, before joint damage) prevents or significantly delays arthropathy development ^[2]^[14].
Once established, arthropathy is managed with physiotherapy, joint splinting, analgesia (COX-2 inhibitors/paracetamol — avoid NSAIDs ^[2]), and in severe cases, synovectomy (radiosynovectomy with yttrium-90 or surgical synovectomy) or total joint replacement.

The most feared acute complication and the leading cause of bleeding-related death.

Feature	Detail
Incidence	~3–5% lifetime risk in severe haemophilia; can occur at any age
Mechanism	Spontaneous (severe disease) or post-traumatic; inadequate Factor IX → failure to form a stable fibrin clot around ruptured cerebral vessels → expanding haematoma → mass effect → raised ICP
Neonatal risk	↑ risk of cephalhaematoma and ICH during delivery ^[2]^[14]; avoid head trauma + brain imaging ≤ 24h of life ^[2]^[14]
Presentation	Headache, vomiting, altered consciousness, seizures, focal neurological deficits
Management	Immediate factor replacement upon suspicion (treat upon suspicion, not diagnosis) ^[2]; urgent CT head; neurosurgical consultation

ICH in Neonatal Haemophilia

Insufficient evidence to recommend routine C/S ^[2]^[14] for pregnant carriers. However, instrumental delivery (forceps, vacuum extraction) and fetal scalp electrodes should be avoided as they increase the risk of cephalhaematoma and ICH in an affected male neonate. Brain imaging should be performed within 24 hours of life in confirmed or suspected haemophilia neonates ^[2]^[14].

Muscle haematoma: common, most commonly in calf, psoas muscles ^[2]^[14]

Complication	Mechanism	Clinical Features
Compartment syndrome	Can occur in acute bleeding into calf muscles → ischaemia, necrosis, fibrosis → subsequent contraction of Achilles tendon ^[2]^[14]	Pain out of proportion to injury; pain on passive stretch; tense swelling; paraesthesia; pulselessness (late sign). This is a surgical emergency — requires fasciotomy if factor replacement does not resolve it rapidly
Femoral nerve compression	Femoral nerve compression in large psoas bleed ^[2]^[14]	Hip flexion posture (flexion relieves compression on the nerve); numbness over anterior thigh (femoral nerve distribution); weakness of knee extension (quadriceps); loss of knee jerk reflex. Psoas bleed can mimic appendicitis or hip pathology
Mimicry of acute abdomen	Bleeding into abdominal wall can produce intense pain → mimic acute abdomen ^[2]^[14]	Important differential: a haemophilia patient with abdominal pain may have abdominal wall haematoma, retroperitoneal haematoma, or intussusception — NOT necessarily a surgical abdomen

Haemophilic pseudotumour: large encapsulated haematoma ^[2]^[14]

Feature	Detail
Site	Large muscle groups in pelvis/lower limbs and bone (long bone, pelvis, cranium) ^[2]^[14]
Pathology	Repeated bleeding with bone involvement → progressive cystic swelling with bone turnover and new bone formation ^[2]^[14]
Radiology	Well-defined, expansile, cystic lesion with calcified walls; may cause cortical thinning, erosion, or pathological fracture on X-ray/CT
Risk	Can erode through bone → pathological fracture; can compress adjacent neurovascular structures; infection of pseudotumour (rare but catastrophic)
Management	Factor IX replacement + observation for small pseudotumours; surgical excision for large, expanding, or complicated pseudotumours (always with full factor cover)

Oropharyngeal bleeding: may occur with minor trauma or dental procedures ^[2]^[14]

Complication	Mechanism	Clinical Significance
Aspiration or upper airway obstruction	Cough/vomiting can induce bleeding into posterior pharynx ^[2]^[14] → expanding pharyngeal/retropharyngeal haematoma can compress the airway	Life-threatening emergency; requires immediate Factor IX replacement; may need intubation or tracheostomy
GI bleeding and bowel wall haematoma	Bowel wall haematoma → obstruction, risk of intussusception ^[2]^[14]	Haemophilia patients with abdominal pain + bleeding tendency → consider intramural haematoma as cause of obstruction; distinguish from surgical causes

Haematuria: common in severe haemophilia but not associated with ↓ renal function ^[2]^[14]

Why does haematuria occur? Renal parenchyma has a rich blood supply; microscopic bleeds in the renal papillae or collecting system occur spontaneously in severe haemophilia.
Key management point: Treat with forced hydration/diuresis — this prevents clot formation in the renal collecting system/ureter that could cause obstruction. Avoid tranexamic acid in haematuria because stabilising clots in the urinary tract → ureteric obstruction → obstructive nephropathy.
Prognosis: Reassuringly, recurrent haemophilia-related haematuria does not lead to chronic kidney disease.

This is the most significant treatment-related complication and has been discussed in detail in the Management section. Key points specific to Haemophilia B:

Inhibitor development: develop alloAb vs exogenous factor due to congenital lack ^[2]^[14]

Feature	Haemophilia A	Haemophilia B
Frequency	~30% in severe disease	~3–5% in severe disease ^[2]^[14]
Risk factors	First 50 exposure days; large inversions in F8; family history	First 50 exposure days; large gene deletions in F9 ^[2]
Unique complications in Haemophilia B	None specific	Anaphylaxis to Factor IX concentrates; Nephrotic syndrome
Presentation	↑ risk of bleeding due to ↓ response to factor infusion ^[2]^[14]	Same + anaphylaxis + proteinuria
Detection	Bethesda assay: positive if ≥ 0.6 BU/mL; high titre if ≥ 5 BU/mL ^[2]	Same

Pathophysiology of inhibitor-associated anaphylaxis (unique to Haemophilia B):

Patients with large F9 gene deletions produce no endogenous Factor IX at all.
Their immune system has never been exposed to Factor IX protein → when exogenous Factor IX is infused, the immune system recognises it as completely foreign.
Initial exposure primes B-cells and T-cells; subsequent exposures trigger a vigorous IgE-mediated (Type I hypersensitivity) response → anaphylaxis.
This does NOT occur in Haemophilia A because most Haemophilia A patients (even those with severe disease due to intron 22 inversions) still produce some non-functional Factor VIII protein → the immune system has partial tolerance.

Pathophysiology of inhibitor-associated nephrotic syndrome (unique to Haemophilia B):

Immune complex deposition (Factor IX–anti-Factor IX antibody complexes) in the glomerular basement membrane during immune tolerance induction → membranous nephropathy → nephrotic syndrome (proteinuria, hypoalbuminaemia, oedema, hyperlipidaemia).
Requires monitoring of renal function and urine protein during ITI.

High Yield: Inhibitor Complications Unique to Haemophilia B

When a Haemophilia B patient develops inhibitors, they are at risk of two complications that do NOT occur in Haemophilia A: (1) anaphylaxis to Factor IX concentrates (IgE-mediated, especially with large gene deletions); (2) nephrotic syndrome (immune complex deposition during ITI). In such patients, FEIBA (aPCC) must also be avoided as it contains Factor IX → use recombinant FVIIa instead.

Bloodborne infections, e.g. HIV, HBV, HCV → ↓ risk with use of recombinant factors ^[2]^[14]

Infection	Historical Context	Current Risk
Hepatitis C (HCV)	Before viral inactivation of plasma-derived products (pre-1985), up to 90% of multiply-transfused haemophilia patients acquired HCV	Extremely low with recombinant products and viral inactivation of plasma-derived products; many older patients remain HCV-positive
Hepatitis B (HBV)	Similar historical risk; now largely preventable	All haemophilia patients should be vaccinated; recombinant products carry no risk
HIV	Devastating epidemic in the 1980s; thousands of haemophilia patients worldwide were infected through contaminated Factor VIII/IX concentrates	Eliminated with current recombinant products and rigorous screening/viral inactivation

Long-term consequences of bloodborne infections:

Cause of mortality: 33% liver failure ^[2] — the single largest cause of death in haemophilia patients historically. This reflects chronic HCV-related cirrhosis and hepatocellular carcinoma.
HCV/HBV co-infection with HIV accelerates liver disease progression.
Ongoing surveillance with liver function tests, fibroscan/elastography, and HCC screening is essential for HCV/HBV-positive patients.

In patients receiving plasma-derived Factor IX concentrates or other blood products (FFP, PCC):

Complication	Mechanism
Transfusion reactions	Febrile non-haemolytic reactions; allergic/urticarial reactions; anaphylaxis (especially in Haemophilia B with inhibitors)
TACO (Transfusion-Associated Circulatory Overload)	Volume overload from frequent large-volume infusions (more relevant for FFP than factor concentrates)
TRALI (Transfusion-Related Acute Lung Injury)	Donor anti-HLA/anti-HNA antibodies → neutrophil activation in pulmonary vasculature → non-cardiogenic pulmonary oedema
Iron overload	Less of a concern with factor concentrates (which are purified proteins, not whole blood); historically relevant for patients receiving repeated red cell transfusions for anaemia from chronic bleeding

Severe Haemophilia B patients (especially children) who require frequent intravenous Factor IX infusions may need a central venous access device (CVAD) such as a Port-a-Cath.

Complication	Details
Catheter-related bloodstream infection (CRBSI)	The most common CVAD complication in haemophilia; organisms: S. aureus, coagulase-negative staphylococci; risk increases with frequency of access
Catheter-related thrombosis	Paradoxical thrombosis can occur around the catheter tip despite the underlying bleeding disorder; may present as arm swelling, SVC syndrome
Mechanical malfunction	Catheter fracture, dislodgement, occlusion

Extended half-life (EHL) Factor IX products have reduced the need for CVADs by decreasing infusion frequency from 2–3× per week to every 7–14 days, and gene therapy aims to eliminate the need for infusions entirely.

Complication	Details
Chronic pain	From arthropathy; can lead to opioid dependence
Depression and anxiety	Chronic illness burden; activity restriction; fear of bleeding
School/work absenteeism	From acute bleeds and hospital visits
Social isolation	Inability to participate in contact sports; peer exclusion
Financial burden	Factor IX concentrates are extremely expensive; gene therapy costs ~$3.5 million per infusion
Family impact	Carrier mothers may experience guilt; genetic counselling is essential; siblings may need screening

Category	Complication	Key Point
Musculoskeletal	Haemophilic arthropathy	Most common chronic complication; up to 50% of severe patients; preventable with primary prophylaxis
	Target joint	≥ 3 bleeds in 6 months; vicious cycle of bleed → synovial hypertrophy → re-bleed
	Pseudotumour	Encapsulated haematoma with bone erosion
	Compartment syndrome	Calf muscle bleed → ischaemia → Achilles contracture
Neurological	ICH	Most feared; leading cause of bleeding death; treat on suspicion
	Femoral nerve palsy	From iliopsoas haematoma
Gastrointestinal	Bowel wall haematoma	→ obstruction, intussusception
Oropharyngeal	Airway obstruction	Posterior pharyngeal haematoma from coughing/vomiting
Urological	Haematuria	Common in severe; treat with hydration; avoid tranexamic acid
Treatment-related	Inhibitors (3–5%)	+ anaphylaxis + nephrotic syndrome (unique to Haemophilia B)
	Bloodborne infections	HCV, HBV, HIV (historical); 33% of mortality from liver failure
	Venous access complications	CRBSI, thrombosis
Psychosocial	Chronic pain, depression, absenteeism, financial burden	Multidisciplinary care essential

High Yield Summary: Complications of Haemophilia B

Haemophilic arthropathy (up to 50% of severe patients) is the most common chronic complication; caused by iron-mediated synovial toxicity → synovial hypertrophy → cartilage destruction → secondary OA → contractures ^[2]^[14].
Target joint (≥ 3 bleeds in 6 months): vicious cycle of bleeding → neovascularisation → re-bleeding; preventable with prophylaxis ^[2]^[14].
ICH is the most feared acute complication and leading cause of bleeding death; treat with Factor IX on suspicion before imaging ^[2].
Compartment syndrome (especially calf) → ischaemia → necrosis → fibrosis → Achilles tendon contracture ^[2]^[14].
Iliopsoas haematoma → femoral nerve compression → hip flexion, anterior thigh numbness, quadriceps weakness ^[2]^[14].
Oropharyngeal bleed → posterior pharyngeal haematoma → airway obstruction (cough/vomiting can trigger) ^[2]^[14].
Bowel wall haematoma → obstruction, intussusception ^[2]^[14].
Inhibitors (~3–5% in Haemophilia B) + anaphylaxis to Factor IX + nephrotic syndrome = complications unique to Haemophilia B ^[2]^[14].
Bloodborne infections (HCV, HBV, HIV) from contaminated products → 33% of mortality from liver failure; risk now minimal with recombinant products ^[2].
Prognosis: life expectancy 63 years (severe), 75 years (mild/moderate); leading cause of death: liver failure (33%), haemorrhage (11–15%) ^[2].

Active Recall - Complications of Haemophilia B

References

^[2] Senior notes: Ryan Ho Haemtology.pdf — Section 4.3.2 Haemophilia A and B, Clinical presentation, Late complications, Inhibitors, Prognosis ^[7] Senior notes: Block A - Hematology Interactive Tutorial.pdf — Case 2, Complications ^[14] Senior notes: Adrian Lui Pediatrics Notes.pdf — Haemophilia clinical presentation, Late complications, Inhibitor development

High Yield Summary

Haemophilia B = X-linked recessive deficiency of Factor IX (Christmas disease), accounting for ~15% of haemophilia cases ^[1]^[2].
Incidence: 1 in 15,000–30,000 live male births; ~1/3–1/2 have severe disease ^[2].
Classified by residual factor activity: Severe (< 1%), Moderate (1–5%), Mild (> 5% to < 40%) — severity determines bleeding pattern ^[1]^[2].
Gene: F9 at Xq26; > 1,100 mutations; ~30–40% sporadic (no family history) ^[2].
Factor IX is part of the intrinsic tenase complex (IXa + VIIIa); deficiency impairs thrombin amplification → weak fibrin clot — explains why bleeding is delayed and deep-seated.
Coagulation-type bleeding: haemarthrosis (hallmark), muscle haematoma, prolonged surgical/dental bleeding, ICH (most feared), rebleeding after initial cessation ^[5]^[6].
Laboratory: Normal PT + Prolonged APTT + Normal platelet count. Specific Factor IX assay confirms diagnosis ^[4]^[5].
In Hong Kong, Factor XII deficiency (~20% prevalence) is the most common cause of isolated prolonged APTT and does NOT cause bleeding — must distinguish from haemophilia ^[4].
Mixing study: corrects in factor deficiency; does NOT correct if inhibitor present ^[4].
Inhibitors develop in ~3–5% of Haemophilia B patients (lower than the ~25–30% in Haemophilia A) but can be associated with anaphylaxis to Factor IX concentrates — unique to Haemophilia B.
Haemophilia B Leyden: specific promoter mutations → Factor IX levels rise at puberty → patients can "outgrow" the disease.

High Yield Summary: Investigations for Haemophilia B

Normal PT + Prolonged APTT + Normal platelets → isolated intrinsic pathway defect ^[5]^[11].
Mixing study corrects → factor deficiency (not inhibitor) ^[4]^[11].
Low Factor IX assay (< 40%) with normal Factor VIII and normal vWF confirms Haemophilia B ^[1]^[2].
APTT may be normal in mild haemophilia → always request specific factor assays if clinical suspicion is high ^[2].
Bethesda assay screens for inhibitory alloantibodies against Factor IX (present in ~3–5% of severe cases) ^[2].
Genetic testing (F9 at Xq26) confirms diagnosis, enables carrier detection, prenatal diagnosis, and predicts inhibitor/anaphylaxis risk ^[2].
Factor XII deficiency (~20% in HK) is the most common isolated prolonged APTT — does NOT cause bleeding ^[4].
Severe haemophilia and DIC are absolute contraindications for bone marrow biopsy ^[2]^[13].
Viral serology (HBV, HCV, HIV) should be performed at baseline — historical risk from plasma-derived products ^[2].

High Yield Summary: Management of Haemophilia B

General measures: avoid trauma, non-contact sports, dental hygiene, avoid antiplatelets/anticoagulants, smallest gauge needles, MedicAlert ^[2]^[14].
Factor IX replacement is the cornerstone — standard rFIX (BeneFIX) or extended half-life rFIX (Alprolix, Idelvion, Rebinyn) for both prophylaxis and on-demand treatment.
DDAVP is NOT effective for Haemophilia B — it releases Factor VIII/vWF, NOT Factor IX ^[7].
Emicizumab is NOT effective for Haemophilia B — it mimics Factor VIIIa and requires Factor IXa, which is absent ^[2].
Prophylaxis is preferred for severe disease: primary (< 3y), secondary (≥ 2 joint bleeds), tertiary (after arthropathy) ^[2]^[14].
Acute bleeding: treat on suspicion for life-threatening bleeds; haemarthrosis = FIX + COX-2/Panadol + RICE; mucosal = FIX + tranexamic acid; haematuria = FIX + forced diuresis (avoid tranexamic acid) ^[2].
Inhibitors (~3–5%): bypassing agents (rFVIIa preferred over FEIBA due to anaphylaxis risk); immune tolerance induction (less successful than in Haem A) ^[2].
Gene therapy (etranacogene dezaparvovec/Hemgenix): single IV infusion of AAV5-FIX Padua variant; FDA-approved 2022; raises FIX to ~39%; paradigm-shifting.
FFP contains Factor IX but large volumes needed; cryoprecipitate does NOT contain Factor IX ^[15].
Prognosis: 63y severe, 75y mild/moderate; leading cause of death: liver failure (33%), haemorrhage (11–15%) ^[2].

High Yield Summary: Complications of Haemophilia B

Haemophilic arthropathy (up to 50% of severe patients) is the most common chronic complication; caused by iron-mediated synovial toxicity → synovial hypertrophy → cartilage destruction → secondary OA → contractures ^[2]^[14].
Target joint (≥ 3 bleeds in 6 months): vicious cycle of bleeding → neovascularisation → re-bleeding; preventable with prophylaxis ^[2]^[14].
ICH is the most feared acute complication and leading cause of bleeding death; treat with Factor IX on suspicion before imaging ^[2].
Compartment syndrome (especially calf) → ischaemia → necrosis → fibrosis → Achilles tendon contracture ^[2]^[14].
Iliopsoas haematoma → femoral nerve compression → hip flexion, anterior thigh numbness, quadriceps weakness ^[2]^[14].
Oropharyngeal bleed → posterior pharyngeal haematoma → airway obstruction (cough/vomiting can trigger) ^[2]^[14].
Bowel wall haematoma → obstruction, intussusception ^[2]^[14].
Inhibitors (~3–5% in Haemophilia B) + anaphylaxis to Factor IX + nephrotic syndrome = complications unique to Haemophilia B ^[2]^[14].
Bloodborne infections (HCV, HBV, HIV) from contaminated products → 33% of mortality from liver failure; risk now minimal with recombinant products ^[2].
Prognosis: life expectancy 63 years (severe), 75 years (mild/moderate); leading cause of death: liver failure (33%), haemorrhage (11–15%) ^[2].

Haemophilia B

1. Definition

2. Epidemiology

2.1 Incidence and Prevalence

2.2 Sex Distribution

2.3 Sporadic vs. Inherited

2.4 Hong Kong Context

3. Risk Factors

4. Anatomy and Function: Factor IX in the Coagulation Cascade

4.1 The Gene: F9

4.2 Factor IX in Coagulation

4.3 Factor IX as a Vitamin K-Dependent Factor

5. Aetiology (with Focus on Hong Kong)

5.1 Inherited Haemophilia B (the Vast Majority)

5.2 Acquired Causes of Factor IX Deficiency (Not True "Haemophilia B" but Important DDx)

5.3 Hong Kong-Specific Points

6. Classification

6.1 By Severity (Based on Residual Factor IX Activity)

6.2 By Presence of Inhibitors

7. Clinical Features

7.1 General Principles

7.2 Symptoms (with Pathophysiological Basis)

A. Age of Onset

B. Pattern of Bleeding

7.3 Signs (with Pathophysiological Basis)

A. Acute Bleeding Signs

B. Chronic Signs (from Recurrent Bleeding — Haemophilic Arthropathy)

C. Other Examination Findings

7.4 History Taking Approach [6][7]

8. Summary of Pathophysiology → Clinical Manifestation Link

Active Recall - Haemophilia B (Definition, Epidemiology, Aetiology, Classification, Clinical Features)

References

Framing: What Clinical Scenario Are We Differentiating?

1. Differential Diagnosis by Clotting Profile Pattern

2. Differential Diagnosis of Isolated Prolonged APTT (Normal PT)

A. Causes that DO Cause Bleeding

B. Causes that DO NOT Cause Bleeding (But Still Prolong APTT)

3. The Mixing Study: Key Differentiating Tool

4. Differentiating Haemophilia B from the Closest Mimics

A. Haemophilia A vs. Haemophilia B

B. Haemophilia B vs. Von Willebrand Disease (vWD)

C. Haemophilia B vs. Liver Disease

D. Haemophilia B vs. Vitamin K Deficiency

5. Differential Diagnosis of Haemarthrosis

6. Diagnostic Algorithm: Approach to Suspected Haemophilia B

7. Summary Table: Key Differentials for Haemophilia B

Active Recall - Haemophilia B Differential Diagnosis

1. Diagnostic Criteria for Haemophilia B

2. Diagnostic Algorithm

3. Investigation Modalities: Detailed Breakdown

3.1 First-Line Screening Investigations

A. Complete Blood Count (CBC)

B. Peripheral Blood Smear (PBS)

C. Clotting Profile: PT and APTT

D. Thrombin Time (TT)

3.2 Second-Line Confirmatory Investigations

A. Mixing Study (1:1 Mix)

B. Specific Factor IX Assay

C. Factor VIII Assay and vWF Panel

D. Factor XI and Factor XII Assays

3.3 Inhibitor Screening

3.4 Genetic Testing

A. F9 Gene Sequencing

B. Carrier Testing

3.5 Ancillary / Supportive Investigations

A. Imaging for Acute Bleeding

B. Baseline and Monitoring Bloodwork

C. Joint Assessment Scores

4. Summary: Diagnostic Investigation Pathway

Active Recall - Haemophilia B Diagnostic Investigations

1. Principles of Management

2. Management Algorithm (Overview)

3. General Measures (All Patients, All Severities)

4. Factor IX Replacement Therapy

4.1 Types of Factor IX Products

4.2 Dosing Principles

4.3 Prophylactic vs. On-Demand Therapy

On-demand therapy: only given during bleeding and invasive procedures [2][14]

Prophylactic therapy: to be given in the absence of bleeding [2][14]

5. DDAVP (Desmopressin) — NOT Effective in Haemophilia B