Polycythemia vera is a chronic myeloproliferative neoplasm characterized by clonal proliferation of myeloid cells, predominantly erythrocytes, typically driven by a JAK2 mutation, leading to increased red blood cell mass and hyperviscosity.

Exam domain	One-glance essentials
Definition / diagnosis	Chronic clonal MPN causing absolute erythrocytosis with trilineage myeloproliferation *JAK2-mutant, EPO-independent erythropoiesis* distinguishes PV from secondary polycythaemia ^[1]
Epidemiology / risks	Usually older adults; thrombotic risk is highest with age > 60 or prior thrombosis ^[2] For secondary erythrocytosis in HK, prioritise hypoxia and ectopic EPO from HCC/RCC ^[1]
Core mechanism	JAK2 V617F (~95–97%) or exon 12 (~3–5%) → constitutive JAK–STAT signalling ^[1] Increased red-cell mass → hypervolaemia/hyperviscosity; high cell turnover → pruritus and hyperuricaemia
Clinical picture	Facial plethora, splenomegaly, headache, dizziness and visual disturbance *Aquagenic pruritus after bathing* and aspirin-responsive erythromelalgia are classic ^[2] Arterial/venous or unusual-site thrombosis may be the first presentation ^[3]
Investigations	Thresholds: *Hb > 16.5/16.0 g/dL or Hct > 49/48% (M/F)* ^[1] *Low EPO → primary; high EPO → secondary; test JAK2 V617F then exon 12 ^[1] BM: hypercellular trilineage panmyelosis*; exclude BCR-ABL1
Management	*All patients: venesection to Hct < 45% + low-dose aspirin unless contraindicated* (for example active bleeding, aspirin intolerance or acquired vWD) ^[1]^[2] High risk: add hydroxyurea or peg-IFN-α; IFN preferred in young/pregnancy Ruxolitinib for hydroxyurea-intolerant/refractory disease
Complications	*Thrombosis is the leading morbidity/mortality: stroke, MI, DVT/PE, Budd–Chiari/splanchnic thrombosis ^[2] Extreme thrombocytosis → acquired vWD/bleeding Transformation: myelofibrosis ~10%, AML ~5%* ^[1]
Prevention / follow-up	Maintain Hct < 45%; control smoking, hypertension, lipids and diabetes Monitor CBC, ferritin, spleen, LDH and transformation clues; investigate secondary EPO sources
Exam traps	Rehydrate before diagnosing erythrocytosis; relative polycythaemia has normal red-cell mass *Do not replace venesection-induced iron deficiency routinely*—it is part of erythropoietic control ^[2] Hold aspirin if extreme thrombocytosis with acquired vWD/bleeding

References

^[1] Lecture slides: GC 086. Splenomegaly.pdf (Polycythaemia Vera section) ^[2] Senior notes: Ryan Ho Haemtology.pdf (pp76–77 — PV clinical features, management and prognosis) ^[3] Senior notes: Block A - Leg swelling and chest pain_ deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (MPN-associated unusual-site thrombosis)

Polycythemia Vera (PV) — Definition, Epidemiology, Etiology, Pathophysiology, Classification, and Clinical Features

Polycythemia vera (PV) is a chronic, clonal myeloproliferative neoplasm (MPN) arising from a somatic gain-of-function mutation in the Janus Kinase 2 (JAK2) gene within a haematopoietic stem cell, resulting in uncontrolled proliferation of the erythroid lineage (and, to a lesser extent, the granulocytic and megakaryocytic lineages), leading to an absolute increase in red cell mass ^[1]^[2]^[3].

Let's break the name down:

Poly- = many
-cyt- = cells
-haemia = blood
Vera = true (Latin)

So "polycythaemia vera" literally means "true increase in blood cells" — distinguishing it from apparent (spurious/relative) polycythaemia where the red cell mass is actually normal but haematocrit appears elevated due to a contracted plasma volume (e.g. dehydration).

Core Concept — MPN Family

PV is one of the four classical myeloproliferative neoplasms (MPNs): CML (BCR-ABL1 positive) and the three BCR-ABL1–negative MPNs — PV, essential thrombocythaemia (ET), and primary myelofibrosis (PMF). All arise from activating mutations in tyrosine kinases that drive growth factor–independent proliferation ^[2]^[3].

Key Distinguishing Feature

JAK2 mutation is present in 100% of PV cases (95–97% JAK2 V617F, 3–5% JAK2 exon 12). If JAK2 is negative, the diagnosis is essentially excluded. This makes PV unique among the BCR-ABL1–negative MPNs: ET and PMF can harbour CALR or MPL mutations instead ^[2]^[3].

Parameter	Detail
Incidence	~1.3 per 100,000/year (varies by geography; slightly higher in Western populations) ^[3]
Age	Median age at diagnosis ~60 years; ~25% diagnosed < 50 y, ~10% < 40 y ^[3]
Sex	Male > Female ≈ 2:1 ^[3]
Familial	Rarely associated with a family history — overwhelmingly sporadic ^[3]
Ethnicity	Higher incidence in individuals of Ashkenazi Jewish descent; lower in Asian populations, but still encountered in Hong Kong

Hong Kong context: While PV is less common in Chinese/Southeast Asian populations compared to Caucasians, it is still the most common primary polycythaemia in Hong Kong. The differential diagnosis in Hong Kong must heavily consider secondary causes — particularly hepatocellular carcinoma (HCC) producing ectopic EPO (given the high prevalence of chronic hepatitis B in HK) ^[1].

3. Anatomy and Function — Relevant Normal Physiology

To understand PV, you need to understand normal red cell production:

Erythropoietin (EPO) is a glycoprotein hormone produced predominantly by peritubular interstitial fibroblasts in the renal cortex in response to tissue hypoxia (detected via the HIF-2α pathway).
EPO binds to the EPO receptor (EpoR) on erythroid progenitors in the bone marrow.
EpoR lacks intrinsic kinase activity — it relies on a cytoplasmic tyrosine kinase called JAK2 (Janus Kinase 2) that is constitutively associated with the receptor.
Upon EPO binding → EpoR dimerisation → JAK2 transphosphorylation and activation → phosphorylation of downstream signalling molecules, principally:
- STAT5 → transcription of erythroid survival and proliferation genes (e.g. Bcl-xL)
- PI3K/AKT → cell survival
- RAS/MAPK → proliferation
This drives erythroid progenitor survival, proliferation, and differentiation into mature red blood cells.
Negative feedback: as Hb rises → improved tissue oxygenation → HIF-2α degradation → EPO production falls → erythropoiesis throttles back.

The spleen is relevant because:

It is a site of extramedullary haematopoiesis (can reactivate in MPNs)
It serves as a reservoir for red cells and platelets (pooling ~30% of platelets)
It is responsible for removal of senescent/abnormal red cells → splenic congestion and work hypertrophy can occur in PV
Splenomegaly is a clinical feature and a diagnostic criterion ^[1]^[2]

4. Etiology and Pathophysiology

PV is caused by a somatic activating mutation in the JAK2 gene located on chromosome 9p24 ^[1]^[2]^[3].

Mutation	Frequency	Mechanism
JAK2 V617F	95–97%	A point mutation (valine → phenylalanine at position 617) in the pseudokinase (JH2) domain of JAK2. The JH2 domain normally acts as an autoinhibitory domain that keeps the kinase inactive in the absence of ligand. V617F disrupts this autoinhibition → constitutive, ligand-independent activation of JAK2
JAK2 exon 12 mutations	~3–5%	Various insertion/deletion mutations in exon 12, affecting the linker region between the SH2 and pseudokinase domains. Also results in constitutive activation but with a somewhat different phenotype (often isolated erythrocytosis without thrombocytosis/leukocytosis)

Why JAK2 V617F is 'Gain-of-Function'

Think of JAK2 like a spring-loaded switch: the JH2 pseudokinase domain acts as the safety catch holding the kinase domain (JH1) in an "off" position. V617F breaks the safety catch → the kinase fires continuously, regardless of whether EPO is bound. The erythroid progenitors proliferate as if EPO were always present — but actual serum EPO is suppressed by the resulting polycythaemia (negative feedback on the kidney).

Risk factors for acquiring JAK2 mutations are poorly defined but may include:

Advanced age (somatic mutations accumulate over time — this is why median age is ~60)
? Environmental exposures (poorly characterised)
Clonal haematopoiesis of indeterminate potential (CHIP): JAK2 V617F can be detected as a CHIP variant in ~0.1–0.2% of the general population; most will never develop PV, but it represents a precursor state

4.2 Pathophysiology — The Three H's

The clinical consequences of PV flow from three interconnected pathophysiological states, sometimes remembered as the "Three H's" ^[3]:

The constitutively active JAK2 drives EPO-independent erythropoiesis → progressively rising Hb and Hct
RBC mass may be > 125% of expected for body mass and sex ^[3]
The expanded blood volume leads to:
- Facial plethora (ruddy, flushed face)
- Hypertension
- Conjunctival suffusion (dilated, congested conjunctival vessels)
- Retinal venous engorgement (seen on fundoscopy)

Blood viscosity increases exponentially with haematocrit (not linearly!) — once Hct > ~50%, viscosity rises steeply
This reduces cerebral blood flow disproportionately (the brain is exquisitely sensitive to viscosity changes):
- Headache, dizziness, tinnitus, visual disturbance ^[1]^[3]
- Transient visual phenomena (amaurosis fugax, scintillating scotoma, ophthalmic migraine) ^[3]
- Loss of concentration, lassitude ^[3]
Sluggish blood flow predisposes to both arterial and venous thrombosis (discussed below)

Overproduction of all three myeloid lineages → increased cell turnover → increased purine metabolism → hyperuricaemia → gout
Cytokine release (IL-31, histamine release from basophils/mast cells) → aquagenic pruritus ^[3]
Constitutional symptoms: night sweats, weight loss, fatigue

Thrombosis is the leading cause of morbidity and mortality in PV. The mechanism is multifactorial:

Factor	Explanation
Hyperviscosity	Sluggish flow → stasis → Virchow's triad
Erythrocyte–platelet interaction	Elevated Hct promotes margination of platelets toward vessel walls, increasing platelet–endothelial contact
Leukocytosis	Activated neutrophils release proteases, ROS, and NETs that promote a prothrombotic endothelial surface; WBC count is an independent risk factor for thrombosis in PV
Platelet activation	JAK2-mutant megakaryocytes produce hyperactivated platelets with increased thromboxane A₂ production
Endothelial dysfunction	Chronic exposure to elevated Hct/viscosity and inflammatory cytokines damages the endothelium
Acquired protein C resistance	Described in some MPN patients

Risk of arterial and venous thrombosis is a key clinical feature ^[1]. Arterial events (stroke, MI) are more common than venous (DVT/PE, portal vein thrombosis, Budd-Chiari syndrome, cerebral venous sinus thrombosis).

Paradoxically, PV (and ET) can also cause bleeding, especially when:

Platelet count > ~1,000–1,500 × 10⁹/L: extreme thrombocytosis causes acquired von Willebrand disease (type 2A) because large vWF multimers are adsorbed onto the excessive platelet surface and cleared → loss of high molecular weight vWF multimers → impaired primary haemostasis
Mucosal bleeding (GI, uterine, cerebral) can occur ^[3]
Epigastric distress, PUD, mucosal erosions — GI symptoms partly related to hyperviscosity-induced mucosal ischaemia + histamine release from basophils ^[3]

Aquagenic pruritus is highly specific to PV (~31%) ^[3]:

Defined as any strong sensation (itch, sting, tickle, burn) in the skin following contact with water without any visible skin changes ^[3]
The mechanism is incompletely understood but is thought to involve:
- ? Mast cell degranulation triggered by water/temperature changes ^[3]
- Elevated basophil counts → increased histamine release
- Increased IL-31 signalling (IL-31 is a pruritogenic cytokine produced by T cells; its receptor is upregulated in PV skin)
Characteristically occurs after a hot bath or shower ^[1]
Often one of the most distressing symptoms for patients — can severely impair quality of life

Erythromelalgia (~29%) ^[3]:

"Erythro-" = red, "-mel-" = limbs, "-algia" = pain
Burning pain in feet/hands associated with colour change (erythema, pallor, cyanosis) but with palpable pulses ^[3]
Caused by microvascular thrombotic complications — arteriolar occlusion by platelet-rich thrombi ^[3]
Responds dramatically to low-dose aspirin or cytoreductive therapy to reduce platelet count ^[3]
Also seen in ET

PV can transform into:

Myelofibrosis ("spent phase" or post-PV myelofibrosis): ~10% at 10–15 years ^[1]
- Progressive marrow fibrosis → extramedullary haematopoiesis → massive splenomegaly → cytopenias
Acute myeloid leukaemia (AML): ~5% overall ^[1]; quoted as 10% at 10 years, 25% at 25 years ^[2]
- Risk increased by use of alkylating agents (e.g. busulfan, chlorambucil) or radioactive phosphorus
- Hydroxyurea (current standard cytoreductive) appears to have a lower leukaemogenic risk

GC Lecture High Yield — Transformation Risk

Transformation to myelofibrosis (~10%), acute leukaemia (~5%) — this exact framing appears on the GC 086 Splenomegaly slide and is likely to be tested ^[1].

5. Classification

Before diagnosing PV, you must exclude secondary causes of erythrocytosis. This is the standard clinical approach framework ^[1]^[2]^[4]:

Key GC slide point: Exclude secondary causes — Absolute polycythaemia (primary vs. secondary EPO-mediated) and Apparent polycythaemia (normal RCM/low plasma volume → elevated HCT) ^[1]

Category	Red Cell Mass	EPO	JAK2	Key Causes
Apparent (Relative)	Normal	Normal	Negative	Dehydration, burns, diuretics, "stress polycythaemia" (Gaisbock syndrome)
Absolute — Primary (PV)	Increased	Low	Positive (100%)	Polycythaemia vera
Absolute — Secondary (Appropriate)	Increased	High	Negative	COPD, OSA, cyanotic CHD, high altitude, CO poisoning, methaemoglobinaemia
Absolute — Secondary (Inappropriate)	Increased	High	Negative	HCC, RCC, cerebellar haemangioblastoma, uterine fibroma, PKD

Hong Kong High Yield — Secondary Causes

In Hong Kong, the most important secondary cause to consider is HCC (hepatoma) producing ectopic EPO, given the high prevalence of chronic hepatitis B. RCC (renal cell carcinoma) is another classic cause. Always check EPO level and consider abdominal imaging ^[1]^[4].

The WHO 2022 (5th edition) classification refines the diagnostic criteria:

Diagnosis requires ALL 3 major criteria, OR the first 2 major + the minor criterion:

	Criterion	Detail
Major 1	Elevated Hb or Hct	Hb > 16.5 g/dL (male) or > 16.0 g/dL (female) OR Hct > 49% (male) or > 48% (female) OR increased red cell mass > 25% above mean normal predicted
Major 2	Bone marrow biopsy	Hypercellular for age with trilineage growth (panmyelosis), including prominent erythroid, granulocytic, and megakaryocytic proliferation with pleomorphic, mature megakaryocytes (differences in size)
Major 3	JAK2 mutation	JAK2 V617F or JAK2 exon 12 mutation
Minor	Subnormal serum EPO level	Below the reference range for normal

GC Slide High Yield — Diagnosis

Diagnosis by BM and JAK2 mutation (95%). Low EPO. ^[1]

Diagnostic thresholds: Hb > 16.5 g/dL / Hct > 49% (male); Hb > 16.0 g/dL / Hct > 48% (female) ^[1]

MPN	Key Mutation(s)	Key Cell Line	Hb	WBC	Plt	AML Risk
PV	JAK2 (100%)	Erythroid (↑↑)	↑↑	↑	↑	10% at 10y, 25% at 25y
ET	JAK2 (60%), CALR (25%), MPL (3%)	Megakaryocytic	N	N	↑↑	< 5%
PMF	JAK2 (60%), CALR (25%), MPL (7%)	Megakaryocytic + fibrosis	↓	Variable	Variable	6–18%
CML	BCR-ABL1 (100%)	Granulocytic	↓	↑↑	↑ (usually)	> 90% if untreated

^[2]^[4]

6. Clinical Features

6.1 Symptoms (with Pathophysiological Basis)

Symptom	Pathophysiological Basis
Headache	Increased blood viscosity → sluggish cerebral microcirculation → cerebral venous distension and impaired oxygen delivery to neurons → headache. The brain is the organ most sensitive to viscosity changes because of its high metabolic demand and reliance on laminar flow in small arterioles ^[1]^[3]
Dizziness / Vertigo	Hyperviscosity reduces blood flow through the vertebrobasilar system and vestibular apparatus → vestibular hypoperfusion → dizziness ^[1]^[3]
Tinnitus	Hyperviscous blood flowing through the cochlear microcirculation → turbulent flow → tinnitus
Visual disturbances — amaurosis fugax, scintillating scotoma, ophthalmic migraine	Retinal and optic nerve blood supply compromised by sluggish flow → transient retinal ischaemia (amaurosis fugax = "fleeting darkness") or cortical spreading depression (migraine variant) ^[3]
Loss of concentration, lassitude	Global cerebral hypoperfusion due to hyperviscosity → reduced cognitive function and fatigue ^[3]
Shortness of breath	Increased pulmonary vascular resistance from viscous blood + reduced oxygen delivery despite high Hb → exercise intolerance ^[3]

Symptom	Pathophysiological Basis
Aquagenic pruritus (~31%, highly specific to PV) — "pruritus particularly after a hot bath"	Any strong sensation (itch, sting, tickle, burn) in skin following contact with water without visible skin changes ^[3]. Mechanism: ? mast cell degranulation triggered by temperature change or water contact → release of histamine + other pruritogenic mediators; elevated basophils and mast cells in PV contribute; IL-31 signalling may also play a role ^[1]^[3]

Exam Pearl — Aquagenic Pruritus

If an exam question describes a patient with polycythaemia who develops itching after bathing — think PV. This is virtually pathognomonic when combined with elevated Hb/Hct. It differentiates PV from secondary polycythaemia (which does NOT cause pruritus).

Symptom	Pathophysiological Basis
Erythromelalgia (~29%) — burning pain in feet/hands with colour change (erythema → pallor → cyanosis) but palpable pulses	Microvascular thrombotic complications — platelet-rich arteriolar microthrombi occlude digital arterioles → ischaemic pain + reactive hyperaemia → erythema. Pulses remain palpable because large vessels are patent. Responds dramatically to low-dose aspirin (inhibits platelet TXA₂ production → prevents microvascular thrombus formation) ^[3]

Symptom	Pathophysiological Basis
Arterial thrombosis (stroke, MI, peripheral arterial occlusion)	Hyperviscosity → stasis; hyperactivated JAK2-mutant platelets → TXA₂ overproduction; leukocytosis → NETs and endothelial activation → Virchow's triad fulfilled ^[1]^[3]
Venous thrombosis (DVT, PE, splanchnic vein thrombosis including portal vein thrombosis, hepatic vein thrombosis / Budd-Chiari syndrome, cerebral venous sinus thrombosis)	Same mechanisms as above. Budd-Chiari syndrome (hepatic vein thrombosis) is a classic association — PV is one of the most common causes of BCS. Any young patient presenting with BCS should be screened for MPNs
Superficial thrombophlebitis	Venous stasis + hypercoagulability → superficial vein thrombosis ^[3]
Bleeding (GI, uterine, cerebral)	Paradoxical: when platelet count is very high (> ~1,000 × 10⁹/L) → acquired von Willebrand disease (type 2A) — excessive platelet surface area adsorbs and cleaves large vWF multimers → impaired primary haemostasis → mucosal bleeding ^[3]

Symptom	Pathophysiological Basis
Night sweats	High cell turnover → cytokine release (IL-1, IL-6, TNF-α) → hypothalamic thermoregulatory resetting ^[3]
Weight loss	Hypermetabolic state from massive marrow proliferative activity
Fatigue / Lassitude	Multifactorial: cytokine-mediated + hyperviscosity-induced cerebral hypoperfusion ^[3]

Symptom	Pathophysiological Basis
Gout / Gouty arthritis	Increased cell turnover → increased purine catabolism → hyperuricaemia → urate crystal deposition in joints ^[3]
Epigastric distress, PUD, mucosal erosions	Elevated histamine from basophilia/mast cells → stimulates gastric parietal cell H₂ receptors → increased HCl secretion → peptic ulceration; also mucosal ischaemia from hyperviscosity ^[3]
Hypertension	Expanded blood volume (hypervolaemia) + increased peripheral vascular resistance from viscous blood ^[3]

Microvascular symptoms refers to a constellation of symptoms driven by platelet-mediated arteriolar microthrombi ^[1]:

Erythromelalgia (as above)
Digital ischaemia
Transient visual disturbances
Atypical TIA-like symptoms

Sign	Pathophysiological Basis
Facial plethora — "ruddy cyanosis"	Expanded RBC mass → increased total Hb concentration → skin appears ruddy-red. When deoxygenated Hb > 5 g/dL (easier to reach when total Hb is very high) → a cyanotic tinge overlays the redness → ruddy cyanosis ^[3]
Conjunctival suffusion	Congested, tortuous conjunctival vessels due to expanded blood volume and hyperviscosity ^[3]
Retinal venous engorgement (on fundoscopy)	Fundoscopy reveals dilated, tortuous retinal veins — the retinal vasculature is directly visible and reflects systemic hyperviscosity/hypervolaemia ^[3]
Splenomegaly (± uncommonly hepatomegaly)	Multiple mechanisms: (1) Extramedullary haematopoiesis — the spleen resumes haematopoietic function (as it did in fetal life); (2) Congestion from increased cell mass filtered through splenic sinusoids; (3) Work hypertrophy from increased removal of abnormal/excess cells ^[1]^[3]
Extensive skin excoriation	Secondary to chronic, severe pruritus (aquagenic pruritus) → patient scratches vigorously → visible excoriations and scratch marks ^[3]
Stigmata of prior thrombotic events	Evidence of previous stroke (residual neurological deficit), DVT (leg swelling, post-thrombotic syndrome), or other vascular events ^[3]
Gouty tophi / Gouty arthritis	Chronic hyperuricaemia → urate crystal deposition → tophi in soft tissues (ears, fingers, elbows) and joints ^[3]

Clinical Examination Pearl

On a clinical exam, the classic PV patient is a middle-aged to elderly male with a red, plethoric face (looks like they've been sunburned), splenomegaly on abdominal examination, and may have scratch marks from aquagenic pruritus. Always check for hepatomegaly, gouty tophi, and signs of prior thrombotic events.

Pathophysiological Mechanism	Symptoms	Signs
Hypervolaemia (↑ RCM)	Headache, SOB	Facial plethora, hypertension, conjunctival suffusion, retinal venous engorgement
Hyperviscosity	Headache, dizziness, visual Sx, tinnitus, fatigue	Retinal vein engorgement
Thrombosis (multi-mechanism)	Stroke, MI, DVT/PE, BCS, erythromelalgia	Stigmata of thrombosis
Bleeding (acquired vWD)	GI/uterine/cerebral bleed	Bruising, mucosal bleeding
Hypermetabolism (↑ cell turnover)	Night sweats, weight loss, gout	Tophi, gouty joints
Cytokine/histamine release	Aquagenic pruritus, epigastric pain	Skin excoriations
Extramedullary haematopoiesis	Abdominal fullness, early satiety	Splenomegaly (± hepatomegaly)

While detailed diagnostic workup will be covered in the next section, the key laboratory features are:

Test	Expected Finding	Rationale
Hb / Hct	↑↑ (Hb > 16.5/16.0 g/dL; Hct > 49/48%) ^[1]	Increased erythropoiesis
WBC	↑ (neutrophilia ± basophilia)	Trilineage proliferation
Platelets	↑	Megakaryocyte proliferation
EPO	↓ (Low / Suppressed)	Negative feedback — high Hb suppresses renal EPO synthesis; also JAK2-mutant erythroid progenitors are hypersensitive to EPO (or EPO-independent) ^[1]
JAK2 V617F	Positive (95–97%)	Pathognomonic mutation
JAK2 exon 12	Positive in remaining ~3–5%	Alternative PV mutation
Serum uric acid	↑	Hypermetabolism / increased purine turnover
LDH	↑	Increased cell turnover
Vitamin B12	↑	Increased transcobalamin production by granulocytes
Iron / Ferritin	↓	Consumed by increased erythropoiesis; also venesection depletes iron
BM biopsy	Hypercellular with trilineage growth (panmyelosis), pleomorphic mature megakaryocytes	Diagnostic criterion ^[2]

High Yield Summary

PV is a clonal MPN caused by JAK2 mutations (100%: 95–97% V617F, 3–5% exon 12) → constitutive activation of JAK2-STAT signalling → EPO-independent erythroid proliferation.
Clinical triad = Hypervolaemia + Hyperviscosity + Hypermetabolism (the "3 H's").
Must exclude secondary polycythaemia — check EPO level: low EPO = primary (PV); high EPO = secondary. In HK, always consider HCC and RCC as causes of ectopic EPO.
Diagnostic thresholds: Hb > 16.5/16.0 g/dL or Hct > 49/48% (M/F) + JAK2 mutation + BM showing trilineage panmyelosis + low EPO.
Aquagenic pruritus (after hot bath) is virtually pathognomonic for PV (~31%).
Erythromelalgia (burning extremity pain with colour change but palpable pulses) responds to low-dose aspirin.
Thrombosis (arterial > venous) is the leading cause of morbidity/mortality — risk factors include hyperviscosity, leukocytosis, and platelet hyperactivation.
Transformation risk: myelofibrosis ~10%, AML ~5% ^[1].
Treatment overview (preview): Venesection (target Hct < 45%), cytoreductive therapy (hydroxyurea, interferon) for high-risk patients, and low-dose aspirin for all patients without contraindications ^[1].

Active Recall - Polycythaemia Vera: Definition to Clinical Features

Differential Diagnosis of Polycythemia Vera (Erythrocytosis)

The clinical challenge with PV is rarely "does this patient have PV specifically?" in isolation. Instead, you are presented with a patient who has elevated haemoglobin/haematocrit, and the question becomes: "Why is the Hb/Hct high?" The differential diagnosis is therefore structured as an approach to erythrocytosis — a systematic framework that allows you to identify PV while ruling out all its mimics.

SAQ Alert — Approach to Erythrocytosis

This framework is explicitly flagged as an SAQ-level topic in the Maksim Medicine Notes ^[4]. It is also the core structure of the GC 086 Splenomegaly slide on PV ^[1]. Master this algorithm — you will be asked to produce it from memory.

The very first branch point is to determine whether the elevated Hb/Hct reflects a true increase in the number of circulating red blood cells or merely a contraction of plasma volume making the existing RBCs appear more concentrated.

Category	Red Cell Mass	Plasma Volume	Core Mechanism
Apparent (Relative) polycythaemia	Normal	Low	Haemoconcentration — the red cells are concentrated in less plasma, so Hct looks artificially high
Absolute polycythaemia	Truly increased	Normal or increased	Genuine overproduction of red blood cells

GC 086 slide: "Apparent polycythaemia — Normal red cell mass / low plasma volume → elevated HCT" ^[1]. This is the first thing to exclude.

Why does this distinction matter? Because apparent polycythaemia requires no haematological workup — it requires rehydration and identification of the cause of volume depletion. Treating apparent polycythaemia with venesection or cytoreductive therapy would be harmful.

These patients have normal total red cell mass but a contracted plasma volume, making the Hct appear elevated.

Cause	Mechanism	Clinical Clues
Dehydration	Fluid loss (vomiting, diarrhoea, poor intake) → ↓ plasma volume → ↑ Hct	History of fluid loss, dry mucous membranes, tachycardia, postural hypotension. Hct corrects after rehydration
Diuretics	Forced diuresis depletes intravascular volume	Medication history — common in hypertensive patients on loop/thiazide diuretics
Burns	Capillary leak → plasma loss into interstitium → haemoconcentration	Obvious burn history; often dramatic Hct elevation acutely
Stress polycythaemia (Gaisbock syndrome)	Chronic low-grade plasma volume contraction in stressed, obese, hypertensive middle-aged men; mechanism not fully understood	Obese, hypertensive male, smoker; no splenomegaly, no JAK2 mutation, normal EPO ^[4]

Clinical Pearl — Gaisbock Syndrome

Gaisbock syndrome (also called "spurious" or "stress" polycythaemia) is a diagnosis of exclusion. The patient typically is a middle-aged, obese, hypertensive male smoker with a mildly elevated Hct. The critical distinction from PV: EPO is normal, JAK2 is negative, and red cell mass measurement (if performed by radiolabelled dilution) is normal. The elevated Hct results from chronically low plasma volume.

Once you have confirmed that the red cell mass is truly increased (clinically suggested by a persistently elevated Hb/Hct that does not correct with hydration), you must next determine whether the erythrocytosis is driven by the bone marrow itself (primary) or by an external signal (secondary).

The key discriminating investigation is the serum EPO level ^[1]^[4]:

EPO Level	Interpretation	Diagnosis
Low / Suppressed	The marrow is overproducing RBCs autonomously — it doesn't need EPO. Negative feedback suppresses renal EPO	Primary polycythaemia = Polycythaemia vera
Normal / Elevated	The marrow is responding to an external EPO stimulus — either physiologically appropriate or pathologically inappropriate	Secondary polycythaemia

GC 086 slide: "Exclude secondary causes → Absolute polycythaemia: (a) Primary e.g. Polycythaemia vera; (b) Secondary (EPO mediated) — (i) Hypoxia e.g. COPD, cyanotic heart disease; (ii) Abnormal EPO secretion, e.g. hepatoma" ^[1]

The only common primary cause is Polycythaemia vera (PV).

Condition	Mechanism	Key Distinguishing Features
Polycythaemia vera	JAK2 V617F (95–97%) or JAK2 exon 12 (3–5%) → constitutive JAK-STAT activation → EPO-independent erythroid proliferation	Low EPO, JAK2 positive (100%), trilineage BM panmyelosis, splenomegaly, aquagenic pruritus, thrombosis history ^[1]^[2]^[3]
Primary familial and congenital polycythaemia (PFCP)	Germline gain-of-function mutations in the EPO receptor (EPOR) gene → hypersensitivity to EPO → erythrocytosis	Rare; presents in childhood/young adulthood; isolated erythrocytosis (no thrombocytosis/leukocytosis); JAK2 negative; EPO low (because the receptor is hyperactive, less EPO is needed); family history present

Exam Tip — PFCP vs PV

Both PFCP and PV have low EPO and erythrocytosis. The distinguishing features are: (1) PFCP is familial (autosomal dominant), presents young, and has JAK2 negative; (2) PV is sporadic, presents older (median 60 y), and is JAK2 positive. PFCP also has isolated erythrocytosis without leukocytosis, thrombocytosis, or splenomegaly because it is an EPO-receptor defect confined to the erythroid lineage, whereas PV involves a stem cell–level mutation affecting all myeloid lineages.

3B. Secondary Absolute Polycythaemia (Normal/High EPO)

This is subdivided into physiologically appropriate (the body genuinely needs more oxygen-carrying capacity) versus pathologically inappropriate (a tumour or renal lesion is autonomously secreting EPO).

The kidney senses tissue hypoxia → HIF-2α stabilised → EPO gene transcription upregulated → more RBCs produced. This is the body working correctly — the problem is the underlying cause of hypoxia.

Cause	Mechanism of Hypoxia	Clinical Clues
COPD	Chronic alveolar hypoventilation + V/Q mismatch → chronic hypoxaemia → ↑ EPO	History of smoking, barrel chest, hyperinflation on CXR, low SpO₂, obstructive spirometry ^[1]^[4]
Obstructive sleep apnoea (OSA)	Intermittent nocturnal hypoxaemia → chronic EPO stimulation	Obesity, loud snoring, daytime somnolence, Epworth score ↑ ^[4]
Pulmonary hypertension	Right-to-left shunting or impaired gas exchange → hypoxaemia	Progressive SOBOE, signs of RV failure ^[4]
Cyanotic congenital heart disease / Eisenmenger syndrome	Right-to-left shunt → deoxygenated blood enters systemic circulation → chronic cyanosis → ↑ EPO	Central cyanosis, clubbing, childhood cardiac history, fixed split S2 (ASD), murmurs ^[1]^[4]
Carbon monoxide (CO) poisoning	CO binds Hb with ~250× affinity of O₂ → carboxyhaemoglobin cannot carry O₂ → functional anaemia + tissue hypoxia → ↑ EPO	Chronic low-level exposure (e.g. indoor gas heater), headache, "cherry-red" skin (classic but unreliable), elevated carboxyHb on co-oximetry ^[4]
Methaemoglobinaemia	Iron in haem ring maintained at ferric (Fe³⁺) state → haemoglobin cannot bind O₂ → tissue hypoxia → ↑ EPO	Congenital (Hb M disease, cytochrome b5 reductase deficiency) or acquired (drugs: dapsone, nitrates); "chocolate brown" blood that does not turn red on exposure to O₂; low SpO₂ on pulse oximetry but normal PaO₂ on ABG ^[5]^[6]
High-altitude residence	Low atmospheric PO₂ → chronic hypoxaemia → ↑ EPO	Travel/residence history; resolves on return to sea level
High-oxygen-affinity haemoglobinopathies	Mutant haemoglobin binds O₂ more tightly → left-shifted O₂ dissociation curve → impaired O₂ release to tissues → tissue-level hypoxia despite normal PaO₂ → ↑ EPO → secondary erythrocytosis	Family history; normal SpO₂/PaO₂ but tissue hypoxia; Hb electrophoresis or p50 measurement (low p50 = left-shifted curve) ^[6]

Why 'Appropriate' Matters

In all these conditions, the elevated EPO is a correct physiological response to genuine tissue hypoxia. The treatment is to fix the underlying cause of hypoxia (e.g. LTOT for COPD, CPAP for OSA, correct the cardiac defect) rather than to treat the erythrocytosis directly — although venesection may occasionally be needed if Hct rises to dangerously hyperviscous levels.

Here, EPO is being secreted by a tumour or pathological renal process — not in response to genuine hypoxia. The patient is not hypoxic (normal SpO₂, normal PaO₂), yet EPO is elevated.

Cause	Mechanism	Clinical Clues
Hepatocellular carcinoma (HCC / hepatoma)	Tumour cells produce EPO autonomously (paraneoplastic)	Extremely important in Hong Kong due to high chronic HBV prevalence; ↑ AFP, liver mass on imaging, chronic liver disease stigmata ^[1]^[2]^[4]
Renal cell carcinoma (RCC)	Tumour cells (clear cell subtype especially) produce EPO; also local renal ischaemia from tumour compression can stimulate normal peritubular cells	Classic triad: flank pain + haematuria + palpable mass (rare complete triad); paraneoplastic hypercalcaemia, hypertension ^[4]^[7]
Cerebellar haemangioblastoma	Highly vascular tumour produces EPO; may be sporadic or part of von Hippel-Lindau (VHL) syndrome	Posterior fossa signs (ataxia, nystagmus, headache), young adults; VHL syndrome includes retinal angiomas + renal cysts/RCC + phaeochromocytoma ^[4]
Uterine fibroma (leiomyoma)	Mechanism uncertain — possibly local tissue hypoxia or direct EPO secretion	Menorrhagia (paradoxically causing iron deficiency), pelvic mass on examination ^[4]
Adrenal tumours	Paraneoplastic EPO production	Incidental adrenal mass on imaging; may co-exist with Cushing's or phaeochromocytoma features ^[4]
Polycystic kidney disease (PKD)	Local renal parenchymal compression/ischaemia by expanding cysts → increased EPO production by surrounding peritubular cells	Bilateral enlarged palpable kidneys, family history (AD-PKD), hypertension, CKD ^[4]
Hydronephrosis	Obstruction → renal parenchymal ischaemia → ↑ EPO	Flank pain, history of stones or ureteric obstruction ^[4]
Renal artery stenosis	Reduced renal perfusion → juxtaglomerular hypoxia → ↑ EPO (+ ↑ renin → renovascular hypertension)	Refractory hypertension, abdominal bruit, flash pulmonary oedema

GC 086 slide specifically names: Hypoxia (COPD, cyanotic heart disease) and Abnormal EPO secretion (hepatoma) ^[1]. These are the must-know examples.

Hong Kong Exam Priority

In Hong Kong, always think HCC when you see secondary polycythaemia with inappropriate EPO elevation. Chronic hepatitis B carrier status is extremely prevalent, and HCC is among the top causes of cancer mortality. An erythrocytotic patient with known HBV, elevated AFP, or a liver lesion on imaging should prompt immediate hepatoma workup.

4. Other Conditions That Can Mimic PV Features

Beyond the pure "elevated Hb/Hct" differential, some conditions can present with overlapping features (e.g. splenomegaly + elevated blood counts) and must be distinguished from PV.

Condition	Key Feature	How to Distinguish from PV
Essential thrombocythaemia (ET)	Platelet count ↑↑, Hb normal	Normal Hb/Hct; JAK2 V617F positive in only 60–65% (also CALR 25%, MPL 3%); BM shows predominant megakaryocyte proliferation without trilineage panmyelosis ^[2]^[3]^[4]
Primary myelofibrosis (PMF)	Marrow fibrosis, splenomegaly, leukoerythroblastic blood film	Hb usually low (not high); tear-drop RBCs + nucleated RBCs on PBS; massive splenomegaly from extramedullary haematopoiesis; dry tap on BM aspirate; JAK2 60%, CALR 25%, MPL 7% ^[3]^[4]
Chronic myeloid leukaemia (CML)	WBC ↑↑, Philadelphia chromosome / BCR-ABL1 positive	Marked leukocytosis with bimodal distribution (neutrophils + myelocytes), basophilia; Hb usually low/normal; BCR-ABL1 positive on FISH/RT-PCR — if BCR-ABL1 positive, it is CML, not PV ^[8]^[4]

Key Sorting Rule Among MPNs

The first step when evaluating any suspected MPN is to check BCR-ABL1: if positive → it's CML. If negative → then differentiate among PV, ET, and PMF based on which lineage predominates (erythroid = PV, megakaryocytic = ET, fibrotic = PMF) and the mutation profile (PV = 100% JAK2; ET/PMF = JAK2 or CALR or MPL) ^[3]^[4].

PV often presents with thrombosis, sometimes in unusual locations (splanchnic veins, cerebral sinuses). The differential for thrombosis in unusual sites includes:

Condition	Distinguishing Feature
PV or other MPN	JAK2 mutation positive — screen for JAK2 in any unexplained splanchnic/cerebral venous thrombosis, even if CBC is normal ("patients with JAK2 mutation can clot even before developing the cytosis") ^[9]
Paroxysmal nocturnal haemoglobinuria (PNH)	Flow cytometry shows loss of GPI-anchored proteins (CD55, CD59); haemolytic anaemia with haemoglobinuria
Antiphospholipid syndrome	Lupus anticoagulant, anti-cardiolipin, anti-β2GP1 antibodies; pregnancy morbidity
Inherited thrombophilia	Protein C/S deficiency, Factor V Leiden, prothrombin G20210A mutation
Behçet disease	Oral/genital ulcers, pathergy, uveitis

Senior note (Leg swelling): "Patient develops a blood clot in an unusual site → screen for JAK2 mutation → MPN. When seeing mesenteric vein thrombosis, will screen for JAK2 mutations." ^[9]

Hyperviscosity is not unique to PV. Other causes of hyperviscosity with similar symptomatology include:

Condition	Cause of Hyperviscosity	Distinguishing Feature
Waldenström macroglobulinaemia	Excessive IgM paraprotein increasing serum viscosity	Lymphoplasmacytic lymphoma; serum protein electrophoresis shows IgM paraprotein; Hb is LOW (not high)
Multiple myeloma	High paraprotein (usually IgA or IgG) can increase viscosity	Bone pain, lytic lesions, renal impairment, M-spike on SPEP; Hb LOW
Leukaemias with hyperleukocytosis	WBC > 100 × 10⁹/L → leukostasis	WBC dramatically elevated, not Hb; often AML or CML in blast crisis

The key distinction is that in PV, the hyperviscosity is driven by elevated Hct (red cell mass), not by paraproteins or WBC — and the Hb is elevated, not depressed.

Feature	PV	Secondary (Appropriate)	Secondary (Inappropriate)	Apparent
Red cell mass	↑	↑	↑	Normal
EPO	Low	High	High	Normal
JAK2	Positive (100%)	Negative	Negative	Negative
SpO₂	Normal	Low	Normal	Normal
WBC / Plt	Often ↑ (trilineage)	Normal	Normal	Normal
Splenomegaly	Often present	Absent	Absent (unless HCC/cirrhosis)	Absent
Aquagenic pruritus	Characteristic	Absent	Absent	Absent
BM biopsy	Panmyelosis	Erythroid hyperplasia only	Erythroid hyperplasia only	Normal

Several red flags should raise your suspicion for PV rather than secondary polycythaemia:

Splenomegaly — secondary polycythaemia does not cause splenomegaly (unless the underlying disease itself does, e.g. CLD with HCC)
Aquagenic pruritus — virtually pathognomonic for PV
Thrombocytosis and/or leukocytosis accompanying the erythrocytosis — secondary polycythaemia only drives the erythroid lineage (EPO acts on erythroid progenitors), whereas PV affects all three myeloid lineages because the JAK2 mutation is in the stem cell
Thrombosis in unusual sites (portal vein, hepatic vein, mesenteric vein, cerebral venous sinus) — highly suggestive of MPN
Low serum EPO — the single most discriminating laboratory test

Why Secondary Polycythaemia Doesn't Cause Thrombocytosis or Leukocytosis

EPO acts specifically on the EPO receptor, which is expressed on erythroid progenitors only. Therefore, elevated EPO drives erythrocytosis in isolation. In PV, the JAK2 mutation is in the haematopoietic stem cell, which gives rise to all three myeloid lineages (erythroid, granulocytic, megakaryocytic). The constitutively active JAK2 affects all downstream progenitors → hence you see increases in Hb, WBC, and platelets simultaneously (trilineage). This is a fundamental first-principles distinction.

High Yield Summary — Differential Diagnosis of PV

Approach to erythrocytosis (SAQ!): First exclude apparent (relative) polycythaemia (dehydration, diuretics, burns, Gaisbock); then differentiate primary (PV, low EPO, JAK2+) from secondary (high EPO).
Secondary — Appropriate: COPD, OSA, cyanotic CHD/Eisenmenger, pulmonary HTN, CO poisoning, high altitude, high-O₂-affinity haemoglobinopathies.
Secondary — Inappropriate: HCC (hepatoma) — #1 in HK; RCC, cerebellar haemangioblastoma, uterine fibroma, adrenal tumours; renal causes (PKD, hydronephrosis, renal artery stenosis).
EPO level is the key discriminating test: low = primary (PV); high = secondary.
Clues favouring PV over secondary: splenomegaly, aquagenic pruritus, trilineage elevation (WBC/Plt also raised), thrombosis in unusual sites, JAK2+.
Other MPNs to distinguish: CML (BCR-ABL1+), ET (isolated thrombocytosis, normal Hb), PMF (fibrosis, low Hb, tear-drop cells).
Any unusual-site thrombosis (splanchnic, cerebral sinus) → screen JAK2 even if CBC is normal — patients can clot before developing overt cytosis.

Active Recall - Differential Diagnosis of Polycythaemia Vera

References

^[1] Lecture slides: GC 086. Splenomegaly.pdf (Polycythaemia Vera section) ^[2] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (Polycythemia Vera section) ^[3] Senior notes: Ryan Ho Haemtology.pdf (Section 3.3.2.1 Polycythaemia Vera, p.76; Section 3.3.2 MPNs overview, p.75) ^[4] Senior notes: Maksim Medicine Notes.pdf (Approach to erythrocytosis, p.170) ^[5] Senior notes: Block A - Many members of the family have anaemia.pdf (Haemoglobinopathies — increased O₂ affinity, methaemoglobinaemia, p.8) ^[6] Senior notes: Block A - Many members of the family have anaemia.pdf (Laboratory diagnosis of haemoglobinopathy, p.30) ^[7] Senior notes: Ryan Ho Urogenital.pdf (Approach to Haematuria — RCC features, p.130) ^[8] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (CML — Philadelphia chromosome, BCR-ABL1, p.22) ^[9] Senior notes: Block A - Leg swelling and chest pain_ deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (MPN-associated thrombosis — JAK2 screening, p.18)

Diagnostic Criteria, Algorithm, and Investigations for Polycythemia Vera

The WHO Classification of Haematolymphoid Tumours (5th edition, 2022) provides the formal diagnostic criteria. Understanding why each criterion exists helps you remember them:

	Criterion	Threshold / Detail	Rationale (First Principles)
Major 1	Elevated Hb or Hct	Hb > 16.5 g/dL or Hct > 49% (male)	These thresholds identify patients whose RBC mass is significantly above the population norm. Hct is an indirect surrogate for red cell mass — it is the proportion of whole blood occupied by red cells. The sex-specific thresholds account for normal testosterone-driven higher erythropoiesis in males
		Hb > 16.0 g/dL or Hct > 48% (female)
		OR increased red cell mass > 25% above mean normal predicted (by ⁵¹Cr-labelled RBC study)	Rarely performed now — only when the diagnosis is genuinely uncertain (e.g. borderline Hct in a dehydrated patient) ^[3]
Major 2	Bone marrow biopsy showing hypercellularity for age with trilineage growth (panmyelosis)	Prominent erythroid, granulocytic, and megakaryocytic proliferation with pleomorphic, mature megakaryocytes (variation in size)	The BM biopsy proves that the erythrocytosis is part of a clonal stem-cell disorder involving all three myeloid lineages — not just an isolated EPO-driven erythroid expansion. PV megakaryocytes are characteristically pleomorphic (mixed sizes) with mature cytoplasm, in contrast to ET (large/giant with hyperlobulated nuclei) or PMF (atypical, clustered, with cloud-like nuclei)
Major 3	JAK2 V617F or JAK2 exon 12 mutation	Detected on peripheral blood or bone marrow	The molecular hallmark — present in 100% of PV cases. Confirms the diagnosis is a clonal neoplasm driven by constitutive JAK-STAT activation, not a reactive process ^[1]^[2]^[3]
Minor	Subnormal serum EPO level	Below the reference range for the laboratory	A suppressed EPO proves the erythrocytosis is EPO-independent (i.e., the marrow is driving itself). In secondary polycythaemia, EPO is elevated (the marrow is responding to an external signal). This is the single most useful discriminating test between primary and secondary causes ^[1]^[4]

Diagnostic rule:

Diagnosis requires ALL 3 major criteria, OR the first 2 major criteria + the minor criterion ^[2]^[3]

In practical terms:

If JAK2 is positive (Major 3 met) → you still need Major 1 (elevated Hb/Hct) + Major 2 (BM biopsy) = 3 Majors → PV diagnosed
If JAK2 is negative (extremely rare, but JAK2 exon 12 can be missed by V617F-only assays) → you need Major 1 + Major 2 + Minor (low EPO) → PV diagnosed

GC Lecture High Yield — Diagnostic Criteria

"Diagnosis by BM and JAK2 mutation (95%). Low EPO." ^[1]

Diagnostic thresholds: Hb > 16.5 g/dL / Hct > 49% (male); Hb > 16.0 g/dL / Hct > 48% (female) ^[1]

The GC slide succinctly encapsulates the three pillars: BM morphology, JAK2 mutation, and low EPO. Know this cold for in-house exams.

Why Is BM Biopsy Required Even with JAK2+?

You might wonder: if JAK2 is present in 100% of PV, why bother with a bone marrow biopsy? Three reasons:

Baseline assessment: The BM establishes the degree of fibrosis at diagnosis. This is critical for later — if fibrosis progresses, it signals transformation to post-PV myelofibrosis.
Exclude other MPNs: JAK2 V617F is also found in 60–65% of ET and PMF. The BM morphology distinguishes PV (trilineage panmyelosis) from ET (predominant megakaryocyte proliferation) and PMF (megakaryocyte atypia + fibrosis).
Prognostic information: Presence of increased reticulin fibrosis at diagnosis carries prognostic implications.

Common Exam Mistake — Confusing PV Criteria Number

The diagnostic formula for PV is 3 Major or 2 Major + 1 Minor (3M or 2M + 1m) ^[2]. Don't confuse this with ET criteria (which require 4 Major or 3 Major + 1 Minor). The senior notes mnemonic: "PV: 3M or 2M+1m" ^[2].

The practical diagnostic pathway starts with detection of elevated Hb/Hct (often incidentally on routine bloods or during workup of thrombosis/hyperviscosity symptoms) and proceeds systematically:

Key logic points in the algorithm:

Always rehydrate first — a patient presenting acutely unwell with vomiting/diarrhoea may have spurious polycythaemia. Repeat CBC after adequate hydration before launching into a haematological workup.
EPO is the pivotal branch point — this single test separates the entire differential into primary vs. secondary. It should be drawn before any therapeutic intervention (e.g. venesection) as venesection-induced iron deficiency can alter EPO dynamics ^[4].
JAK2 V617F can be tested on peripheral blood — no need for bone marrow for the mutation test itself. BM is needed for morphological confirmation and baseline fibrosis assessment ^[3].
If EPO is normal/high → the workup shifts to finding the source of EPO — pulse oximetry, ABG, chest imaging (for lung disease), echocardiography (for cardiac shunts), abdominal imaging (for tumours), and tumour markers.

3. Investigation Modalities — Key Findings and Interpretation

Parameter	Expected in PV	Interpretation
Haemoglobin	↑↑ (typically > 16.5/16.0 g/dL M/F)	Reflects absolute increase in red cell mass — the hallmark of PV
Haematocrit	↑↑ (typically > 49/48% M/F)	Proportion of blood volume occupied by RBCs; directly correlates with viscosity. Hct > 45% is the target for treatment — above this, viscosity rises exponentially and thrombotic risk increases sharply
WBC	↑ (~49% of patients at diagnosis)	Trilineage proliferation: JAK2-mutant stem cell drives granulopoiesis as well. Predominantly neutrophilia ± basophilia
Platelets	↑ (~53% of patients at diagnosis)	Megakaryocyte proliferation from the JAK2-mutant clone; important for thrombosis risk stratification
MCV	Normal or ↓	If low, reflects iron deficiency from increased erythropoiesis consuming iron stores (or prior venesection). Importantly, MCV may be low even before treatment is started
RDW	May be ↑	Reflects anisocytosis — variation in RBC size, especially if concurrent iron deficiency exists

Senior notes: "CBC: polycythaemia (↑Hb, ↑Hct) ± leukocytosis (49%), thrombocytosis (53%)" ^[3]

Finding	Interpretation
Excessive normochromic normocytic (NcNc) RBCs	The overproduced red cells are morphologically normal — they are not dysplastic. PV produces too many normal-looking cells (unlike MDS, which produces abnormal-looking cells) ^[3]
Thrombocytosis with platelet anisocytosis	Giant platelets + small platelets reflect abnormal megakaryopoiesis
Neutrophilic leukocytosis ± basophilia	Trilineage proliferation; basophilia is particularly characteristic of MPN
No blasts, no dysplastic features	Presence of blasts would suggest transformation to AML or MDS; dysplasia would suggest MDS/MPN overlap
Leukoerythroblastic picture → if present, concerning for post-PV myelofibrosis	Nucleated RBCs + immature granulocytes appearing in peripheral blood indicates extramedullary haematopoiesis — the marrow is being replaced by fibrosis, forcing haematopoiesis into the spleen/liver ^[3]
Tear-drop cells (dacrocytes) → if present	Also indicates myelofibrosis transformation — RBCs are deformed by squeezing through fibrotic marrow spaces

PBS Pearl

In a standard PV presentation, the PBS is often unremarkable except for the sheer number of red cells — they look normal. This is in contrast to conditions like MDS or thalassaemia where the morphology itself is abnormal. The PBS becomes dramatic only when transformation occurs (leukoerythroblastic picture, tear-drops = spent phase myelofibrosis).

Result	Interpretation	Next Step
Low / Suppressed	Primary polycythaemia (PV) — the marrow is producing RBCs autonomously (JAK2-driven), and the resulting high Hb/Hct suppresses renal EPO through negative feedback	Check JAK2 V617F mutation → if negative, check JAK2 exon 12 ^[1]^[4]
Normal / High	Secondary polycythaemia — the marrow is responding to an external EPO stimulus	Investigate cause: pulse oximetry, ABG, imaging for tumours, renal assessment ^[4]

GC 086: "Low EPO" is listed as a diagnostic feature ^[1]

Maksim Notes: "EPO level: to differentiate primary (low) vs secondary (elevated)" ^[4]

Why is EPO low in PV? Two reasons working together:

Negative feedback: The high Hb delivers abundant oxygen to renal peritubular cells → HIF-2α is degraded → EPO gene transcription is suppressed
EPO-independence: JAK2 V617F activates the signalling cascade downstream of the EPO receptor, so the mutant clone doesn't need EPO to proliferate — the EPO is literally unnecessary

Test	Method	Result in PV	Notes
JAK2 V617F	Allele-specific PCR or quantitative RT-PCR on peripheral blood	Positive in 95–97% of PV	Can be performed on peripheral blood — no need for BM. The allele burden (% of mutant allele) can also be quantified and may correlate with disease severity and risk of transformation ^[1]^[3]
JAK2 exon 12	Sequencing of exon 12 on peripheral blood	Positive in ~3–5% of V617F-negative PV	These patients often present with isolated erythrocytosis (without leukocytosis/thrombocytosis) and have a more erythroid-predominant BM pattern. Still diagnostic of PV ^[3]

GC High Yield

"Diagnosis by BM and JAK2 mutation (95%)" ^[1]. The "95%" refers to JAK2 V617F specifically. The remaining ~5% have JAK2 exon 12, meaning 100% of PV is JAK2-mutant.

JAK2 is Not Exclusive to PV

JAK2 V617F is also found in ET (~60–65%) and PMF (~60–65%). A positive JAK2 V617F does not automatically mean PV — you must correlate with the CBC pattern (erythrocytosis = PV, thrombocytosis only = ET, cytopenias + fibrosis = PMF) and BM morphology. The distinguishing feature of PV is that JAK2 is positive in 100% — not that JAK2 is specific to PV ^[3]^[4].

The BM biopsy provides morphological confirmation and critical baseline assessment.

Component	Technique	Key Findings in PV	Interpretation
Aspirate	Smear for cytology; specimens for flow cytometry, cytogenetics, molecular studies	Hypercellular marrow with erythroid hyperplasia; increased megakaryocytes; no increase in blasts (< 5%)	Permits assessment of blast %, lineage morphology, and provision of material for molecular/cytogenetic studies ^[3]^[10]
Trephine biopsy	Core biopsy for histology; reticulin staining for fibrosis grading	Hypercellular for age with trilineage growth (panmyelosis): erythroid, granulocytic, and megakaryocytic expansion; pleomorphic, mature megakaryocytes (varying sizes); reticulin fibrosis grade 0–1 at diagnosis (grade ≥ 2 suggests early MF transformation)	The trephine is essential — it reveals architecture, cellularity, and fibrosis that aspirate alone cannot assess. Reticulin grading at baseline is critical for monitoring future MF transformation ^[3]

Morphological features that distinguish PV from other MPNs on BM:

Feature	PV	ET	PMF
Cellularity	Hypercellular (panmyelosis)	Normal/mildly ↑	Hyper → hypocellular (fibrotic)
Erythroid	↑↑	Normal	Variable
Granulocytic	↑	Normal	Variable
Megakaryocytes	Pleomorphic, mature, varying sizes	Large/giant, hyperlobulated, staghorn nuclei	Atypical, clustered, cloud-like/balloon-like nuclei
Fibrosis	Grade 0–1 (at diagnosis)	Grade 0–1	Grade 2–3 (overt)

Ryan Ho notes: "BM: to be done only when peripheral blood findings doubtful or suspect disease progression. Morphology: hypercellular marrow with trilineage proliferation (erythroid, granulocytic, megakaryocytic)" ^[3]

When Is BM Biopsy Performed?

According to WHO criteria, BM is a major criterion and should ideally be performed in all suspected PV cases. However, pragmatically, some centres may defer BM if all three of the following are met: (1) Hb/Hct meets threshold, (2) JAK2 V617F is positive, and (3) EPO is low — since these three together are virtually diagnostic. The 2022 WHO criteria still formally require BM for a complete diagnosis, particularly because it provides baseline fibrosis assessment and excludes early fibrotic transformation.

Test	Expected Finding	Clinical Significance
Serum LDH	↑	Reflects increased cell turnover and intramedullary haematopoietic activity. A markedly elevated LDH may suggest disease progression or transformation ^[3]
Serum uric acid	↑	Hyperuricaemia from increased purine catabolism due to high cell turnover — predisposes to gout and urate nephropathy
Serum vitamin B12	↑	Elevated due to increased transcobalamin I/III (haptocorrin) production by granulocytes. Historically, elevated B12 was used as a diagnostic clue for MPNs before molecular testing was available ^[11]
Serum ferritin / Iron studies	Ferritin ↓, Iron ↓	Iron stores are consumed by the voracious erythropoiesis even before treatment. After venesection, iron depletion is expected and even desirable (it helps limit erythropoiesis). Ferritin should be checked at diagnosis ^[4]
Serum protein electrophoresis	Usually normal	To exclude paraproteinaemia (Waldenström's, myeloma) as a cause of hyperviscosity — different mechanism, different disease
Coagulation profile	Usually normal; check if extreme thrombocytosis (> 1,000 × 10⁹/L) or bleeding symptoms	To screen for acquired von Willebrand disease — check vWF:RCo (ristocetin cofactor activity) and vWF multimer analysis
BCR-ABL1 (FISH / RT-PCR)	Negative	Essential to exclude CML, which is a separate MPN with a completely different treatment paradigm (TKIs). If BCR-ABL1 is positive → it's CML, not PV ^[8]

Maksim Notes: "If primary: JAK2 mutation, ferritin level" — ferritin is part of the standard initial workup ^[4]

Test	Purpose	Expected in PV
Pulse oximetry (SpO₂)	First-line screening to exclude hypoxia-driven secondary polycythaemia	Normal in PV (SpO₂ > 92%). If low → consider COPD, cyanotic CHD, OSA, pulmonary HTN
ABG with co-oximetry	Detects carboxyhaemoglobin (CO poisoning) and methaemoglobin; confirms oxygenation status	Normal PaO₂, normal carboxyHb (< 3% non-smoker, < 10% smoker), normal metHb (< 1%)

Maksim Notes: "Pulse oximetry" is listed as the first investigation in the approach to erythrocytosis ^[4]

These are not investigations for PV per se, but are part of the diagnostic algorithm to exclude secondary polycythaemia when EPO is normal/high:

Investigation	Purpose	Relevance
CXR	Screen for lung pathology (COPD, fibrosis, mass lesion)	Hyperexpanded lungs = COPD; mass lesion = possible lung cancer
Spirometry	Confirm obstructive/restrictive lung disease	If FEV1/FVC < 70% → COPD contributing to secondary polycythaemia
Sleep study (polysomnography)	Diagnose OSA	OSA causes intermittent nocturnal hypoxaemia → EPO-driven erythrocytosis
Echocardiography	Detect intracardiac shunts (Eisenmenger), pulmonary hypertension	Elevated RVSP or right-to-left shunt → secondary polycythaemia
Abdominal ultrasound / CT	Screen for HCC, RCC, PKD, hydronephrosis	Crucial in Hong Kong — AFP + hepatitis serology + abdominal imaging to exclude HCC in HBV carriers ^[1]^[2]
Serum AFP	Tumour marker for HCC	If elevated → imaging confirmation of HCC
CT/MRI brain (posterior fossa)	Screen for cerebellar haemangioblastoma	If young patient with polycythaemia + cerebellar signs → consider VHL syndrome
Renal Doppler ultrasound	Detect renal artery stenosis	Renovascular hypertension + polycythaemia → suspect RAS-driven EPO elevation

Test	Method	When Used
⁵¹Cr-labelled RBC mass measurement	Patient's RBCs are labelled with radioactive chromium-51, re-injected, and the dilution measured to calculate total red cell mass	NOT routinely used unless there is genuine diagnostic uncertainty — e.g. borderline Hb/Hct in a patient where dehydration cannot be reliably excluded, or in patients with concurrent conditions that confound Hb/Hct interpretation (e.g. iron deficiency masking true polycythaemia) ^[3]

Ryan Ho: "↑RBC mass by ⁵¹Cr-labelled RBC study → NOT used unless uncertain diagnosis of true polycythaemia" ^[3]

Test	Principle	Status
EEC assay	Marrow or peripheral blood progenitors are cultured without EPO. In PV, erythroid colonies form spontaneously (EPO-independent growth) — this is a hallmark of the JAK2-driven autonomous proliferation	NOT routinely performed — molecular testing for JAK2 has superseded this. Was historically important as a functional assay proving EPO-independence ^[3]

Ryan Ho: "Culture: classically form endogenous erythroid colony (EEC) when placed in culture, but NOT routinely done" ^[3]

A logical, stepwise investigation strategy (from least to most invasive, consistent with general principles of endocrine/haematology workup ^[10]):

Step	Investigation	Purpose
1	History & Physical Examination	Look for hyperviscosity symptoms, pruritus, plethora, splenomegaly, thrombosis history; elicit smoking/COPD history, cardiac history, family history
2	CBP + PBS	Confirm elevated Hb/Hct; assess WBC/Plt (trilineage?); evaluate RBC morphology
3	Pulse oximetry	Quick bedside screen for hypoxaemia → exclude common secondary causes ^[4]
4	Serum EPO level	The pivotal test — low = primary (PV); high = secondary ^[1]^[4]
5	JAK2 V617F (peripheral blood)	If EPO low → confirm PV. Can be ordered simultaneously with EPO if clinical suspicion high
6	JAK2 exon 12 (if V617F negative)	Catch the ~3–5% of PV with exon 12 mutations
7	Bone marrow biopsy (aspirate + trephine)	Morphological confirmation: panmyelosis; baseline reticulin fibrosis grading; exclude other MPNs/MDS; cytogenetics
8	Ancillary bloods: LDH, uric acid, ferritin, B12, LRFT, coagulation profile, BCR-ABL1	Assess metabolic consequences; exclude CML; baseline organ function
9	If EPO high: Imaging (CXR, ABG, echo, abdominal US/CT, AFP, sleep study)	Identify the source of EPO — lung disease, cardiac shunt, tumour, renal pathology

Investigation	PV	Secondary Polycythaemia	Apparent Polycythaemia
Hb/Hct	↑↑	↑↑	↑ (corrects with hydration)
WBC	Often ↑	Normal	Normal
Platelets	Often ↑	Normal	Normal
SpO₂	Normal	Low (appropriate) or Normal (inappropriate)	Normal
EPO	Low	High	Normal
JAK2	Positive (100%)	Negative	Negative
BCR-ABL1	Negative	Negative	Negative
BM biopsy	Panmyelosis, pleomorphic megakaryocytes, low fibrosis	Erythroid hyperplasia only	Normal
Ferritin	Low (iron consumed)	Variable	Normal
LDH	↑	Normal	Normal
Uric acid	↑	Normal	Normal

High Yield Summary — Diagnosis of PV

WHO 2022 criteria: 3 Major or 2 Major + 1 Minor ^[2].
- Major 1: Hb > 16.5/16.0 or Hct > 49/48% (M/F)
- Major 2: BM showing trilineage panmyelosis with pleomorphic mature megakaryocytes
- Major 3: JAK2 V617F or exon 12 mutation
- Minor: Subnormal EPO
Investigation sequence: CBP → SpO₂ → EPO level (pivotal test) → JAK2 V617F (peripheral blood) → BM biopsy (confirmation + baseline fibrosis).
EPO is the key branching test: low = primary (PV); high = secondary.
JAK2 V617F can be done on peripheral blood — BM is needed for morphology and fibrosis, not for the mutation test itself.
BCR-ABL1 must be checked to exclude CML — a JAK2-negative MPN with marked leukocytosis could be CML.
BM biopsy at diagnosis is essential to: (a) confirm morphology; (b) grade baseline fibrosis (reticulin); (c) distinguish PV from ET/PMF.
Ancillary tests: ferritin (often low), LDH (↑), uric acid (↑), B12 (↑), coag profile (if plt > 1000 → screen acquired vWD).
If EPO is high → investigate secondary causes: SpO₂/ABG for hypoxia; imaging for tumours (HCC, RCC); renal assessment.

Active Recall - Diagnostic Criteria and Investigations for PV

References

^[1] Lecture slides: GC 086. Splenomegaly.pdf (Polycythaemia Vera section) ^[2] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (Diagnostic criteria for MPN, p.24; Polycythemia Vera section, p.27) ^[3] Senior notes: Ryan Ho Haemtology.pdf (Section 3.3.2.1 Polycythaemia Vera — Laboratory findings, p.76–77; Section 3.3.2 MPNs overview, p.75) ^[4] Senior notes: Maksim Medicine Notes.pdf (Approach to erythrocytosis — Investigations, p.170) ^[8] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (CML — BCR-ABL1, p.22) ^[10] Senior notes: Ryan Ho Fundamentals.pdf (Bone marrow examination — techniques and indications, p.391) ^[11] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (B12 in MPN/CML, p.19)

Management of Polycythemia Vera

Before discussing individual treatments, it is essential to understand what we are trying to achieve — and why these goals matter from a pathophysiological standpoint:

Goal	Rationale (First Principles)
Prevent thrombosis	Thrombosis is the leading cause of morbidity and mortality in PV. The primary driver is hyperviscosity (Hct-dependent, rises exponentially above ~45%) combined with hyperactivated JAK2-mutant platelets and leukocyte-mediated endothelial damage. Reducing Hct and suppressing platelet/WBC counts directly lowers thrombotic risk
Control symptoms	Hyperviscosity symptoms (headache, dizziness, visual disturbance), aquagenic pruritus, erythromelalgia, and constitutional symptoms significantly impair quality of life
Minimise bleeding risk	Paradoxical bleeding occurs with extreme thrombocytosis (> ~1,000 × 10⁹/L) due to acquired von Willebrand disease. Cytoreduction to normalise platelet count mitigates this
Delay or prevent transformation	PV can transform to post-PV myelofibrosis (~10%) or AML (~5%). Avoiding leukaemogenic agents (alkylating agents, ³²P) is critical, especially in younger patients ^[1]^[2]^[3]
Target Hct < 45%	The landmark CYTO-PV trial (2013) demonstrated that maintaining Hct < 45% (vs. 45–50%) significantly reduced cardiovascular death and major thrombotic events. This is now the universal treatment target regardless of age or risk

Why Hct < 45% Specifically?

Blood viscosity increases exponentially with haematocrit — not linearly. Below 45%, the viscosity curve is relatively flat. Above 45%, each percentage-point rise produces a disproportionately large viscosity increase. The CYTO-PV trial provided clinical proof that this physiological inflection point translates into real patient outcomes: patients maintained at Hct < 45% had ~4× fewer thrombotic events than those allowed to run at 45–50%.

Treatment intensity is determined by thrombotic risk stratification. The classification is straightforward:

Risk Category	Definition	Treatment Approach
Low risk	Age ≤ 60 years AND no history of thrombosis	Venesection + low-dose aspirin
High risk	Age > 60 years OR any history of thrombosis	Venesection + low-dose aspirin + cytoreductive therapy

Ryan Ho notes: "Risk stratification: ≤60y + no Hx of thrombosis (low risk) vs others (high risk)" ^[3]

Why these two risk factors?

Age > 60: Cumulative cardiovascular comorbidities (atherosclerosis, hypertension, diabetes) compound the prothrombotic state of PV. Older patients have endothelial dysfunction that synergises with hyperviscosity.
Prior thrombosis: A previous thrombotic event is the single strongest predictor of recurrence. The patient has already demonstrated that their haemostatic balance has been tipped into a prothrombotic state.

Additional Risk Factors (Refined Risk Models)

More recent risk models also incorporate leukocytosis (WBC > 11 × 10⁹/L), JAK2 V617F allele burden, and cardiovascular risk factors (smoking, hypertension, diabetes) as modifiers. However, for exam purposes, the core stratification remains age and thrombosis history ^[3].

4. Treatment Modalities — Detailed

GC 086 slide: "Venesection" — listed as first-line treatment ^[1]

Block A Splenomegaly notes: "Venesection → remove blood" — for hyperviscosity symptoms ^[2]

Aspect	Detail
What it is	Physical removal of whole blood (like donating blood) to reduce Hct
Mechanism	Removing blood directly reduces the red cell mass → Hct falls → viscosity decreases → thrombotic risk falls. Over time, repeated venesection depletes iron stores → iron-deficient erythropoiesis is impaired → provides a "brake" on new RBC production
Method	Removal of 1 unit (~450–500 mL) of blood ≈ 3% decrease in haematocrit. Frequency: usually 0.5–2 units per week initially until target Hct < 45% is achieved, then as needed to maintain target ^[3]
Volume replacement	± Normal saline (NS) volume replacement to prevent hypovolaemia, especially in elderly or cardiovascularly compromised patients ^[3]
Target	Hct < 45% for ALL patients — both sexes, all ages
Indications	All PV patients — this is the universal first-line treatment regardless of risk category
Advantages	Rapid, effective, no drug side effects, no leukaemogenic risk
Limitations	Does not control WBC or platelet counts; does not address constitutional symptoms or splenomegaly; repeated venesection causes iron deficiency (which is actually partly therapeutic but can worsen fatigue); requires frequent hospital/clinic visits
Contraindications	Severe iron deficiency anaemia (Hb paradoxically already low — occurs in late-stage PV where iron depletion limits erythropoiesis); severe cardiovascular instability

Why Not Use Iron Replacement After Venesection?

This is a common exam question trap. After venesection, patients become iron-deficient — and this is intentional. Iron deficiency limits the marrow's ability to produce new red cells, acting as a natural brake on erythropoiesis. If you replace iron, you are essentially refuelling the very production line you are trying to slow down. Iron supplements are therefore contraindicated in PV unless there is severe, symptomatic iron deficiency that cannot be managed otherwise. Never give iron to a PV patient without explicit haematology consultation.

GC 086 slide: "Aspirin" — listed alongside venesection and cytoreduction ^[1]

Block A Splenomegaly notes: "Aspirin" — for risk of arterial and venous thrombosis + microvascular symptoms ^[2]

Aspect	Detail
What it is	Low-dose acetylsalicylic acid (aspirin)
Dose	40–100 mg PO daily or BD ^[3]
Mechanism	Irreversibly inhibits cyclooxygenase-1 (COX-1) in platelets → blocks thromboxane A₂ (TXA₂) synthesis → reduces platelet aggregation and activation. Since platelets are anucleate (no nucleus, no new protein synthesis), the inhibition lasts the lifetime of the platelet (~7–10 days). In PV, JAK2-mutant megakaryocytes produce hyperactivated platelets with excessive TXA₂ — aspirin directly counteracts this
Indications	All PV patients unless contraindicated. Evidence from the ECLAP trial (2004) showing ~60% reduction in combined risk of non-fatal MI, non-fatal stroke, and cardiovascular death
Benefits beyond thromboprophylaxis	Controls microvascular symptoms including erythromelalgia (responds dramatically to aspirin), transient visual disturbances, and digital ischaemia ^[2]^[3]. Also partially ameliorates pruritus and GI symptoms in some patients
Contraindications / Cautions	Extreme thrombocytosis (plt > ~1,000–1,500 × 10⁹/L) — paradoxically, aspirin can worsen bleeding in these patients because they already have acquired von Willebrand disease (loss of large vWF multimers adsorbed onto excess platelets); adding antiplatelet therapy on top increases haemorrhagic risk. Reduce platelet count with cytoreduction first, then start aspirin. Also: active peptic ulcer disease, aspirin hypersensitivity

Why BD Aspirin in Some Patients?

The standard aspirin dose is once daily. However, in PV, the platelet turnover is accelerated (JAK2-driven megakaryopoiesis produces new platelets faster than normal). Newly produced platelets are not yet inhibited by the previous day's aspirin. Some haematologists therefore use BD dosing (e.g. 40 mg twice daily) to ensure continuous COX-1 inhibition across the full platelet lifespan. This is a nuanced point for clinical practice.

4.3 Cytoreductive Therapy

Cytoreduction means reducing the production of blood cells at the marrow level — essentially "turning down the tap" rather than just draining the overflow (which is what venesection does).

GC 086 slide: "Cytoreductive: hydroxyurea, Interferon" ^[1]

Block A Splenomegaly notes: "Cytoreductive: oral hydroxyurea, Interferon" ^[2]

Aspect	Detail
Drug name breakdown	Hydroxy-urea → "hydroxylated urea" — a simple chemical modification of urea
Mechanism of action	Ribonucleotide reductase inhibitor → blocks the conversion of ribonucleotides to deoxyribonucleotides → inhibits DNA synthesis → suppresses proliferation of all rapidly dividing cells, including erythroid, granulocytic, and megakaryocytic precursors ^[3]
Why it works in PV	By inhibiting DNA synthesis in the hyper-proliferative JAK2-mutant clone, HU reduces production of all three cell lines → lowers Hb/Hct, WBC, and platelet counts simultaneously
Dose	Starting dose typically 15–20 mg/kg/day PO, titrated to response and CBC
Indications	First-line cytoreductive agent for high-risk PV patients ^[3]; also used in low-risk patients with poor venesection tolerance, progressive splenomegaly, or aspirin-refractory symptoms
Side effects	Cytopaenias (dose-limiting — the therapeutic effect and toxicity are on the same spectrum); mucocutaneous ulcers (oral ulcers, leg ulcers); diarrhoea; peripheral neuropathy; skin cancer (long-term use); teratogenicity (absolutely contraindicated in pregnancy); macrocytosis/megaloblastic anaemia (because it inhibits DNA synthesis — same mechanism as folate/B12 deficiency, producing large RBCs) ^[3]
Leukaemogenic risk	HU is not definitively proven to be leukaemogenic as a single agent — this is a longstanding debate. Current consensus is that HU has a low leukaemogenic potential, especially compared to alkylating agents or ³²P. It is acceptable for long-term use in most patients ^[3]
Monitoring	Regular CBP (every 2–4 weeks initially, then every 1–3 months once stable) to avoid over-suppression (neutropenia, thrombocytopenia, anaemia); renal and hepatic function; watch for skin lesions

Aspect	Detail
Drug name breakdown	"Peg-" = PEGylated (polyethylene glycol conjugated to the protein, extending its half-life so it can be given less frequently); "Interferon-alpha" = a cytokine normally produced by immune cells in response to viral infection
Mechanism of action	Multiple: (1) Antiproliferative — directly inhibits proliferation of JAK2-mutant haematopoietic stem cells; (2) Immunomodulatory — activates NK cells and cytotoxic T cells that can preferentially target the mutant clone; (3) Pro-apoptotic — promotes apoptosis of JAK2-mutant progenitors. Uniquely, IFN-α can reduce JAK2 V617F allele burden over time — suggesting it may preferentially suppress the mutant clone rather than just indiscriminately suppressing all haematopoiesis
Dose	Peg-IFN-α2a (Pegasys) or ropeginterferon-α2b (Besremi): subcutaneous injection, typically every 2 weeks (dose varies)
Indications	Preferred in younger patients (< 40 years) — no leukaemogenic risk, potential for molecular response (JAK2 allele burden reduction); preferred in pregnancy/women of childbearing age — HU is teratogenic; IFN-α is considered safe in pregnancy; patients intolerant of HU ^[1]^[2]^[3]
Side effects	Flu-like symptoms (fever, myalgia, fatigue — especially early in treatment); depression/neuropsychiatric effects; autoimmune phenomena (thyroiditis, hepatitis); injection-site reactions; fatigue
Contraindications	Active severe depression or psychosis; active autoimmune hepatitis; decompensated liver disease
Advantages over HU	No leukaemogenic risk; potential for molecular remission (↓ JAK2 allele burden); safe in pregnancy

Ropeginterferon-α2b (Besremi) — Modern IFN

Ropeginterferon-α2b is a mono-PEGylated interferon specifically approved for PV (FDA 2021, EMA 2019). It has a longer half-life than peg-IFN-α2a, allowing every-2-week or even monthly dosing with improved tolerability. It is increasingly used as a first-line alternative to HU, particularly in younger patients. The PROUD-PV/CONTINUATION-PV trials showed non-inferiority to HU with better molecular responses.

Agent	Mechanism	When Used	Important Caveats
Ruxolitinib	JAK1/JAK2 inhibitor → directly targets the constitutively active JAK2 pathway. "Ruxo-" from its chemical name; "-tinib" = tyrosine kinase inhibitor	Reserved for patients refractory/intolerant to HU and IFN ^[3]. Particularly effective for splenomegaly reduction and symptom control (pruritus, constitutional symptoms). The RESPONSE trial showed superiority over best available therapy in HU-resistant/intolerant PV	Side effects: cytopaenias (especially anaemia and thrombocytopenia — ironically, since you're inhibiting the very pathway driving overproduction); increased infection risk (herpes zoster reactivation, opportunistic infections); weight gain; hyperlipidaemia. Does not eradicate the JAK2-mutant clone — disease returns on discontinuation
Busulfan	Alkylating agent → cross-links DNA → cytotoxic to rapidly dividing cells	Mainly reserved for elderly patients (> 75–80 years) where other options are poorly tolerated. Low doses are effective in controlling counts ^[3]	Leukaemogenic — increases AML transformation risk with prolonged use. Avoid in younger patients ^[3]. Pulmonary fibrosis (rare). "Busulfan lung" is a classic toxicity
Anagrelide	Inhibits megakaryocyte maturation → selectively reduces platelet production without significantly affecting RBC or WBC counts	If isolated thrombocytosis is the dominant problem (unusual in PV where erythrocytosis predominates, but can occur) ^[3]	Does not control Hb/Hct or WBC; cardiac side effects (palpitations, heart failure — positive inotropic effect); headache; fluid retention
Radioactive phosphorus (³²P)	Incorporated into DNA of rapidly dividing cells → beta radiation causes DNA damage → cytotoxic	Rarely used now — historically important but largely abandoned due to high leukaemogenic risk. May still be considered in very elderly patients with limited life expectancy who cannot tolerate other treatments ^[3]	Strongly leukaemogenic — significantly increases AML risk. Avoid in younger patients ^[3]

Ryan Ho notes: "Avoid leukemogenic agents, eg. alkylating agents, radioactive phosphorus (³²P), esp in young patients" ^[3]

GC Lecture High Yield — Treatment Summary

The GC 086 slide lists the three treatment pillars:

"Venesection" — to control Hct

"Cytoreductive: hydroxyurea, Interferon" — for high-risk patients

"Aspirin" — for thromboprophylaxis and microvascular symptoms

This is the exact level of detail expected for in-house exams ^[1].

An often-overlooked but critically important aspect of PV management is aggressive cardiovascular risk factor control. Since thrombosis is the main killer, reducing atherosclerotic risk factors synergises with PV-specific treatment:

Intervention	Rationale
Smoking cessation	Smoking causes endothelial dysfunction, platelet activation, and chronic CO exposure (worsening functional polycythaemia). It independently increases thrombotic risk on top of PV
Hypertension control	PV patients are often hypertensive (hypervolaemia). Uncontrolled HTN accelerates atherosclerosis and increases stroke risk
Lipid management	Statin therapy per standard cardiovascular guidelines if indicated
Diabetes management	Tight glycaemic control reduces microvascular and macrovascular disease
Weight management	Obesity promotes venous stasis (Virchow's triad) and is associated with worse MPN outcomes

Symptom	Management	Mechanism
Aquagenic pruritus	First-line: Antihistamines (limited efficacy); More effective: ruxolitinib (highly effective for pruritus); peg-IFN-α; phototherapy (narrow-band UVB); SSRIs (paroxetine, fluoxetine — modulate serotonin-mediated itch pathways)	Pruritus in PV is likely driven by mast cell degranulation, basophil histamine release, and IL-31. Standard antihistamines often disappoint because the itch is not purely histamine-mediated; ruxolitinib addresses the upstream JAK2-driven cytokine dysregulation
Erythromelalgia	Low-dose aspirin (40–100 mg); cytoreduction if refractory	Microvascular platelet thrombi → aspirin inhibits TXA₂ → prevents arteriolar thrombosis. Responds dramatically ^[2]^[3]
Gout / Hyperuricaemia	Allopurinol (xanthine oxidase inhibitor); ensure adequate hydration; avoid thiazide diuretics	Reduced uric acid production; allopurinol blocks the final step of purine catabolism
Peptic ulcer disease	PPI (proton pump inhibitor)	Histamine from basophils/mast cells stimulates parietal cell H₂ receptors → HCl hypersecretion. PPI blocks the final common pathway (H⁺/K⁺-ATPase)
Constitutional symptoms (fatigue, night sweats)	Ruxolitinib is the most effective agent; peg-IFN-α also helps	JAK inhibition reduces inflammatory cytokine production (IL-6, TNF-α, IL-1β) driving constitutional symptoms

7. Management in Special Populations

Principle	Detail
Risk	PV in pregnancy is high risk for both maternal thrombosis and placental insufficiency (IUGR, pre-eclampsia, miscarriage) due to hyperviscosity in the uteroplacental circulation
Venesection	Continue — safe in pregnancy
Aspirin	Low-dose aspirin (75–150 mg) throughout pregnancy — recommended for thromboprophylaxis and to reduce pre-eclampsia risk
Cytoreduction	Peg-IFN-α is the only cytoreductive agent safe in pregnancy. HU is teratogenic (absolutely contraindicated). Ruxolitinib — insufficient safety data, avoid
Anticoagulation	LMWH (e.g. enoxaparin) may be added for patients with prior thrombosis or additional risk factors

Principle	Detail
Treatment horizon	These patients will require treatment for decades → minimising long-term toxicity is paramount
Preferred cytoreduction	Peg-IFN-α — no leukaemogenic risk, potential for molecular remission (JAK2 allele burden reduction), safe for fertility
Avoid	Alkylating agents (busulfan), radioactive phosphorus (³²P) — leukaemogenic with prolonged use ^[3]
HU	Acceptable if IFN-α intolerant, but long-term safety concerns remain debated

Principle	Detail
Goals	Focus on symptom control and thrombosis prevention; quality of life prioritised
Preferred cytoreduction	HU (well-tolerated, oral, cheap); busulfan (intermittent low-dose) acceptable given limited life expectancy
IFN-α	Poorly tolerated in the elderly (flu-like symptoms, depression, fatigue)

Parameter	Frequency	Purpose
CBP (Hb, Hct, WBC, Plt)	Every 2–4 weeks during initiation; every 1–3 months once stable	Ensure Hct < 45%; monitor for over-suppression (cytopaenias from cytoreduction); track WBC/Plt trends
Serum ferritin	Every 3–6 months	Monitor iron status; iron deficiency is expected and partly therapeutic; intervene only if severely symptomatic
LDH, uric acid	Every 3–6 months	Rising LDH may indicate disease progression or transformation
JAK2 V617F allele burden	Every 6–12 months (mainly if on IFN-α)	Molecular response assessment; declining allele burden suggests preferential suppression of mutant clone
BM biopsy	Not routine; repeat if clinical suspicion of transformation (rising LDH, new cytopenias, leukoerythroblastic PBS, progressive splenomegaly)	Assess for post-PV myelofibrosis (increasing fibrosis grade) or AML transformation (blast % ≥ 20%)
Spleen size	Clinical examination ± ultrasound at each visit	Progressive splenomegaly suggests disease progression or myelofibrotic transformation

Measure	Detail
Median survival	10–16 years in treated patients (historical data; may be improving with modern therapies) ^[3]
Untreated	~18 months — primarily from thrombotic complications ^[3]
Leading cause of death	Thrombosis (stroke, MI, PE) > transformation to AML > haemorrhage > myelofibrosis-related complications
Transformation risk	Myelofibrosis ~10%, AML ~5% ^[1]^[2]

GC 086: "Transformation to myelofibrosis (10%), acute leukaemia (5%)" ^[1]

Ryan Ho: "fair with median survival 10-16y in treated patients (18mo in [untreated])" ^[3]

Clinical Scenario	Treatment
All PV patients	Venesection (target Hct < 45%) + low-dose aspirin (40–100 mg/d) + CVS risk factor modification
Low risk (age ≤ 60, no thrombosis Hx)	Venesection + aspirin. No cytoreduction unless symptoms/counts difficult to control
High risk (age > 60 OR thrombosis Hx)	Venesection + aspirin + cytoreductive therapy (1st line: HU or peg-IFN-α)
Young patient (< 40 years)	Prefer peg-IFN-α over HU; avoid alkylating agents and ³²P
Pregnant	Venesection + aspirin + peg-IFN-α (only safe cytoreductive in pregnancy); ± LMWH
HU-resistant/intolerant	Ruxolitinib (especially for splenomegaly, pruritus, constitutional symptoms)
Elderly (> 75–80 years)	HU or low-dose busulfan; IFN-α often poorly tolerated
Extreme thrombocytosis (> 1,000 × 10⁹/L)	Cytoreduction first to lower plt count; hold aspirin until plt < ~1,000 (risk of acquired vWD → bleeding)

High Yield Summary — Management of PV

Three treatment pillars (GC 086): Venesection, Cytoreductive (hydroxyurea, interferon), Aspirin ^[1].
Target Hct < 45% for ALL patients — venesection is universal first-line; typically 1 unit (~500 mL) removed per session, reducing Hct by ~3%.
Low-dose aspirin (40–100 mg/d) for all patients — thromboprophylaxis + microvascular symptom control. Hold if plt > ~1,000 × 10⁹/L (acquired vWD risk).
Risk stratification: Low risk = age ≤ 60 + no thrombosis Hx → venesection + aspirin only. High risk = age > 60 OR thrombosis Hx → add cytoreduction.
First-line cytoreduction: Hydroxyurea (ribonucleotide reductase inhibitor) for most; Peg-IFN-α preferred in young (< 40 y) and pregnancy.
Second-line: Ruxolitinib (JAK1/2 inhibitor) for HU/IFN-refractory disease; highly effective for splenomegaly and pruritus.
Avoid leukaemogenic agents (alkylating agents, ³²P) especially in young patients.
Do NOT replace iron after venesection — iron deficiency is therapeutic in PV.
Prognosis: Median survival 10–16 years treated; 18 months untreated. Transformation: MF ~10%, AML ~5%.

Active Recall - Management of Polycythaemia Vera

References

^[1] Lecture slides: GC 086. Splenomegaly.pdf (Polycythaemia Vera — Treatment section) ^[2] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (PV clinical features and treatment, p.27; PV treatment algorithm, p.28) ^[3] Senior notes: Ryan Ho Haemtology.pdf (Section 3.3.2.1 Polycythaemia Vera — Management, prognosis, p.77; MPN overview — complications, p.75)

Complications of Polycythemia Vera

The complications of PV can be organised into two broad groups: disease-related complications (direct consequences of the underlying pathophysiology) and treatment-related complications (iatrogenic). Understanding each complication requires tracing it back to first principles — the constitutively active JAK2 driving hypervolaemia, hyperviscosity, and hypermetabolism.

1.1 Thrombotic Complications — The Leading Cause of Morbidity and Mortality

GC 086 slide: "Risk of arterial and venous thrombosis" ^[1]

Ryan Ho: "↑risk of arteriovenous thrombosis (eg. stroke, MI, supf thrombophlebitis, VTE)" ^[3]

Thrombosis accounts for ~45% of all deaths in PV and is the single most important complication to prevent. The mechanism is multifactorial (reviewed in the pathophysiology section) — a perfect storm of hyperviscosity, platelet hyperactivation, leukocyte-mediated endothelial damage, and stasis.

Complication	Pathophysiology	Clinical Presentation
Ischaemic stroke	Hyperviscosity → sluggish cerebral blood flow → stasis in cerebral arteries; hyperactivated platelets → thrombus formation on atherosclerotic plaques in carotid/cerebral vessels; leukocytosis contributes via NETs and endothelial activation	Sudden-onset focal neurological deficit (hemiparesis, aphasia, visual field loss). PV patients presenting with stroke should always have Hct checked and JAK2 screened if not previously known ^[3]
Myocardial infarction	Same mechanism as stroke but in coronary arteries — platelet-rich thrombus forming on coronary plaque, exacerbated by increased myocardial oxygen demand (hypervolaemia-driven hypertension, tachycardia) against a background of hyperviscous coronary flow	Typical ACS presentation: chest pain, ECG changes, troponin rise
Peripheral arterial occlusion	Thrombosis in limb arteries → acute limb ischaemia	Acutely painful, pale, pulseless limb (the 6 P's)
Erythromelalgia (microvascular arterial)	Microvascular thrombotic complications — platelet-rich arteriolar microthrombi in digital vessels	Burning pain in feet/hands with colour change but palpable pulses. Responds dramatically to low-dose aspirin ^[3]
Transient visual symptoms	Transient retinal or optic nerve ischaemia from microvascular thrombi or hyperviscosity-related flow reduction	Amaurosis fugax, scintillating scotoma, ophthalmic migraine ^[3]

Complication	Pathophysiology	Clinical Presentation	Why It Matters
Deep vein thrombosis / Pulmonary embolism	Venous stasis from hyperviscosity + hypercoagulability from platelet/leukocyte activation → Virchow's triad fulfilled in deep veins	Leg swelling + pain (DVT); sudden dyspnoea + pleuritic chest pain + haemodynamic compromise (PE)	Standard VTE but in a younger-than-expected patient → should prompt MPN screening
Splanchnic vein thrombosis	PV has a particular predilection for splanchnic veins (portal, hepatic, mesenteric, splenic). The mechanism is not entirely clear but likely relates to the high concentration of JAK2-mutant cells in the splanchnic circulation + local endothelial factors	Portal vein thrombosis: abdominal pain, ascites, splenomegaly, variceal bleeding. Mesenteric vein thrombosis: severe abdominal pain, bloody diarrhoea, risk of bowel infarction	Any patient presenting with splanchnic vein thrombosis should be screened for JAK2 mutation — PV/MPN is one of the commonest acquired causes ^[9]
Budd-Chiari syndrome (hepatic vein thrombosis)	Thrombosis of hepatic veins → impaired hepatic venous outflow → hepatic congestion, sinusoidal hypertension, centrizonal necrosis	Acute: severe RUQ pain, hepatomegaly, ascites, jaundice, fulminant hepatic failure. Chronic: progressive ascites, hepatomegaly, portal hypertension	PV is one of the most common causes of Budd-Chiari syndrome. A young patient presenting with BCS should have JAK2 screened even if CBC is normal (JAK2 mutation can precede overt cytosis) ^[9]
Cerebral venous sinus thrombosis (CVST)	Thrombosis of dural venous sinuses → impaired CSF absorption → raised intracranial pressure; venous infarction of brain parenchyma	Headache (progressive, worst on waking/lying down), papilloedema, seizures, focal neurological deficits	Another "unusual-site thrombosis" that should trigger MPN screening
Superficial thrombophlebitis	Thrombosis of superficial veins → localised inflammation	Tender, erythematous, palpable cord along a superficial vein ^[3]	Often dismissed as minor, but in PV context signals the prothrombotic state

Why Does PV Favour Splanchnic Veins?

The exact mechanism is debated, but hypotheses include: (1) Local haematocrit effect — blood in the portal circulation is concentrated (post-absorptive state), and PV exacerbates this further; (2) JAK2-mutant endothelial cells — some studies suggest the JAK2 mutation can be expressed in splanchnic endothelial cells, directly creating a prothrombotic endothelial surface; (3) Hepatic congestion from splenomegaly — increased splenic blood flow into the portal system raises portal pressure and promotes stasis.

Ryan Ho: "↑risk of bleeding, eg. GI, uterine, cerebral" ^[3]

Complication	Pathophysiology	Clinical Presentation
GI bleeding (upper and lower)	Multiple mechanisms: (1) Acquired von Willebrand disease (type 2A) when platelet count > ~1,000 × 10⁹/L — excessive platelet surface area adsorbs and proteolyses large vWF multimers → impaired primary haemostasis; (2) PUD from histamine-driven gastric acid hypersecretion (basophilia → H₂ receptor stimulation on parietal cells); (3) Mucosal erosions from hyperviscosity-induced mucosal ischaemia	Haematemesis, melaena, haematochezia
Uterine bleeding	Acquired vWD → mucosal bleeding tendency; also exacerbated by aspirin use	Menorrhagia, intermenstrual bleeding
Cerebral haemorrhage	Acquired vWD + aspirin use + hypertension (hypervolaemia-driven) → haemorrhagic stroke	Sudden-onset headache, focal neurological deficit, depressed consciousness
Mucosal bleeding	Acquired vWD affecting mucosal surfaces	Epistaxis, gum bleeding, easy bruising

The Thrombosis–Bleeding Paradox

PV can cause both thrombosis and bleeding — and this is a frequent exam pitfall. The key determinant is the platelet count:

Moderate thrombocytosis (< ~1,000 × 10⁹/L) → net prothrombotic → aspirin is beneficial
Extreme thrombocytosis (> ~1,000–1,500 × 10⁹/L) → acquired vWD → net bleeding tendency → aspirin is contraindicated until plt is reduced by cytoreduction

This is because vWF multimer loss is proportional to platelet surface area. More platelets = more adsorption = less functional vWF.

1.3 Transformation Complications

GC 086 slide: "Transformation to myelofibrosis (10%), acute leukaemia (5%)" ^[1]

Block A Splenomegaly notes: "Transformation to myelofibrosis, 10%; to acute leukemia, 5%" ^[2]

Ryan Ho: AML risk quoted as "10% at 10y, 25% at 25y" ^[3]

These represent the natural history of PV — the acquisition of additional mutations over time by the JAK2-mutant clone, driving disease evolution.

Aspect	Detail
Incidence	~10% of PV patients over 10–15 years ^[1]^[2]
Pathophysiology	The JAK2-mutant megakaryocytes release platelet-derived growth factor (PDGF), transforming growth factor-β (TGF-β), and other fibrogenic cytokines → reactive proliferation of fibroblasts (which are NOT part of the neoplastic clone) → progressive reticulin and collagen deposition in the bone marrow → marrow fibrosis → marrow failure. As the marrow becomes increasingly fibrotic, haematopoiesis is forced to relocate to the spleen and liver (extramedullary haematopoiesis) — though this is largely non-functional and does not adequately compensate for marrow failure
Clinical features	The clinical picture reverses: instead of polycythaemia, the patient develops progressive cytopenias (anaemia, thrombocytopenia, leukopenia); massive splenomegaly (extramedullary haematopoiesis + congestion); constitutional symptoms worsen (fatigue, night sweats, weight loss, cachexia); portal hypertension (from hepatic extramedullary haematopoiesis and increased splenic blood flow)
Laboratory clues	Leukoerythroblastic blood film — nucleated RBCs + immature granulocytes escaping from the stressed marrow into peripheral blood; tear-drop cells (dacrocytes) — RBCs deformed by squeezing through fibrotic marrow sinusoids; rising LDH; falling Hb ^[3]
BM biopsy	Increased reticulin/collagen fibrosis (grade 2–3); residual megakaryocyte clusters; reduced cellularity (hypocellular for age)
Management	Ruxolitinib for splenomegaly and symptoms; supportive care (transfusions); consider allogeneic haematopoietic stem cell transplant (allo-HSCT) in younger, fit patients with poor prognostic features

Why 'Spent Phase'?

The term "spent phase" captures the idea that the hyperproliferative marrow has eventually exhausted itself — burnt out through years of overproduction and progressive fibrosis. The marrow is now spent (depleted), and haematopoiesis fails. Clinically, the patient transitions from a plethoric, polycythaemic state to a pale, anaemic, massively splenomegalic state — a dramatic reversal.

Aspect	Detail
Incidence	~5% overall; ~10% at 10 years, ~25% at 25 years ^[1]^[2]^[3]
Pathophysiology	The JAK2-mutant clone is genetically unstable — years of autonomous proliferation generate replication errors and additional somatic mutations (e.g. in TP53, TET2, ASXL1, EZH2, IDH1/2). Accumulation of these secondary "hits" can cause a blast transformation — maturation arrest occurs, blasts accumulate (≥ 20% in marrow or blood), and the disease behaves as acute leukaemia. The AML that arises from PV is typically therapy-related or secondary and carries a poor prognosis compared to de novo AML
Risk factors for transformation	Use of leukaemogenic agents — alkylating agents (busulfan, chlorambucil), radioactive phosphorus (³²P) — these damage DNA and promote secondary mutations. Prolonged disease duration. Older age. Additional cytogenetic abnormalities ^[3]
Clinical features	Rapid clinical deterioration: new cytopenias (especially neutropenia, thrombocytopenia), rising blast count on PBS, infections, bleeding, fatigue, bone pain
Laboratory clues	Blasts ≥ 20% in peripheral blood or bone marrow; new cytogenetic abnormalities (e.g. complex karyotype, del(17p)/TP53 mutations)
Prognosis	Very poor — secondary/post-MPN AML is resistant to standard chemotherapy. Median survival is measured in months. Allo-HSCT is the only potentially curative option but is limited by age, fitness, and donor availability

Why Is Post-PV AML So Aggressive?

Post-MPN AML typically arises from a clone that has accumulated multiple high-risk mutations (especially TP53, which is the "guardian of the genome"). TP53-mutant AML is inherently chemoresistant — TP53 normally triggers apoptosis in response to DNA damage from chemotherapy. Without functional TP53, the leukaemic cells survive chemotherapy. Additionally, years of prior cytoreductive therapy may have already selected for resistant subclones.

Beyond acute thrombotic events, chronic hyperviscosity causes progressive end-organ damage:

Organ	Complication	Mechanism
Brain	Cognitive decline, chronic headaches, TIA-like episodes	Chronic cerebral hypoperfusion from viscous blood → subcortical white matter ischaemia
Heart	Hypertension, left ventricular hypertrophy, heart failure	Hypervolaemia → increased preload + afterload; hyperviscosity → increased systemic vascular resistance → chronic pressure overload on the LV
Eyes	Retinal vein occlusion, chronic visual impairment	Sluggish retinal venous flow → thrombosis; chronic retinal venous engorgement → oedema and haemorrhage
Kidneys	Urate nephropathy, renal impairment	Hyperuricaemia → uric acid crystal deposition in renal tubules and interstitium; hyperviscosity-related renal hypoperfusion

Complication	Pathophysiology
Gout and gouty arthritis	Increased cell turnover → excessive purine catabolism → hyperuricaemia → monosodium urate crystal deposition in joints (especially 1st MTP, ankles, knees) → acute inflammatory arthritis ^[3]
Urate nephropathy	Uric acid precipitation in renal collecting ducts and interstitium → tubulointerstitial nephritis → progressive renal impairment
Urate nephrolithiasis	Uric acid stones forming in the renal pelvis/ureters → renal colic, obstruction
Peptic ulcer disease and GI mucosal erosions	Basophilia → excess histamine release → stimulation of gastric parietal cell H₂ receptors → gastric acid hypersecretion → erosion of gastroduodenal mucosa. Also mucosal ischaemia from hyperviscosity ^[3]

The spleen progressively enlarges due to extramedullary haematopoiesis, congestion, and work hypertrophy:

Complication	Mechanism
Early satiety and weight loss	Massive spleen compresses the stomach → reduced gastric capacity → early satiety → poor nutritional intake
Left upper quadrant pain	Splenic capsular stretching from progressive enlargement; splenic infarction from ischaemia within the congested spleen
Splenic infarction	Splenic artery branches occluded by thrombi (hyperviscous blood) or outgrowing their blood supply → wedge-shaped infarcts → severe LUQ pain, fever, left pleural effusion
Hypersplenism	Overactive splenic sequestration of formed blood elements → cytopenias (anaemia, thrombocytopenia, leukopenia) despite marrow hyperactivity — an additional mechanism contributing to cytopenias in the spent phase
Portal hypertension	Increased splenic blood flow into the portal vein + possible portal vein thrombosis → portal hypertension → ascites, variceal bleeding, hepatic encephalopathy

Treatment	Complication	Mechanism
Venesection	Iron deficiency (intentional but can become symptomatic — fatigue, pica, koilonychia, glossitis)	Repeated removal of iron-containing haemoglobin depletes total body iron stores. Iron deficiency limits erythropoiesis (therapeutic), but severe deficiency impairs all iron-requiring enzymes (cytochromes, myoglobin) causing systemic symptoms
	Hypovolaemia (acute)	Acute blood volume reduction without adequate NS replacement → hypotension, syncope (especially in elderly)
Hydroxyurea	Cytopaenias (neutropenia, anaemia, thrombocytopenia)	Excessive suppression of marrow proliferation — dose-dependent, reversible on dose reduction ^[3]
	Mucocutaneous ulcers (oral ulcers, leg ulcers)	Direct cytotoxicity to rapidly dividing mucosal/cutaneous epithelium ^[3]
	Macrocytosis / Megaloblastic changes	HU inhibits ribonucleotide reductase → impairs DNA synthesis → cells grow large without dividing (same mechanism as B12/folate deficiency causing megaloblastic change) ^[3]
	Skin cancers (squamous cell carcinoma, actinic keratosis)	Long-term DNA synthesis inhibition may impair skin cell DNA repair → photodamage-induced carcinogenesis ^[3]
	Teratogenicity	DNA synthesis inhibition is catastrophic to the rapidly dividing embryonic cells → absolutely contraindicated in pregnancy ^[3]
Peg-IFN-α	Depression / neuropsychiatric effects	IFN-α modulates serotonin and dopamine metabolism in the CNS; directly neurotoxic to certain neuronal populations
	Autoimmune phenomena (thyroiditis, hepatitis)	Immune activation/dysregulation → breaking of self-tolerance
	Flu-like symptoms	IFN-α induces the same inflammatory cytokine cascade (IL-1, IL-6, TNF-α, prostaglandins) that produces fever and myalgia during viral infection — because that is its normal physiological role
Ruxolitinib	Cytopaenias (especially anaemia)	Inhibiting JAK2 reduces ALL haematopoietic signalling, not just the mutant clone — EPO, TPO, and G-CSF all signal through JAK2
	Herpes zoster reactivation	JAK inhibition impairs NK cell and T cell function (JAK-STAT signalling is critical for IFN-γ and IL-12 pathways that control latent VZV)
	Weight gain, hyperlipidaemia	Mechanism not fully understood; possibly related to changes in leptin/adipokine signalling via JAK-STAT
	Rebound phenomenon on discontinuation	Abrupt withdrawal → cytokine storm from unopposed JAK-STAT signalling in inflammatory cells → severe systemic inflammatory response; disease flare with splenomegaly and constitutional symptoms
Alkylating agents / ³²P	Leukaemogenic transformation (AML/MDS)	DNA cross-linking/radiation-induced double-strand breaks → mutagenesis → secondary MDS/AML. Risk is cumulative and dose-dependent ^[3]

Category	Complication	Approximate Risk / Timing	Key Mechanism
Thrombosis	Stroke, MI, DVT/PE, splanchnic vein thrombosis, Budd-Chiari, CVST, erythromelalgia	~40–45% lifetime without treatment; highest in first 2 years	Hyperviscosity + platelet hyperactivation + leukocyte-mediated endothelial damage
Haemorrhage	GI, uterine, cerebral bleeding	Increased with plt > 1,000 × 10⁹/L	Acquired von Willebrand disease (type 2A)
Myelofibrosis	Post-PV MF (spent phase)	~10% over 10–15 years	Megakaryocyte-derived fibrogenic cytokines (PDGF, TGF-β) → reactive marrow fibrosis
AML	Blast transformation	~5% overall (10% at 10y, 25% at 25y)	Accumulation of secondary mutations (TP53, TET2, etc.) in the JAK2-mutant clone
Gout	Acute gouty arthritis, tophi	Common, proportional to disease duration	Hyperuricaemia from increased cell turnover
PUD	Peptic ulcers, mucosal erosions	Common	Basophil-derived histamine → gastric acid hypersecretion
Splenic complications	Infarction, hypersplenism, portal HTN	Proportional to spleen size	Splenic congestion, ischaemia, increased portal flow
Treatment-related	Cytopaenias, mucosal ulcers, skin cancers (HU); depression (IFN); infections (ruxolitinib); AML (alkylating agents/³²P)	Variable	Agent-specific mechanisms as above

High Yield Summary — Complications of PV

Thrombosis is the #1 cause of morbidity and mortality — arterial (stroke, MI) and venous (DVT/PE, splanchnic veins, Budd-Chiari, CVST). Driven by hyperviscosity, platelet hyperactivation, and leukocytosis.
Bleeding occurs paradoxically when platelets are extremely high ( > 1,000 × 10⁹/L) due to acquired von Willebrand disease — aspirin is contraindicated until cytoreduction brings platelets down.
Transformation (GC 086 high yield): Myelofibrosis ~10%, AML ~5% ^[1]^[2]. Post-PV AML has very poor prognosis due to TP53 and other high-risk mutations.
Post-PV myelofibrosis represents the "spent phase" — progressive marrow fibrosis → cytopenias, massive splenomegaly, leukoerythroblastic picture, tear-drop cells.
Avoid leukaemogenic agents (alkylating agents, ³²P) especially in young patients — they increase AML transformation risk.
Hypermetabolic complications: gout (hyperuricaemia), PUD (basophil histamine), urate nephropathy.
Splenic complications: infarction, hypersplenism, portal hypertension (especially with splanchnic vein thrombosis).
Treatment complications: HU → cytopaenias, mucosal ulcers, skin cancers, teratogenicity; IFN-α → depression, autoimmunity; ruxolitinib → infections (zoster), rebound on withdrawal.

Active Recall - Complications of Polycythaemia Vera

References

^[1] Lecture slides: GC 086. Splenomegaly.pdf (Polycythaemia Vera — transformation risk) ^[2] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (Complications of PV, p.28; Clinical features and treatment, p.27) ^[3] Senior notes: Ryan Ho Haemtology.pdf (Section 3.3.2.1 Polycythaemia Vera — clinical features, laboratory findings, management, prognosis, p.76–77; Section 3.3.2 MPNs overview — complications, p.75) ^[9] Senior notes: Block A - Leg swelling and chest pain_ deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf (MPN-associated thrombosis — JAK2 screening in unusual-site thrombosis, p.18)

High Yield Summary

PV is a clonal MPN caused by JAK2 mutations (100%: 95–97% V617F, 3–5% exon 12) → constitutive activation of JAK2-STAT signalling → EPO-independent erythroid proliferation.
Clinical triad = Hypervolaemia + Hyperviscosity + Hypermetabolism (the "3 H's").
Must exclude secondary polycythaemia — check EPO level: low EPO = primary (PV); high EPO = secondary. In HK, always consider HCC and RCC as causes of ectopic EPO.
Diagnostic thresholds: Hb > 16.5/16.0 g/dL or Hct > 49/48% (M/F) + JAK2 mutation + BM showing trilineage panmyelosis + low EPO.
Aquagenic pruritus (after hot bath) is virtually pathognomonic for PV (~31%).
Erythromelalgia (burning extremity pain with colour change but palpable pulses) responds to low-dose aspirin.
Thrombosis (arterial > venous) is the leading cause of morbidity/mortality — risk factors include hyperviscosity, leukocytosis, and platelet hyperactivation.
Transformation risk: myelofibrosis ~10%, AML ~5% ^[1].
Treatment overview (preview): Venesection (target Hct < 45%), cytoreductive therapy (hydroxyurea, interferon) for high-risk patients, and low-dose aspirin for all patients without contraindications ^[1].

High Yield Summary — Differential Diagnosis of PV

Approach to erythrocytosis (SAQ!): First exclude apparent (relative) polycythaemia (dehydration, diuretics, burns, Gaisbock); then differentiate primary (PV, low EPO, JAK2+) from secondary (high EPO).
Secondary — Appropriate: COPD, OSA, cyanotic CHD/Eisenmenger, pulmonary HTN, CO poisoning, high altitude, high-O₂-affinity haemoglobinopathies.
Secondary — Inappropriate: HCC (hepatoma) — #1 in HK; RCC, cerebellar haemangioblastoma, uterine fibroma, adrenal tumours; renal causes (PKD, hydronephrosis, renal artery stenosis).
EPO level is the key discriminating test: low = primary (PV); high = secondary.
Clues favouring PV over secondary: splenomegaly, aquagenic pruritus, trilineage elevation (WBC/Plt also raised), thrombosis in unusual sites, JAK2+.
Other MPNs to distinguish: CML (BCR-ABL1+), ET (isolated thrombocytosis, normal Hb), PMF (fibrosis, low Hb, tear-drop cells).
Any unusual-site thrombosis (splanchnic, cerebral sinus) → screen JAK2 even if CBC is normal — patients can clot before developing overt cytosis.

High Yield Summary — Diagnosis of PV

WHO 2022 criteria: 3 Major or 2 Major + 1 Minor ^[2].
- Major 1: Hb > 16.5/16.0 or Hct > 49/48% (M/F)
- Major 2: BM showing trilineage panmyelosis with pleomorphic mature megakaryocytes
- Major 3: JAK2 V617F or exon 12 mutation
- Minor: Subnormal EPO
Investigation sequence: CBP → SpO₂ → EPO level (pivotal test) → JAK2 V617F (peripheral blood) → BM biopsy (confirmation + baseline fibrosis).
EPO is the key branching test: low = primary (PV); high = secondary.
JAK2 V617F can be done on peripheral blood — BM is needed for morphology and fibrosis, not for the mutation test itself.
BCR-ABL1 must be checked to exclude CML — a JAK2-negative MPN with marked leukocytosis could be CML.
BM biopsy at diagnosis is essential to: (a) confirm morphology; (b) grade baseline fibrosis (reticulin); (c) distinguish PV from ET/PMF.
Ancillary tests: ferritin (often low), LDH (↑), uric acid (↑), B12 (↑), coag profile (if plt > 1000 → screen acquired vWD).
If EPO is high → investigate secondary causes: SpO₂/ABG for hypoxia; imaging for tumours (HCC, RCC); renal assessment.

High Yield Summary — Management of PV

Three treatment pillars (GC 086): Venesection, Cytoreductive (hydroxyurea, interferon), Aspirin ^[1].
Target Hct < 45% for ALL patients — venesection is universal first-line; typically 1 unit (~500 mL) removed per session, reducing Hct by ~3%.
Low-dose aspirin (40–100 mg/d) for all patients — thromboprophylaxis + microvascular symptom control. Hold if plt > ~1,000 × 10⁹/L (acquired vWD risk).
Risk stratification: Low risk = age ≤ 60 + no thrombosis Hx → venesection + aspirin only. High risk = age > 60 OR thrombosis Hx → add cytoreduction.
First-line cytoreduction: Hydroxyurea (ribonucleotide reductase inhibitor) for most; Peg-IFN-α preferred in young (< 40 y) and pregnancy.
Second-line: Ruxolitinib (JAK1/2 inhibitor) for HU/IFN-refractory disease; highly effective for splenomegaly and pruritus.
Avoid leukaemogenic agents (alkylating agents, ³²P) especially in young patients.
Do NOT replace iron after venesection — iron deficiency is therapeutic in PV.
Prognosis: Median survival 10–16 years treated; 18 months untreated. Transformation: MF ~10%, AML ~5%.

High Yield Summary — Complications of PV

Thrombosis is the #1 cause of morbidity and mortality — arterial (stroke, MI) and venous (DVT/PE, splanchnic veins, Budd-Chiari, CVST). Driven by hyperviscosity, platelet hyperactivation, and leukocytosis.
Bleeding occurs paradoxically when platelets are extremely high ( > 1,000 × 10⁹/L) due to acquired von Willebrand disease — aspirin is contraindicated until cytoreduction brings platelets down.
Transformation (GC 086 high yield): Myelofibrosis ~10%, AML ~5% ^[1]^[2]. Post-PV AML has very poor prognosis due to TP53 and other high-risk mutations.
Post-PV myelofibrosis represents the "spent phase" — progressive marrow fibrosis → cytopenias, massive splenomegaly, leukoerythroblastic picture, tear-drop cells.
Avoid leukaemogenic agents (alkylating agents, ³²P) especially in young patients — they increase AML transformation risk.
Hypermetabolic complications: gout (hyperuricaemia), PUD (basophil histamine), urate nephropathy.
Splenic complications: infarction, hypersplenism, portal hypertension (especially with splanchnic vein thrombosis).
Treatment complications: HU → cytopaenias, mucosal ulcers, skin cancers, teratogenicity; IFN-α → depression, autoimmunity; ruxolitinib → infections (zoster), rebound on withdrawal.

Polycythemia Vera

1. Definition

2. Epidemiology

3. Anatomy and Function — Relevant Normal Physiology

3.1 Normal Erythropoiesis and the EPO–JAK2–STAT Axis

3.2 The Spleen

4. Etiology and Pathophysiology

4.1 Etiology

4.2 Pathophysiology — The Three H's

4.2.1 Hypervolaemia (Increased Red Cell Mass → Expanded Blood Volume)

4.2.2 Hyperviscosity

4.2.3 Hypermetabolism (Increased Cell Turnover)

4.3 Pathophysiology of Thrombosis in PV

4.4 Pathophysiology of Bleeding in PV

4.5 Pathophysiology of Aquagenic Pruritus

4.6 Pathophysiology of Erythromelalgia

4.7 Natural History and Transformation

5. Classification

5.1 Classification of Erythrocytosis (The Clinical Approach)

5.2 WHO 2022 Diagnostic Criteria for PV

5.3 Classification within MPNs

6. Clinical Features

6.1 Symptoms (with Pathophysiological Basis)

A. Hyperviscosity Symptoms

B. Aquagenic Pruritus

C. Erythromelalgia

D. Thrombohaemorrhagic Symptoms

E. Hypermetabolic / Constitutional Symptoms

F. Other Symptoms

G. Microvascular Symptoms

6.2 Signs (with Pathophysiological Basis)

7. Summary of Pathophysiology → Clinical Features Mapping

8. Laboratory Findings (Preview)

Active Recall - Polycythaemia Vera: Definition to Clinical Features

References

1. Overarching Framework: Is the Red Cell Mass Truly Increased?

2. Differential Diagnosis of Apparent (Relative) Polycythaemia

3. Differential Diagnosis of Absolute Polycythaemia

3A. Primary Absolute Polycythaemia (Low EPO)

3B. Secondary Absolute Polycythaemia (Normal/High EPO)

3B-i. Appropriate EPO Elevation (Response to Tissue Hypoxia)

3B-ii. Inappropriate EPO Elevation (Ectopic/Autonomous EPO Production)

4. Other Conditions That Can Mimic PV Features

4A. Other Myeloproliferative Neoplasms

4B. Conditions Presenting with Thrombosis in Unusual Sites

4C. Hyperviscosity — Conditions with Overlapping Symptoms

5. Differential Diagnosis Algorithm (Mermaid)

6. Summary Table: Key Differentiating Features

7. Clinical Approach in Practice — When to Suspect PV Over Secondary Causes

Active Recall - Differential Diagnosis of Polycythaemia Vera

1. WHO 2022 Diagnostic Criteria for PV

2. Stepwise Diagnostic Algorithm

3. Investigation Modalities — Key Findings and Interpretation

3.1 Complete Blood Picture (CBP / FBC)

3.2 Peripheral Blood Smear (PBS)

3.3 Serum Erythropoietin (EPO) Level

3.4 JAK2 Mutation Analysis

3.5 Bone Marrow Examination

3.6 Additional Blood Tests

3.7 Pulse Oximetry and Arterial Blood Gas

3.8 Additional Investigations to Exclude Secondary Causes

3.9 Red Cell Mass Study (⁵¹Cr-Labelled RBC)

3.10 Endogenous Erythroid Colony (EEC) Assay

4. Putting It All Together: Practical Investigation Sequence

5. Interpretation Summary Table

Active Recall - Diagnostic Criteria and Investigations for PV

1. Goals of Therapy

2. Risk Stratification

3. Management Algorithm

4. Treatment Modalities — Detailed

4.1 Therapeutic Phlebotomy (Venesection)

4.2 Low-Dose Aspirin

4.3 Cytoreductive Therapy

4.3.1 Hydroxyurea (Hydroxycarbamide) — First-Line Cytoreduction

4.3.2 Pegylated Interferon-Alpha (Peg-IFN-α) — First-Line Alternative

4.3.3 Second-Line / Refractory Options

5. Cardiovascular Risk Factor Modification

6. Management of Specific Symptoms

7. Management in Special Populations

7.1 Pregnancy