Polycythemia — Absolute and Relative

Absolute Polycythemia

Primary Polycythemia (Polycythemia Vera)

• JAK2 V617F mutation (95% of cases) or alternative mutations in CALR or MPL genes lead to constitutive activation of the JAK-STAT signaling pathway
• Results in erythropoietin-independent erythropoiesis—bone marrow progenitor cells (BFU-E and CFU-E) proliferate without physiologic EPO stimulation
• Autonomous RBC production occurs with suppressed serum EPO levels (<2 mIU/mL)
• Associated abnormal megakaryopoiesis and granulopoiesis contribute to thrombocytosis and leukocytosis
• Progressive bone marrow fibrosis occurs due to cytokine-mediated stromal changes and transforming growth factor-β (TGF-β) release from megakaryocytes

Secondary Absolute Polycythemia

• Tissue hypoxia stimulates the hypoxia-inducible factor (HIF) pathway, increasing renal production of erythropoietin (EPO)
• EPO binds to EPO receptor on erythroid progenitors, promoting proliferation and differentiation
• Chronic hypoxemia (altitude, lung disease, cyanotic heart disease) produces sustained EPO elevation (>25 mIU/mL)
• Ectopic EPO production from malignancy (renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma) or benign renal lesions bypasses physiologic feedback
• Abnormal hemoglobins (high oxygen affinity, left-shifted oxygen-hemoglobin dissociation curve) reduce tissue oxygen delivery despite normal PO₂
• Androgen excess directly stimulates erythropoiesis independent of EPO

Relative Polycythemia

• Plasma volume depletion without increase in total RBC mass
• Results from severe dehydration, burns, diuretic use, or hemoconcentration
• Fluid redistribution from intravascular to extravascular compartments elevates hemoglobin and hematocrit artifactually
• Pathophysiology is hemoconcentration rather than increased erythropoiesis
• Bone marrow morphology remains normal with appropriate EPO levels

Primary Polycythemia

• Polycythemia vera (PV): JAK2 V617F (>95%), CALR mutations (5%), MPL mutations (<1%); median age 60-65 years; slight male predominance

Secondary Absolute Polycythemia

Hypoxia-Driven

• High altitude (>8,000 feet, chronic exposure)
• Chronic lung disease (COPD, idiopathic pulmonary fibrosis, cystic fibrosis)
• Cyanotic congenital heart disease (right-to-left shunt)
• Sleep apnea (nocturnal hypoventilation)
• Chronic carbon monoxide exposure

Ectopic EPO Production

• Renal cell carcinoma (most common paraneoplastic syndrome from RCC)
• Hepatocellular carcinoma
• Cerebellar hemangioblastoma
• Uterine fibroid
• Benign renal cysts

Abnormal Hemoglobin/Oxygen Delivery

• High-affinity hemoglobins (e.g., Hb Chesapeake)
• 2,3-BPG deficiency
• Methemoglobinemia
• Carbon monoxide poisoning (chronic)

Androgenic Stimulation

• Anabolic steroid use
• Testosterone replacement therapy
• Renal transplantation (renal artery stenosis from anastomosis)

Relative Polycythemia

• Severe dehydration
• Thermal burns
• Diuretic overuse
• Diarrheal illness
• Smoking (carboxyhemoglobin reduces oxygen-carrying capacity, triggering EPO response)
• Chronic stress (Gaisböck syndrome)

Polycythemia Vera

Constitutional/Systemic Symptoms

• Pruritus (aquagenic pruritus—severe itching after hot bath/shower, pathognomonic but occurs in ~50%); attributed to increased mast cell activation and histamine release
• Fatigue and malaise from increased blood viscosity
• Headache, dizziness, vertigo from cerebral hyperviscosity
• Night sweats and weight loss (paraneoplastic phenomenon)

Vascular Complications

• Thrombosis (arterial and venous): deep vein thrombosis, pulmonary embolism, stroke, myocardial infarction; paradoxically increased bleeding risk due to platelet dysfunction despite thrombocytosis
• Erythromelalgia (burning pain and erythema of extremities from microvascular thrombosis)
• Splenic infarction

Physical Examination

• Plethora (ruddy complexion, deep facial flushing)
• Splenomegaly (75% of patients; from extramedullary hematopoiesis and myeloproliferation)
• Hepatomegaly (50%)
• Hypertension (from increased blood volume and viscosity)

Constitutional Features of Progression

• Progressive splenomegaly with pain
• Cachexia and weight loss
• Transformation to acute myeloid leukemia (3-5% of patients over 10 years) or myelofibrosis ("spent phase," 10-15%)

Secondary Polycythemia

Variable Presentation

• Often asymptomatic if mild
• Symptoms referable to underlying disease (dyspnea in COPD, neurologic symptoms in cerebellar hemangioblastoma)
• Less likely to have pruritus, thrombosis, or splenomegaly compared to PV
• Clinical features depend on rapidity of RBC elevation

Relative Polycythemia

• Often asymptomatic
• Symptoms of dehydration or underlying cause
• May have mild headache or dizziness
• No splenomegaly
• Normal oxygen saturation

Laboratory Studies

Complete Blood Count (CBC)

• Hemoglobin: >16.5 g/dL (men), >16 g/dL (women)
• Hematocrit: >49% (men), >48% (women)
• RBC count: elevated (>6.0 × 10¹²/L)
• Mean corpuscular volume (MCV): typically low-normal in PV (due to iron depletion from increased RBC production); helps distinguish from secondary causes
• White blood cell count: often elevated (leukocytosis with left shift) in PV
• Platelet count: elevated (often >400,000/μL) in PV

• Absolute polycythemia: true increase in RBC mass (>36 mL/kg in men, >32 mL/kg in women)
• Relative polycythemia: normal RBC mass (<36 mL/kg in men, <32 mL/kg in women)
• Performed using chromium-51 labeled RBCs or other isotopic methods
• Essential for diagnosis of polycythemia vera and differentiation from relative forms

Erythropoietin (EPO) Level

• Polycythemia vera: suppressed, typically <2 mIU/mL (due to feedback inhibition from high RBC mass)
• Secondary polycythemia: elevated, usually >25 mIU/mL
• Intermediate levels (2-25 mIU/mL) require further investigation
• Most useful when normal or low (strongly suggests PV)

Oxygen Saturation

• Normal (≥92%) in PV and most secondary causes
• Low (<92%) suggests hypoxia-driven secondary polycythemia
• Arterial blood gas helpful if borderline

Molecular/Genetic Testing

JAK2 Mutation Analysis

• JAK2 V617F: present in ~95% of PV cases; constitutively active tyrosine kinase
• Highly specific for diagnosis of PV when present
• Rarely found in secondary polycythemia or relative polycythemia
• Detected by allele-specific PCR or sequencing

• CALR (calreticulin): exon 9 mutations in 5% of PV
• MPL: W515L or W515K mutations in <1% of PV
• Testing if JAK2 negative and clinical suspicion remains high

Bone Marrow Examination

Histomorphology in Polycythemia Vera

• Hypercellularity with cellularity >80% (trilineage hyperplasia)
• Prominent erythropoiesis with increased normoblasts throughout marrow
• Megakaryocytic hyperplasia with abnormal morphology (large, hyperlobulated nuclei, increased ploidy)
• Granulocytic hyperplasia
• Reticulin fibrosis ranging from absent in early disease to marked in spent-phase myelofibrosis (graded 0-3)
• Reduced or absent iron stores (from increased iron utilization in RBC production)

Gross Appearance of Bone Marrow Aspirate/Core

• Hypercellular (red or hemorrhagic appearance due to erythroid predominance)
• In spent phase: dry tap or fibrotic marrow with increased collagen deposition

Other Secondary Polycythemia

• Bone marrow shows appropriate erythroid hyperplasia but normal maturation without abnormal morphology
• No increase in blasts
• Normal cytogenetics

Diagnostic Criteria for Polycythemia Vera (WHO 2016)

1. 1.Elevated hemoglobin (>16.5 g/dL men, >16 g/dL women) OR elevated hematocrit (>49% men, >48% women) OR increased RBC mass
2. 2.Bone marrow biopsy showing hypercellularity with trilineage growth (erythroid, granulocytic, megakaryocytic)
3. 3.Presence of JAK2, CALR, or MPL mutation

Minor Criterion

1. 1.Subnormal serum EPO level

Imaging & Additional Studies

Abdominal Ultrasound or CT

• Assess splenomegaly (common in PV, rare in secondary causes)
• Evaluate for portal or mesenteric vein thrombosis

Chest Imaging

• Rule out underlying lung disease in patients with hypoxia
• Identify cardiac shunts if congenital disease suspected

Uric Acid and LDH

• Often elevated in PV (from RBC turnover)
• Useful for prognosis and monitoring
• May indicate transformation to acute leukemia if sudden elevation

Polycythemia Vera

First-Line Management

Phlebotomy

• Target hematocrit <45% (men) or <42% (women) to reduce thrombotic risk
• Remove 250-500 mL of whole blood at weekly intervals initially
• Continue periodically as needed (typically every 3-6 months)
• Rationale: reduces blood viscosity, improves tissue perfusion, and significantly decreases thrombotic complications
• Iron depletion (therapeutic goal) reduces iron availability for erythropoiesis
• Oral iron supplementation should be avoided
• Most patients develop iron deficiency, which self-limits RBC production
• Low-risk patients (age <60, no thrombotic history, JAK2 wild-type) may be managed with phlebotomy alone

Antiplatelet Therapy

• Low-dose aspirin (75-100 mg daily) for all patients
• Reduces arterial thrombotic events, particularly myocardial infarction and stroke
• May increase bleeding risk if platelets extremely elevated (>1,000,000/μL)
• Consider adding for microvascular symptoms (erythromelalgia)

Second-Line Cytoreductive Therapy

Indicated for

• High-risk disease (age ≥60 or history of thrombosis)
• Intolerant to phlebotomy
• Symptomatic splenomegaly
• Rapidly accelerating course

Hydroxyurea

• JAK2 inhibitor-like mechanism: reduces granulocyte and platelet production; induces fetal hemoglobin; antineoplastic effects
• Initial dose 15-20 mg/kg/day, titrated to achieve hematocrit goal and platelet count <400,000/μL
• Generally well-tolerated; primary concern is leukemogenic potential with long-term use (though controversial)
• Monitor CBC, renal, and hepatic function monthly initially

JAK2 Inhibitors

• Ruxolitinib FDA-approved for PV in patients inadequately controlled or intolerant to hydroxyurea
• Mechanism: selective JAK1/JAK2 inhibitor blocks pathologic signaling
• Effective for symptom control (pruritus resolution in ~60%), splenomegaly reduction, and thrombotic event reduction
• Does NOT achieve cytoreduction comparable to hydroxyurea; hematocrit control may be challenging
• Adverse effects: anemia, leukopenia, thrombocytopenia, increased infection risk
• Expensive; reserve for specific indications

Interferon-Alpha

• Used as alternative cytoreductive agent, particularly in young patients desiring to avoid hydroxyurea
• Mechanism: inhibits hematopoietic stem cell proliferation
• More difficult tolerability (flu-like symptoms, depression)
• Increasingly replaced by ruxolitinib in modern practice

Anagrelide

• Primarily for platelet reduction in essential thrombocythemia; less effective in PV
• Reserved for specific scenarios due to cardiac toxicity and lesser efficacy

Management of Complications

Thrombosis

• Aggressive risk factor modification (smoking cessation, blood pressure control)
• Consider therapeutic anticoagulation with warfarin or DOACs for recurrent thrombotic events
• Phlebotomy and cytoreduction essential

Pruritus

• Aspirin, ruxolitinib, or hydroxycarbamide
• H2-receptor antagonists (cimetidine) or selective serotonin reuptake inhibitors (SSRIs) for refractory cases

Transformation to AML or Myelofibrosis

• Supportive care and chemotherapy as appropriate for transformed disease
• No current preventive therapy proven to reduce transformation risk

Secondary and Relative Polycythemia

Treatment of Underlying Cause

• Supplemental oxygen for hypoxia-driven polycythemia (altitude exposure, COPD, sleep apnea)
• Treatment of malignancy producing EPO
• Correction of abnormal hemoglobin if possible
• Discontinuation of androgenic agents

Relative Polycythemia

• Fluid repletion and management of underlying dehydration
• Diuretic discontinuation when appropriate
• No specific phlebotomy needed once intravascular volume restored

Thrombotic Complications

Arterial Thrombosis

• Myocardial infarction: most common arterial event; occurs due to increased blood viscosity and platelet dysfunction
• Stroke: from cerebral artery thrombosis
• Splenic infarction: from microvascular occlusion
• **Mesent

The buzzword-to-diagnosis shortcuts

• Aquagenic pruritus (itching after a hot shower) plus facial plethora plus splenomegaly is polycythemia vera until proven otherwise; secondary and relative polycythemia essentially never produce this triad.
• Erythromelalgia — burning, red, warm hands and feet relieved by aspirin — reflects platelet-mediated microvascular occlusion and is shared by PV and essential thrombocythemia.
• Budd–Chiari or portal/splanchnic vein thrombosis in a young patient: test for JAK2 V617F even if the hemoglobin looks normal, since splenic pooling and GI bleeding can mask the erythrocytosis. This is the single association examiners test most often.

Single best next step

• Serum EPO plus JAK2 V617F is the first branch point after confirming a true elevation in hemoglobin/hematocrit. Suppressed EPO points to PV; elevated EPO points to hypoxia or an EPO-secreting lesion. Red cell mass studies are the conceptual gold standard but are rarely available and are not the practical first test.
• Check pulse oximetry/ABG and a smoking history: normal saturation with high hematocrit in a smoker should prompt a carboxyhemoglobin level, since CO shifts the curve left and drives EPO despite a normal PaO₂.

Therapy pearls

• Phlebotomy to hematocrit <45% plus low-dose aspirin is the backbone for all PV patients; the CYTO-PV trial established the <45% target, and cytoreduction (hydroxyurea, or ruxolitinib per its FDA label after hydroxyurea failure) is reserved for high-risk disease — age ≥60 or prior thrombosis — as reflected in NCCN myeloproliferative neoplasm guidance.
• Never give iron to a phlebotomized PV patient; the induced microcytic iron deficiency is therapeutic, not a deficiency to correct.

Distractors to avoid

• Extreme thrombocytosis does not mean safety from bleeding — acquired von Willebrand syndrome can cause hemorrhage, so aspirin is used cautiously at very high platelet counts.
• Relative polycythemia (Gaisböck syndrome, diuretics, burns, dehydration) has a normal red cell mass, normal EPO, no splenomegaly, and no JAK2 mutation — treat the volume, not the marrow.