How is polycythemia vera diagnosed in a middle‑aged to older adult presenting with elevated hemoglobin or hematocrit?

Diagnosis of Polycythemia Vera

Polycythemia vera is diagnosed by demonstrating elevated hemoglobin/hematocrit (≥16.5 g/dL in men or ≥16.0 g/dL in women; hematocrit ≥49% in men or ≥48% in women) combined with the presence of a JAK2 mutation (found in >95% of cases), plus supportive bone marrow histology or a subnormal serum erythropoietin level. 1

Major Diagnostic Criteria

The modern WHO-based diagnostic framework requires both major criteria plus at least one minor criterion to establish the diagnosis 1:

Major Criteria:

• Elevated erythrocytosis: Hemoglobin >16.5 g/dL in men or >16.0 g/dL in women, OR hematocrit >49% in men or >48% in women, OR increased red cell mass >25% above predicted normal 1

• Presence of JAK2V617F or functionally similar JAK2 mutation (including exon 12 mutations): This mutation is present in approximately 95-97% of polycythemia vera patients and is absent in secondary or spurious polycythemia 2, 1

Minor Criteria:

• Bone marrow histology showing panmyelosis with prominent erythroid and megakaryocytic proliferation, hypercellularity for age with trilineage growth 1, 3

• Serum erythropoietin level below the normal reference range: Low EPO has >90% specificity for polycythemia vera, though sensitivity is only 64-70% 2, 1

• Endogenous erythroid colony formation in vitro (rarely used due to limited availability and need for expertise) 2, 1

Practical Diagnostic Algorithm

Step 1: Confirm True Polycythemia

• Obtain repeat hemoglobin/hematocrit measurements to confirm sustained elevation 4
• Exclude apparent (relative) polycythemia from plasma volume depletion due to dehydration, diuretics, burns, or capillary leak syndrome 4

Step 2: Order JAK2 Mutation Testing and Serum EPO Level Simultaneously

• JAK2V617F mutation testing is the cornerstone: Present in >95% of polycythemia vera cases 2, 1
• Serum EPO level helps distinguish primary from secondary polycythemia 2, 1, 4:
  • Low EPO (below reference range) → Strongly suggests polycythemia vera (specificity >90%) 2, 1
  • Normal EPO → Does not exclude polycythemia vera; proceed with JAK2 testing 2, 1
  • Elevated EPO → Suggests secondary polycythemia; systematically evaluate for hypoxia-driven or tumor-related causes 4

Step 3: Exclude Secondary Causes When EPO is Elevated or Normal

• Detailed smoking history: Smoker's polycythemia from chronic carbon monoxide exposure is the most common secondary cause and resolves with cessation 4, 5
• Assess for hypoxia: Arterial blood gas or pulse oximetry, chest X-ray for chronic lung disease (COPD), sleep study if clinically indicated 4, 5
• Screen for EPO-producing tumors: Abdominal ultrasound or CT for renal cell carcinoma, hepatocellular carcinoma, uterine leiomyomas, pheochromocytoma, meningioma, parathyroid carcinoma 4, 5
• Medication review: Exogenous erythropoietin, testosterone/androgen preparations 5
• Post-renal transplant erythrocytosis in transplant recipients 5

Step 4: Bone Marrow Examination

• Perform bone marrow biopsy with cytogenetic studies when diagnosis remains uncertain after Steps 1-3 2
• Characteristic histology: Hypercellularity, panmyelosis with prominent erythroid and megakaryocytic proliferation, increased megakaryocyte number with cluster formation, giant megakaryocytes, pleomorphism, mild reticulin fibrosis (12% of cases), decreased bone marrow iron stores 2
• Cytogenetic abnormalities (trisomies 9 and 8, deletions of chromosomes 13q and 20q) are present in only 13-18% at diagnosis and have limited diagnostic value 2

Step 5: Specialized Testing for Equivocal Cases (<10% of patients)

• Bone marrow immunohistochemistry for c-mpl (thrombopoietin receptor): Markedly decreased expression supports polycythemia vera 2
• Neutrophil PRV-1 expression assay: High expression in polycythemia vera vs. not detectable in secondary polycythemia (limited availability) 2, 4
• Spontaneous erythroid colony assay: Distinguishes polycythemia vera from secondary polycythemia but requires expertise and is rarely used 2, 4

Masked Polycythemia Vera: A Critical Diagnostic Pitfall

Approximately 15-35% of polycythemia vera patients present with hemoglobin levels below the traditional WHO thresholds (hemoglobin 16.0-18.4 g/dL in men, 15.0-16.4 g/dL in women) due to blood dilution, coincidental blood loss anemia, or iron deficiency 3, 6. These patients have "masked PV" and may present with:

• Unusual site thrombosis (portal vein, hepatic vein, mesenteric vein) as the first manifestation 3
• Normal or near-normal blood counts despite underlying myeloproliferative disease 3
• JAK2V617F mutation positivity with bone marrow hypercellularity and trilineage growth 3, 6
• Higher platelet counts and increased bone marrow reticulin fibrosis compared to overt polycythemia vera 6
• Worse overall survival, particularly in patients >65 years or with white blood cell count >15 × 10⁹/L 6

Clinical pearl: Investigate for myeloproliferative neoplasm with JAK2 testing in any patient presenting with unusual site thrombosis (portal, hepatic, mesenteric veins), even with normal hemoglobin/hematocrit 3

Critical Diagnostic Pitfalls to Avoid

• Do not assume normal EPO excludes polycythemia vera: EPO sensitivity for polycythemia vera is only 64-70%; normal EPO with elevated hemoglobin still requires JAK2 testing 2, 1, 4

• Do not overlook smoking as a reversible cause: Smoker's polycythemia is a real condition caused by chronic carbon monoxide exposure; risk reduction begins within 1 year of cessation and returns to baseline after 5 years 4, 5

• Beware of false reassurance from "normal" EPO in secondary polycythemia: In chronic hypoxic states, EPO may normalize after hemoglobin stabilizes at a compensatory higher level, potentially mimicking polycythemia vera 4

• Do not miss JAK2-negative polycythemia vera: Approximately 3-5% of polycythemia vera cases carry JAK2 exon 12 mutations instead of JAK2V617F 5

• Do not assume COPD explains erythrocytosis without excluding JAK2 mutations: Patients can have both conditions simultaneously 5

• Do not perform unnecessary red cell mass measurements: Modern JAK2 testing and EPO levels have largely replaced the need for these measurements except in truly equivocal cases 4, 7, 8

Clinical Features That Trigger Immediate Workup

• Sustained hemoglobin increase ≥2 g/dL from baseline, even if within normal reference range 4
• Thrombocytosis, leukocytosis, microcytosis (from iron deficiency due to phlebotomy or occult bleeding) 2
• Splenomegaly 2
• Aquagenic pruritus (itching after warm water exposure) 4
• Unusual thrombosis (portal, hepatic, mesenteric veins, Budd-Chiari syndrome) 3
• Erythromelalgia (burning pain in hands/feet with erythema) 4