What test is used to confirm polycythemia vera?

Diagnostic Testing for Polycythemia Vera

The diagnosis of polycythemia vera requires JAK2 mutation testing (V617F followed by exon 12 if negative) combined with hemoglobin/hematocrit measurement, serum erythropoietin level, and bone marrow biopsy when needed to satisfy WHO criteria. 1

Primary Diagnostic Tests

JAK2 Mutation Testing (Essential First-Line Test)

• JAK2 V617F mutation testing is the mandatory initial molecular test, detecting >90-95% of polycythemia vera cases and serving as a major WHO diagnostic criterion 1, 2
• If JAK2 V617F is negative, proceed immediately to JAK2 exon 12 mutation analysis, which identifies an additional 2-3% of cases 2
• The combination of both JAK2 tests captures >97% of polycythemia vera patients when paired with elevated hemoglobin/hematocrit 1

Hemoglobin/Hematocrit Measurement (Major Criterion)

• Men require hemoglobin ≥18.5 g/dL or hematocrit >49% to meet the first major WHO criterion 1, 2
• Women require hemoglobin ≥16.5 g/dL or hematocrit >48% 1, 2
• A sustained increase of ≥2 g/dL from baseline (reaching ≥17 g/dL in men or ≥15 g/dL in women) also satisfies this criterion 1

Serum Erythropoietin Level (Minor Criterion)

• Measure serum erythropoietin in all suspected cases—a level below the laboratory reference range serves as a WHO minor criterion 1, 2
• Low or low-normal erythropoietin supports primary polycythemia, while elevated levels mandate evaluation for secondary causes 1
• Erythropoietin may fall within normal range in up to 30% of confirmed polycythemia vera cases, so a normal value does not exclude the diagnosis 1

Bone Marrow Biopsy (Minor Criterion)

• Perform bone marrow biopsy when JAK2 is negative or diagnosis remains equivocal 1
• Characteristic findings include hypercellularity with trilineage (panmyelosis) proliferation, increased pleomorphic megakaryocytes with cluster formation, and decreased iron stores 1, 2
• This histology satisfies a WHO minor criterion and is particularly valuable in JAK2-negative cases 1

WHO Diagnostic Algorithm

Two pathways establish the diagnosis: 1, 2

Pathway 1 (Captures >97% of Cases)

• Both major criteria (elevated hemoglobin/hematocrit AND JAK2 mutation)
• PLUS at least 1 minor criterion (low erythropoietin, characteristic bone marrow histology, or endogenous erythroid colony formation)

Pathway 2 (For JAK2-Negative or Low-Burden Cases)

• First major criterion (elevated hemoglobin/hematocrit)
• PLUS at least 2 minor criteria

Specialized Tests (Limited Role)

• Endogenous erythroid colony (EEC) assay is a WHO minor criterion but has limited availability and requires specialized expertise 1
• Red cell mass measurement is no longer required by WHO criteria and has been largely abandoned due to cost, complexity, and availability of JAK2 testing 3, 4
• Red cell mass may be considered only when hemoglobin/hematocrit values are borderline, JAK2 is negative, and diagnosis remains unclear after bone marrow examination 3

Critical Diagnostic Pitfalls

• Never diagnose polycythemia vera based solely on low erythropoietin—this violates WHO criteria and risks missing secondary causes 2
• Check iron studies (ferritin, transferrin saturation) before finalizing diagnosis, as iron deficiency can mask true erythrocytosis by lowering hemoglobin while red cell mass remains elevated 2, 3
• A hemoglobin of 16 g/dL in men does NOT meet the major criterion threshold of ≥18.5 g/dL; such cases require JAK2 testing and evaluation via the minor criteria pathway 2
• JAK2 V617F positivity alone is insufficient for diagnosis—it occurs in 50% of essential thrombocythemia and primary myelofibrosis cases, so WHO criteria must be fully satisfied 1, 5
• Cytogenetic studies have limited diagnostic value, showing abnormalities in only 13-18% of cases at diagnosis 1

Practical Diagnostic Sequence

1. Measure hemoglobin/hematocrit and order JAK2 V617F testing in any patient meeting threshold values 2, 4
2. If JAK2 V617F is negative, immediately order JAK2 exon 12 testing 2
3. Obtain serum erythropoietin level (low = minor criterion; high = evaluate for secondary polycythemia) 1
4. Perform bone marrow biopsy if:
   • JAK2 is negative 1
   • Erythropoietin is normal or high 1
   • Diagnosis remains uncertain after initial testing 1
5. Check iron studies to exclude masked erythrocytosis from iron deficiency 2, 3