What is the definition of polycythemia?

Definition of Polycythemia

Polycythemia is defined by elevated hemoglobin ≥18.5 g/dL in men or ≥16.5 g/dL in women, or other evidence of increased red cell volume (such as hematocrit ≥99th percentile for age, sex, and altitude, or red cell mass >25% above mean normal predicted value). 1

Context-Specific Definitions

The definition varies based on clinical context:

Polycythemia Vera (Primary Polycythemia)

• Hemoglobin threshold: ≥18.5 g/dL in men, ≥16.5 g/dL in women serves as the first major diagnostic criterion 1
• Alternative criteria: Hemoglobin ≥17 g/dL in men or ≥15 g/dL in women if associated with a documented and sustained increase of at least 2 g/dL from baseline that cannot be attributed to correction of iron deficiency 1
• Red cell mass: Elevated >25% above mean normal predicted value can substitute for hemoglobin/hematocrit criteria 2, 1
• Molecular marker: The presence of JAK2 V617F mutation (found in >90-95% of PV cases) or other functionally similar JAK2 mutations (such as exon 12 mutations) constitutes the second major diagnostic criterion 2, 1

Neonatal Polycythemia

• Venous hematocrit >65% defines polycythemia in newborns 3, 4
• The hematocrit peaks at 2 hours of age and decreases gradually thereafter 3, 4
• The relationship between hematocrit and viscosity is linear until 65%, then becomes exponential, leading to hyperviscosity syndrome 3, 4

Diagnostic Framework for Polycythemia Vera

The 2008 WHO revision requires both major criteria plus at least one minor criterion for PV diagnosis 1:

Major Criteria:

1. Hemoglobin elevation as defined above 1
2. Presence of JAK2 V617F or functionally similar mutation 1

Minor Criteria:

1. Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis) 1
2. Serum erythropoietin level below normal reference range 1
3. Endogenous erythroid colony formation in vitro 1

Key Distinguishing Features

Primary vs. Secondary Polycythemia:

• JAK2 mutations are absent in secondary or spurious polycythemia, making molecular testing critical for differentiation 2
• Secondary polycythemia results from hypoxia, high oxygen affinity hemoglobin, inappropriate erythropoietin production, or other reactive causes 2
• Inflammatory markers (NLR, PLR, SII) are significantly elevated in PV compared to secondary polycythemia and may serve as adjunctive diagnostic tools when JAK2 testing is unavailable 5

Clinical Implications

• Thrombotic risk: PV carries a 56% lifetime thrombotic event rate, with both arterial (58%) and venous (42%) events 6
• Transformation risk: PV can progress to myelofibrosis or acute myeloid leukemia (blast phase) 7
• Neonatal context: Polycythemia-hyperviscosity syndrome affects multiple organ systems through hypoperfusion 3, 4