Polycythemia: Definition and Diagnostic Hemoglobin Thresholds
Polycythemia vera (PV) is diagnosed when hemoglobin is ≥18.5 g/dL in men or ≥16.5 g/dL in women, according to the revised WHO criteria. 1, 2
Understanding Polycythemia
Polycythemia refers to an elevated red blood cell mass above normal ranges for age and sex. It exists in two main forms:
Primary Polycythemia (Polycythemia Vera)
- A clonal myeloproliferative neoplasm caused by JAK2 mutations (present in >95% of cases) 3
- Results in autonomous red blood cell proliferation independent of erythropoietin
- Affects approximately 65,000 people in the US with annual incidence of 0.5-4.0 per 100,000 3
Secondary Polycythemia
- Reactive increase in red blood cells due to external factors
- Common causes: chronic hypoxia (smoking, sleep apnea, COPD), erythropoietin-secreting tumors, high altitude exposure
- At high altitude (>2,500 meters), defined as hemoglobin ≥210 g/dL in males and ≥190 g/dL in females 4
WHO Diagnostic Criteria for Polycythemia Vera
The 2007 revised WHO criteria require both major criteria plus 1 minor criterion OR the first major criterion plus 2 minor criteria 1, 2:
Major Criteria:
- Hemoglobin ≥18.5 g/dL in men, ≥16.5 g/dL in women OR other evidence of increased red cell volume (hematocrit >99th percentile for age/sex/altitude, OR hemoglobin ≥17 g/dL in men/≥15 g/dL in women with documented sustained increase of ≥2 g/dL from baseline)
- Presence of JAK2 V617F mutation or functionally similar mutation (JAK2 exon 12)
Minor Criteria:
- Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis)
- Serum erythropoietin level below normal reference range
- Endogenous erythroid colony formation in vitro
Important Clinical Nuances
Iron deficiency confounds diagnosis: Hemoglobin may be falsely normal or low in PV patients with concurrent iron deficiency. In routine practice, you can make a working diagnosis of PV despite not meeting hemoglobin thresholds if iron deficiency is present, though formal research criteria require demonstrating elevated hemoglobin after iron replacement 1.
Masked PV exists: A subset of patients have JAK2-positive PV with normal hemoglobin/hematocrit due to blood dilution or coincidental blood loss, yet still carry thrombotic risk and require treatment 5. These patients may present with unusual site thrombosis (splanchnic veins) as their first manifestation.
Alternative hemoglobin thresholds for early disease: The criteria also recognize hemoglobin ≥17 g/dL in men or ≥15 g/dL in women as diagnostic if there is a documented sustained increase of ≥2 g/dL from the individual's baseline that cannot be attributed to iron deficiency correction 1.
Clinical Manifestations
Common presenting features include:
- Thrombocytosis (53%) and leukocytosis (49%) 3
- Pruritus, especially after warm baths (33%) 3
- Erythromelalgia—burning pain in hands/feet (5.3%) 3
- Splenomegaly (36%) 3
- Arterial thrombosis (16% at or before diagnosis) and venous thrombosis (7%), including unusual sites like splanchnic veins 3
Critical pitfall: When evaluating polycythemia with normal hemoglobin but with unusual site thrombosis, leukocytosis, or thrombocytosis, always test for JAK2 mutations—do not be falsely reassured by "normal" blood counts 5, 6.
Prognosis and Complications
Median survival ranges from 14.1 to 27.6 years across cohorts 3. Long-term risks include: