What is the typical blood picture in polycythemia vera?

Blood Picture in Polycythemia Vera

The typical blood picture in polycythemia vera shows elevated hemoglobin (>16.5 g/dL in men, >16.0 g/dL in women), often accompanied by thrombocytosis (present in 53% of cases) and leukocytosis (present in 49% of cases), creating a characteristic panmyelosis pattern. 1

Complete Blood Count Findings

Red Blood Cell Parameters

• Hemoglobin elevation is the defining feature, with diagnostic thresholds of >16.5 g/dL in men or >16.0 g/dL in women according to WHO 2016 criteria 2, 3
• Hematocrit is correspondingly elevated, typically >49% in men or >48% in women 3
• Red cell mass is increased >25% above mean normal predicted value, though direct measurement is no longer routinely required for diagnosis 4
• Microcytosis may be present due to iron deficiency from chronic phlebotomy or increased iron utilization 3

White Blood Cell Findings

• Leukocytosis occurs in approximately 49% of patients at presentation, with white blood cell counts often >12 × 10⁹/L 1
• Neutrophil predominance is typical, reflecting the granulocytic proliferation that characterizes the disease 4
• Basophilia may be observed as part of the myeloproliferative process 5

Platelet Abnormalities

• Thrombocytosis is present in approximately 53% of cases, with platelet counts frequently >400 × 10⁹/L 1
• Extreme thrombocytosis (≥1000 × 10⁹/L) can occur and paradoxically increases bleeding risk due to acquired von Willebrand disease 1

Peripheral Blood Smear Characteristics

• The smear typically shows normochromic, normocytic red cells unless iron deficiency has developed 5
• Increased red cell mass creates a crowded appearance with numerous erythrocytes per high-power field 5
• Platelets may appear increased in number and occasionally show anisocytosis 5
• Mature granulocytes predominate when leukocytosis is present, without significant left shift in uncomplicated cases 4

Bone Marrow Findings (Major Criterion)

• Hypercellularity for age with trilineage growth (panmyelosis) is characteristic, showing prominent erythroid, granulocytic, and megakaryocytic proliferation 2, 3
• Pleomorphic, mature megakaryocytes with hyperlobulated nuclei are a hallmark feature, often appearing in loose clusters 4
• Mild reticulin fibrosis is present in approximately 12% of newly diagnosed cases 3
• Iron stores are typically absent or markedly reduced on bone marrow examination 4

Supporting Laboratory Features

Serum Erythropoietin

• Low or subnormal serum erythropoietin (below the reference range) is found in the majority of cases and serves as a WHO minor criterion 2, 3
• This finding has >90% specificity but only 64–70% sensitivity for polycythemia vera 3
• A normal erythropoietin level does not exclude the diagnosis 3

Molecular Markers

• JAK2 V617F mutation is present in >95% of cases and is a WHO major criterion 2, 1, 6
• JAK2 exon 12 mutations account for an additional 2–3% of cases when V617F is negative 2
• The variant allele frequency correlates with disease burden and prognosis 6

Clinical Pitfalls

• Masked polycythemia can occur when increased plasma volume dilutes the hemoglobin/hematocrit, keeping values in the normal range despite elevated red cell mass; this "inapparent polycythemia vera" represents approximately 17% of cases 7
• Iron deficiency can falsely lower hemoglobin values, masking the true degree of erythrocytosis; formal diagnosis should be deferred until after iron replacement 2, 3
• The presence of isolated thrombocytosis or leukocytosis without obvious erythrocytosis should still prompt evaluation for polycythemia vera, particularly if splenomegaly is present 3, 7
• Normal oxygen saturation (>92%) must be documented to exclude secondary causes of erythrocytosis from hypoxia 8

Diagnostic Algorithm Based on Blood Picture

When the blood picture shows elevated hemoglobin/hematocrit:

1. First-line testing: Order JAK2 V617F mutation analysis 2, 3
2. If JAK2 V617F positive: Diagnosis requires only one additional minor criterion (low erythropoietin, bone marrow hypercellularity, or endogenous erythroid colonies) 2
3. If JAK2 V617F negative: Test for JAK2 exon 12 mutations 2
4. If all JAK2 testing negative: Require two minor criteria plus exclusion of secondary causes 2

When thrombocytosis, leukocytosis, or splenomegaly is present without meeting hemoglobin thresholds, red cell mass measurement or bone marrow biopsy should be considered to detect masked polycythemia vera 7, 8