CBC with Differential Findings in Polycythemia Vera
Polycythemia vera presents on CBC with differential as elevated hemoglobin (≥18.5 g/dL in men, ≥16.5 g/dL in women) and hematocrit (≥49% in men, ≥48% in women), frequently accompanied by thrombocytosis (in 53% of patients) and leukocytosis (in 49% of patients). 1, 2
Primary Red Blood Cell Abnormalities
The hallmark finding is erythrocytosis, which manifests as:
- Hemoglobin elevation to ≥18.5 g/dL in men or ≥16.5 g/dL in women 1
- Hematocrit elevation to ≥49% in men or ≥48% in women 1, 3
- A sustained increase of ≥2 g/dL hemoglobin from baseline, even if values remain within normal reference ranges, should trigger investigation 1, 4
Critical pitfall: Iron deficiency can mask the erythrocytosis in PV. Look for low MCHC (<32%), which suggests concurrent iron deficiency that may be hiding the true extent of red cell elevation. 5, 4 This is why some patients present with "masked PV" where hemoglobin values are lower than typical diagnostic thresholds but still represent true PV. 6
White Blood Cell Findings
Leukocytosis is present in approximately 49% of PV patients at diagnosis. 2
- White blood cell count is often elevated, typically >10 × 10⁹/L 1
- Leukocytosis >15 × 10⁹/L is a risk factor for inferior survival and increased thrombotic risk 1, 6
- The differential typically shows increased neutrophils without significant left shift 1
- Important consideration: Progressive leukocytosis may indicate need for cytoreductive therapy even in otherwise low-risk patients 1
Platelet Abnormalities
Thrombocytosis occurs in approximately 53% of PV patients. 2
- Platelet counts are frequently elevated, often >400 × 10⁹/L 1
- Platelet counts >1,500 × 10⁹/L indicate need for cytoreductive therapy 1
- Critical warning: Extreme thrombocytosis (≥1,000 × 10⁹/L) paradoxically increases bleeding risk due to acquired von Willebrand disease 1, 7, 2
- Platelet size abnormalities may be present, with altered mean platelet volume 7
Additional CBC Parameters to Evaluate
Mean corpuscular hemoglobin concentration (MCHC) deserves special attention:
- Low MCHC (<32%) suggests iron deficiency masking the full extent of erythrocytosis 5, 4
- Iron deficiency is common in PV due to chronic phlebotomy or occult bleeding
- Checking serum ferritin and iron studies is essential if MCHC is low 4
Distinguishing PV from Secondary Polycythemia on CBC
The CBC alone cannot definitively distinguish PV from secondary causes, but certain patterns are suggestive:
- PV typically shows: Erythrocytosis + thrombocytosis + leukocytosis (panmyelosis) 1
- Secondary polycythemia typically shows: Isolated erythrocytosis without significant platelet or white cell elevation 5
- The presence of thrombocytosis and leukocytosis alongside erythrocytosis strongly favors PV over secondary causes 1, 2
What the CBC Cannot Tell You
The CBC findings must be interpreted with additional testing:
- JAK2 mutation testing (present in >95% of PV cases) is essential for diagnosis 1, 5
- Serum erythropoietin levels (low EPO has >90% specificity for PV) 5, 4
- Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis) confirms the diagnosis 1, 4
Common pitfall: Do not rely on traditional markers like vitamin B12 levels or leukocyte alkaline phosphatase, as these lack sensitivity and specificity for PV diagnosis. 5