What are the typical laboratory findings in essential thrombocythemia (ET) and polycythemia vera (PV)?

Laboratory Findings in Essential Thrombocythemia and Polycythemia Vera

Essential Thrombocythemia (ET)

The hallmark laboratory finding in ET is a sustained platelet count ≥450 × 10⁹/L, with JAK2V617F mutation present in approximately 50-60% of cases, CALR mutation in 25%, and MPL mutation in 3%, while 17% remain triple-negative. 1, 2

Complete Blood Count Findings

• Platelet count: Sustained elevation ≥450 × 10⁹/L (revised WHO criteria lowered threshold from 600 × 10⁹/L) 1
• Hemoglobin: Normal or below PV range (men <18.5 g/dL, women <16.5 g/dL) 1
• White blood cell count: Usually normal, without significant left shift or granulocytic proliferation 1
• Red blood cell mass: Normal (critical to exclude PV) 1

Molecular and Genetic Testing

• JAK2V617F mutation: Present in 50-70% of ET patients 3, 4, 2
• CALR mutation: Present in approximately 25% of cases 2, 5
• MPL mutation: Present in approximately 3% of cases 2, 5
• Triple-negative status: Approximately 17% lack all three driver mutations 5

Bone Marrow Findings

• Megakaryocyte morphology: Proliferation mainly of megakaryocytic lineage with increased numbers of enlarged, mature megakaryocytes with deeply lobulated and hyperlobulated nuclei 1
• Cellularity: Normally or only slightly hypercellular for age 1
• Fibrosis: Minimal or absent reticulin fibrosis; collagen fibrosis absent 1
• Other lineages: No significant increase or left-shift of neutrophil granulopoiesis or erythropoiesis (distinguishes from PV) 1

Additional Laboratory Tests

• Serum ferritin: Normal or elevated (helps exclude iron deficiency as cause of reactive thrombocytosis) 1
• von Willebrand factor studies: Should be checked when platelet count >1,000 × 10⁹/L to screen for acquired von Willebrand syndrome 3, 4, 2
• BCR-ABL: Must be negative to exclude chronic myeloid leukemia 1

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Polycythemia Vera (PV)

The defining laboratory feature of PV is elevated hemoglobin (≥18.5 g/dL in men, ≥16.5 g/dL in women) or increased red cell mass, with JAK2 mutation present in virtually all cases (either JAK2V617F or JAK2 exon 12 mutation). 1, 3, 2

Complete Blood Count Findings

• Hemoglobin: ≥18.5 g/dL in men, ≥16.5 g/dL in women 1, 3
• Hematocrit: Elevated (treatment target is <45%) 1, 4, 2
• Red blood cell mass: Increased >25% above mean normal predicted value 1
• Platelet count: Elevated in approximately 50% of patients (thrombocytosis) 1
• White blood cell count: Elevated in approximately 50% of patients (leukocytosis) 1, 3

Molecular and Genetic Testing

• JAK2V617F mutation: Present in approximately 95% of PV cases 3, 4, 2
• JAK2 exon 12 mutation: Present in JAK2V617F-negative cases 1, 2
• Absence of JAK2 mutation combined with normal/increased serum erythropoietin excludes PV diagnosis 3, 4

Bone Marrow Findings

• Cellularity: Hypercellular for age with trilineage growth (panmyelosis) 1
• Proliferation pattern: Prominent erythroid, granulocytic, and megakaryocytic proliferation 1
• Megakaryocytes: Pleomorphic with hyperlobulated nuclei 1
• Iron stores: Absent or markedly decreased 1

Serum Erythropoietin

• Level: Below the reference range for normal (subnormal) 1, 3
• Diagnostic utility: Normal or increased EPO combined with absence of JAK2 mutation excludes PV 3, 4

Additional Laboratory Findings

• Serum ferritin: Often low or normal (iron deficiency may mask elevated hemoglobin) 1
• Mean corpuscular volume (MCV): May be normal or low if iron deficient 1
• Endogenous erythroid colony formation: Present in vitro (minor diagnostic criterion) 1

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Key Distinguishing Features Between ET and PV

Critical Differentiators

• Hemoglobin threshold: PV requires hemoglobin ≥18.5 g/dL (men) or ≥16.5 g/dL (women); ET must have hemoglobin below these thresholds 1
• Bone marrow pattern: PV shows panmyelosis (trilineage proliferation); ET shows predominantly megakaryocytic proliferation without significant erythroid or granulocytic increase 1
• Serum erythropoietin: Subnormal in PV; normal or elevated in ET 1, 3
• JAK2 mutation frequency: Nearly 100% in PV (including exon 12 mutations); 50-70% in ET 3, 4, 2

Common Pitfall to Avoid

Iron deficiency can mask PV by normalizing hemoglobin levels. In suspected PV with borderline hemoglobin and low ferritin, iron replacement therapy should be attempted; failure to increase hemoglobin to PV range after iron repletion helps exclude PV, while increase to PV range confirms the diagnosis. 1