Which myeloproliferative neoplasm is most likely with extreme thrombocytosis (platelets ~1180 x10^9/L) and JAK2 positivity with BCR-ABL negative?

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Multiple Choice

Which myeloproliferative neoplasm is most likely with extreme thrombocytosis (platelets ~1180 x10^9/L) and JAK2 positivity with BCR-ABL negative?

Extreme thrombocytosis with a JAK2 mutation and a BCR-ABL–negative test strongly points to essential thrombocythemia. The hallmark of essential thrombocythemia is clonal megakaryocyte proliferation driving very high platelet counts, often exceeding 1,000 x 10^9/L. The presence of a JAK2 mutation supports a myeloproliferative process driven by constitutive JAK-STAT signaling; many ET cases harbor JAK2, CALR, or MPL mutations. Importantly, the absence of BCR-ABL distinguishes this from chronic myeloid leukemia, which would be BCR-ABL positive and typically not show isolated extreme thrombocytosis as the defining feature. Consider how the other neoplasms would present differently. Polycythemia vera centers on increased red cell mass (elevated hematocrit/hemoglobin), though platelets can rise, the dominant abnormality is erythrocytosis rather than isolated thrombocytosis. Primary myelofibrosis often features splenomegaly, anemia, and a leukoerythroblastic smear with marrow fibrosis, rather than pure, marked thrombocytosis as the defining clue. Aplastic anemia shows pancytopenia due to marrow failure, not elevated platelets.

Extreme thrombocytosis with a JAK2 mutation and a BCR-ABL–negative test strongly points to essential thrombocythemia. The hallmark of essential thrombocythemia is clonal megakaryocyte proliferation driving very high platelet counts, often exceeding 1,000 x 10^9/L. The presence of a JAK2 mutation supports a myeloproliferative process driven by constitutive JAK-STAT signaling; many ET cases harbor JAK2, CALR, or MPL mutations. Importantly, the absence of BCR-ABL distinguishes this from chronic myeloid leukemia, which would be BCR-ABL positive and typically not show isolated extreme thrombocytosis as the defining feature.

Consider how the other neoplasms would present differently. Polycythemia vera centers on increased red cell mass (elevated hematocrit/hemoglobin), though platelets can rise, the dominant abnormality is erythrocytosis rather than isolated thrombocytosis. Primary myelofibrosis often features splenomegaly, anemia, and a leukoerythroblastic smear with marrow fibrosis, rather than pure, marked thrombocytosis as the defining clue. Aplastic anemia shows pancytopenia due to marrow failure, not elevated platelets.

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