Bone marrow (BM) morphologic features remain the cornerstone of diagnosis in both essential thrombocythemia (ET) and polycythemia vera (PV). In addition, recently discovered mutations, such as JAK2, CALR, and MPL, have proven useful in facilitating the diagnostic process. A JAK2 mutation is expected in PV, and its absence makes the diagnosis unlikely. However, JAK2 mutations also occur in about 60% of patients with ET, which underlines the need for BM examination in distinguishing JAK2-mutated ET from PV when the hemoglobin/hematocrit level is diagnostically equivocal (ie, as in "masked" PV). Most patients with JAK2-unmutated ET express CALR or MPL mutations, with respective estimated incidences of 22% and 3%, while approxmately 15% are wild-type for all 3 mutations (ie, they are triple-negative). As such, CALR first, followed by MPL if CALR is absent, mutation screening is appropriate in the diagnostic work-up of JAK2-unmutated ET but does not replace the need for BM morphologic examination in (1) confirming the diagnosis in triple-negative ET and (2) distinguishing ET from other myeloproliferative neoplasms that share the same mutations, including masked PV and early/prefibrotic myelofibrosis. Young patients (aged <60 years) with ET or PV and no history of thrombosis are conventionally regarded as having "low-risk" disease. First-line treatment in low-risk PV is phlebotomy to achieve a hematocrit target of 45% and low-dose aspirin, and first-line treatment in ET is observation alone in the absence of additional risk factors for arterial thrombosis (ie, JAK2 mutation and cardiovascular risk factors) or low-dose aspirin therapy, once or twice daily, in the presence of one or both of these risk factors, respectively. Cytoreductive therapy is indicated in high-risk (patients aged ≥60 years or a history of thrombosis) PV or ET in the form of hydroxyurea as first-line and interferon alfa or busulfan as second-line drugs of choice. We do not use ruxolitinib in patients with PV unless they have severe pruritus or symptomatic splenomegaly that is proved to be refractory to hydroxyurea, interferon alfa, and busulfan.
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