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The proposed revised World Health Organization criteria for the diagnosis of essential thrombocythemia requires all four criteria.
Sustained platelet count of at least 450 � 109 /L.
Bone marrow biopsy showing predominant proliferation of enlarged mature megakaryocytes; no significant increase of granulocytic or erythroid precursors.
Not meeting criteria for polycythemia vera (p. vera), primary myelofibrosis, chronic myelogenous leukemia, myelodysplastic syndrome, or other myeloid neoplasm.
Demonstration of JAK2 617V greater than F or other clonal marker. In the absence of a clonal marker, there must be no evidence for reactive thrombocytosis. In particular, with a decreased serum ferritin, there must be no increase in hemoglobin level to p. vera range with iron replacement therapy.
Patients older than 60 years or those with a prior thrombotic episode have as much as a 25% chance of developing cerebral, cardiac, or peripheral arterial thromboses and, less often, a chance of developing a pulmonary embolism or deep venous thrombosis. Similar to the other myeloproliferative syndromes, conversion to acute leukemia is found in a small percentage of patients (<10%) with long-term follow-up.
There is no staging system for this disease.
Untreated essential thrombocythemia means that a patient is newly diagnosed and has had no prior treatment except supportive care.
Controversy is considerable regarding whether asymptomatic patients with essential thrombocythemia require treatment. A randomized trial of patients with essential thrombocythemia and a high risk of thrombosis compared treatment with hydroxyurea titrated to attain a platelet count below 600,000/mm3 with a control group that received no therapy. Hydroxyurea was found to be effective in preventing thrombotic episodes (4% vs. 24%).[Level of evidence: 1iiDiv] A retrospective analysis of this trial found that antiplatelet drugs had no significant influence on the outcome. Resistance to hydroxyurea is defined as a platelet count of greater than 600,000/mcL after 3 months of at least 2 g per day of hydroxyurea or a platelet count greater than 400,000/�L and a white blood count of less than 2,500/�L or a hemoglobin less than 10 g/dL at any dose of hydroxyurea.
In a case-controlled observational study of 65 low-risk patients (<60 years of age, platelet count <1,500 � 109 /L, and no history of thrombosis or hemorrhage) with a median follow-up of 4.1 years, the thrombotic risk of 1.91 cases per 100 patient years and hemorrhagic risk of 1.12 cases per 100 patient years was not increased over the normal controls. A prospective randomized trial of 809 patients compared hydroxyurea plus aspirin versus anagrelide plus aspirin. Although the platelet-lowering effect was equivalent, the anagrelide group had significantly more thrombotic and hemorrhagic events (hazard ratio [HR] = 1.57; P = .03) and more myelofibrosis (HR = 2.92; P = .01). No differences were seen for myelodysplasia or acute leukemia.[Level of evidence: 1iiA] Many clinicians use hydroxyurea or platelet apheresis prior to elective surgery to reduce the platelet count and to prevent postoperative thromboembolism. No prospective or randomized trials document the value of this approach.