Primary thrombocythemia is one of the myeloproliferative disorders, in which the dominant abnormal proliferation involves megakaryocytes. It is important in the diagnosis to exclude other causes of thrombocytosis, especially the other myeloproliferative disorders, which are more common. The following criteria are those of the Polycythemia Vera Study Group.


Features required for the diagnosis of primary thrombocythemia:

(1) a platelet count > 600,000 /µL on 2 occasions at least one month apart

(2) megakaryocytic hyperplasia often with aggregates of megakaryocytes

(3) absence of an identifiable condition associated with secondary thrombocytosis (exclude secondary thrombocytosis)

(4) normal or reduced red cell mass (exclude polycythemia rubra vera)

(5) either no marrow fibrosis; or fibrosis of the marrow with less than 33% involvement + no leukoerythroblastic response + no significant splenomegaly (exclude myelofibrosis)

(6) patient negative for the Philadelphia chromosome or bcr/abl fusion gene (exclude chronic myelogenous leukemia)

(7) iron deficiency excluded by stainable iron in the bone marrow or normal serum ferritin levels (exclude iron deficiency)


Additional findings:

(1) splenomegaly, usually mild

(2) abnormal platelet function tests, with reduced aggregation to epinephrine and ADP

(3) history of thrombosis

(4) history of bleeding, and so may be associated with iron deficiency, but iron replacement does not correct platelet count

(5) abnormal platelets in peripheral blood smear, sometimes with megakaryocyte fragments


Differential Diagnosis of primary thrombocythemia:

(1) secondary thrombocytosis

(1a) history of chronic infection, chronic inflammatory condition, splenectomy or non-hematologic malignancy

(1b) correction of underlying disorder results in return of platelet count to normal

(2) iron deficiency

(2a) absent marrow iron stores or reduced serum ferritin levels

(2b) correction of deficiency results in return of platelet count to normal

(3) chronic myelogenous leukemia

(3a) presence of Philadelphia chromosome or bcr/abl fusion gene

(4) polycythemia rubra vera

(4a) increased red cell mass (>36 mL/kg for males, > 32 mL/kg for females)

(4b) elevated hemoglobin

(5) myelofibrosis

(5a) increased marrow fibrosis

(5b) splenomegaly which may be moderate or severe

(5c) leukoerythroblastic response with immature granulocytes and nucleated red blood cells in peripheral blood


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