Isolated thrombocytosis in chronic myeloid leukemia without significant leukocytosis

Chronic myeloid leukemia (CML) typically causes leukocytosis rather than thrombocytosis. We encountered two women in their thirties with remarkable thrombocytosis, whose platelet counts were over 3,000×10³/µl, and without significant leukocytosis. Although their clinical findings resembled that of essential thrombocythemia (ET), they were diagnosed with CML because of the presence of Philadelphia chromosome. JAK2, CALR, and MPL were unmutated. On fluorescence in situ hybridization analysis, only 19.8% of granulocytes in case 2 were found to be BCR/ABL positive in peripheral blood (PB). We reviewed 11 CML cases whose platelet counts were over 2,000×10³/µl, but their WBC counts were not significantly elevated (<12,000/µl). Most of them were young females with a normal or a high neutrophil alkaline phosphatase score and without immature myeloid cells in PB. These findings suggested that there is a subgroup of CML patients with marked thrombocytosis and without significant leukocytosis, which may be misdiagnosed as ET.