Isolated Extreme Thrombocytosis as the Presenting Feature of Chronic Myeloid Leukemia-An Unusual Phenotype Treated With Plateletpheresis.

Chronic myeloid leukemia (CML) can rarely present with extreme thrombocytosis as the initial manifestation, a phenotype that can mimic essential thrombocythemia, leading to delayed diagnosis. We describe the case of a 54-year-old woman who was evaluated by outpatient hematology for isolated thrombocytosis with normal iron studies and leukocyte counts. Molecular testing confirmed BCR::ABL1 fusion, consistent with chronic phase CML. She was started on hydroxyurea, but her platelet count continued to rise, requiring hospitalization for close monitoring, where she was initiated on dasatinib, a tyrosine kinase inhibitor (TKI). As her platelets peaked above 4,000 × 109/L (Reference Range [RR]: 150-450 × 109/L), given the high risk of both thrombotic and bleeding complications, she underwent plateletpheresis, which effectively reduced counts before TKI therapy could achieve disease control. This case highlights the importance of considering the diagnosis of CML in patients with isolated thrombocytosis, particularly in women, and emphasizes the role of plateletpheresis as a potential preventive bridge until TKI therapy takes effect.