Extreme Thrombocytosis in Patients with Overt Myelofibrosis and Its Clinical Associations

<b>Background/Objectives</b>: Overt primary myelofibrosis (PMF), secondary post-polycythemia vera (post-PV), and post-essential thrombocythemia (post-ET) myelofibrosis (SMF) are chronic myeloproliferative neoplasms (MPN) that sometimes present with extreme thrombocytosis (ExTh, platelet count > 1000 × 10⁹/L), a phenomenon of uncertain clinical significance since there are no published data available. <b>Methods</b>: We retrospectively investigated the clinical correlations and associated outcomes of ExTh in a cohort of 172 patients with overt myelofibrosis diagnosed in six Croatian hematology centers. <b>Results</b>: ExTh was present in 5.8% of patients and was associated with post-ET etiology of myelofibrosis, older age, smaller spleen size, and the presence of arterial hypertension (<i>p</i> < 0.05 for all analyses). No significant associations were observed with sex, degree of bone marrow fibrosis, or driver mutation status. Over the follow-up period, patients with ExTh experienced a favorable course regarding survival (<i>p</i> < 0.001) and bleeding risk (<i>p</i> = 0.034), whereas no significant association with thrombotic risk was observed (<i>p</i> = 0.682). <b>Conclusions</b>: In contrast to its context in ET, ExTh in overt fibrotic MPN does not appear to confer higher bleeding or thrombotic risk. Instead, it is associated with more favorable survival outcomes and reduced bleeding risk.