Immature Platelet Fraction as a Potential Biomarker of Dysregulated Thrombopoiesis in Philadelphia-Negative Myeloproliferative Neoplasms.

Background/Objectives: Thrombotic events represent the leading cause of morbidity and mortality in patients with Philadelphia chromosome-negative myeloproliferative neoplasms (Ph- MPNs), particularly in those aged > 60 years. The immature platelet fraction (IPF) reflects the proportion of newly released, reticulated, highly reactive platelets and has emerged as a marker of thrombopoietic activity in various prothrombotic conditions. Methods: We prospectively measured IPF in 45 patients with newly diagnosed Ph- MPNs (24 with essential thrombocythemia, 13 with polycythemia vera, 5 with MPN-unclassified, and 3 with primary myelofibrosis) and 27 controls without MPN. Results: IPF was significantly higher in patients with Ph- MPN than in controls (median 27 vs. 10.9, p < 0.0001). Within the MPN cohort, IPF values differed significantly across subtypes (p = 0.027), being highest in essential thrombocythemia and primary myelofibrosis, intermediate in unclassified MPN, and lowest in polycythemia vera. Patients older than 60 years exhibited higher IPF independently of platelet count (p = 0.021). No significant difference was observed between JAK2V617F-positive and -negative cases. Conclusions: These results indicate that IPF captures accelerated and dysregulated thrombopoiesis characteristics of Ph- MPNs and may provide additional insight into subtype-specific biology and age-related prothrombotic risk beyond conventional complete blood count parameters.