Ghirardi, Arianna

ABSTRACT This study aimed to distinguish between essential thrombocythemia (ET) and prefibrotic primary myelofibrosis (pre‐PMF) using routine blood tests, with a focus on white blood cell (WBC) and platelet (PLT) levels. We evaluated the predicted probability of a pre‐PMF diagnosis based on the interaction between WBC and PLT levels using data from a logistic regression model involving 891 patients with ET and 180 patients with pre‐PMF. Patients were divided into four groups based on whether their WBC and PLT values were below or above the respective thresholds of 8.85 × 10 9 /L and 793 × 10 9 /L. The results showed that patients with low levels of both WBCs and PLTs had the lowest predicted probability of pre‐PMF (6%), indicating a profile more typical of ET. When either WBC or PLT levels were elevated, the probability increased to approximately 18%–19%, indicating a potential shift toward pre‐PMF features. Notably, patients with elevated levels of both WBCs and PLTs had the highest probability of a pre‐PMF diagnosis (26%), which was more than four times higher than that of the low–low group. The association between combined WBC/PLT levels and a pre‐PMF diagnosis remained significant ( p < 0.001), even when adjusting for serum lactate dehydrogenase (LDH) and splenomegaly. These findings suggest that elevated WBC and PLT levels together can serve as a practical and accessible diagnostic tool for supporting the differential diagnosis of ET or pre‐PMF in cases of uncertainty, and for deferring bone marrow biopsy in cases of early disease presenting with isolated thrombocytosis.