[Primary myelofibrosis with Factor V Leiden diagnosed following portal vein thrombosis].

Primary myelofibrosis (PMF), a subtype of myeloproliferative neoplasms, is frequently associated with thrombosis, particularly in the presence of the JAK2 V617F mutation. In contrast, Factor V Leiden (FVL) is a hereditary thrombophilic mutation commonly observed in Caucasian populations, and clinical encounters with FVL carriers are extremely rare in Japan. Here, we report a case of an American patient who presented with portal hypertension and esophageal variceal bleeding, which led to the diagnosis of PMF. Based on family history, the patient was also diagnosed as a carrier of FVL. Treatment with ruxolitinib resulted in a marked improvement in splenomegaly and symptoms related to portal hypertension. Additionally, the patient safely underwent invasive surgery for a fracture with appropriate perioperative thrombosis risk management. This case represents a rare instance of PMF with FVL mutation encountered in Japan. In cases of atypical-site thrombosis, particularly among Caucasian patients, comprehensive evaluation including MPN driver mutations and other thrombophilic factors is essential to avoid underdiagnosis.