JAK2-Positive Myeloproliferative Neoplasm: Polycythaemia Vera Presenting As Budd-Chiari Syndrome.

Budd-Chiari syndrome (BCS), caused by hepatic venous outflow obstruction, is a critical diagnosis often masking an underlying myeloproliferative neoplasm (MPN). We present the case of a 33-year-old female whose diagnosis was driven by hepatic decompensation rather than typical haematological symptoms of MPN. Initial imaging confirmed the anatomical pathology, revealing chronic BCS on computed tomography (non-visualised hepatic veins, hepatic vein stenosis, and classic caudate lobe hypertrophy). The subsequent work-up noted a paradoxical haematological picture (significant neutrophilic leucocytosis (up to 55,570 cells/mm³) and relative thrombocytosis) despite splenomegaly and portal hypertension. This finding, combined with the CT evidence, prompted definitive genetic screening, which confirmed the diagnosis of an MPN by detecting the JAK2 exon 14 c.1849G>T (p.Val617Phe) mutation (variant allele frequency of 79.2%). This case underscores the importance of utilising radiological evidence in the initial stages and recognising the combination of an atypical blood picture alongside hepatic decompensation as a key indicator for an underlying MPN, allowing for prompt molecular diagnosis and targeted therapy.