Functional Antithrombin (AT) Deficiency Presenting as Multiterritorial Splanchnic Vein Thrombosis: A Case Report.

Functional antithrombin (AT) deficiency is an inherited thrombophilic disorder that can present as venous thrombosis at atypical sites. We report the case of a 47-year-old man with a family history of venous thromboembolism who presented with thrombosis involving the splenoportal confluence and the superior mesenteric vein. Initial laboratory evaluation revealed normal AT activity levels and a negative standard thrombophilia panel. Due to the unusual thrombotic location and family history, extended genetic testing was performed, which identified a pathogenic heterozygous variant in SERPINC1 NM_000488.3:c.749C > A (p.Thr250Asn), consistent with functional AT deficiency. After the resolution of gastrointestinal bleeding complications, long-term anticoagulation therapy was reintroduced. This case underscores the importance of considering hereditary thrombophilia in patients with thrombosis at unusual sites even when standard laboratory studies are unremarkable.