Fortuitous Discovery of Asymptomatic Homozygous Antithrombin Deficiency: A Rare p.Leu131Phe Heparin‐Binding Site Mutation: A Case Report

Homozygous Type II heparin‐binding site antithrombin deficiency is a rare inherited thrombophilia associated with a high risk of early thrombotic events. We report a 6‐week‐old infant homozygous for the SERPINC1 c.391C>T (p.Leu131Phe) variant (Antithrombin Budapest III), with markedly reduced antithrombin activity (12%). Despite this high‐risk genotype, the patient remains asymptomatic without thrombotic or vascular abnormalities and has not required anticoagulant treatment to date. This case highlights the phenotypic heterogeneity of homozygous antithrombin Budapest III deficiency and the importance of genetic diagnosis, family screening, and careful long‐term follow‐up. Trial Registration: The authors have confirmed clinical trial registration is not needed for this submission