A 42-year-old man is admitted with bleeding from esophageal varices. Investigation reveals that he has an occlusion of the portal vein. There is no evidence of liver cirrhosis. Which test will most likely reveal an underlying predisposing factor for this condition?

Antithrombin III counteracts excess of thrombin formation. The excess of thrombin facilitates conversion of fibrinogen to fibrin. Portal vein thrombosis will lead to portal hypertension but not hepatic congestion, as seen in Budd-Chiari syndrome. Portal vein thrombosis may occur in cirrhosis, trauma, in patients on contraceptive tablets, and in those who have an increased propensity for thrombus formation. It is also a direct complication of periumbilical infection in the neonate.