Antithrombin Phe229Leu: a new homozygous variant leading to spontaneous antithrombin polymerization in vivo associated with severe childhood thrombosis.
Before heparinisation, he was discovered to have a low antithrombin III level (biological activity (B) 60%, immunological level (I) 50) and a further inquiry showed the same abnormality in 4 members of the family, leading to a diagnosis of a congenital deficit: a 35 year old sister with a bilateral post-DVT changes had antithrombin III levels of 70% (B) and 45% (I); two nephews, sons of the affected sister: the one aged 5 years was asymptomatic despite antithrombin III levels of 50% (I) and 70% (B); the other had experience DVT at the age of 2 and, on oral anti-vitamin K drugs, had antithrombin III levels of 55% (I) and 67% (B) at the age of 15 years; the patient's brother died at the age of 29 of cerebral vein thrombosis after pulmonary embolism.