Antithrombin in infancy and childhood.

Antithrombin III was measured immunologically, and antithrombin activity was measured with aid of a new synthetic tripeptide substrate for serine proteases in plasma of healthy infants and children, 0-14 years of age. Both methods gave decreased values in the youngest infants as compared with adults. The antithrombin increased with age and reached adult values about 6 months of age. In infants up to one month of age, antithrombin III measured immunologically was significantly lower than antithrombin activity, whereas there was no difference between the antithrombin methods in the higher ages. Antithrombin III concentration and antithrombin activity as measured were poorly correlated.