Antithrombin (AT)*
Spontaneous thromboembolic diseases are observed in presence of congenital or acquired deficiencies of antithrombin. These deficiencies are classified in 4 different groups:
• Type I: Decreased AT concentration and decreased AT activity; this is the most frequent case.
• Type II RS (Reactive Site): Normal AT concentration and decreased biological activity; a protein abnormality is present at the active site.
• Type II HBS (Heparin Binding Site): Normal AT concentration, normal AT activity in the absence of heparin, but decreased in its presence.
• Type II (Pleiotropic): Decreased AT concentration and activity; nonfunctional protein and at a lowered level.
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