Antithrombin III (ATIII) is a protein produced by the liver that helps prevent the inappropriate formation of blood clots. Individuals with an ATIII deficiency have a higher risk of blood clot formation, particularly phlebitis. ATIII deficiencies may result from a variety of conditions including heredity conditions, liver synthesis deficiency, excessive alcohol consumption or increased loss (kidney disease, bleeding). In a type 1 ATIII deficiency, the protein functions normally but is present in insufficient quantities. In type 2, there is enough protein, but it does not function normally. The antithrombin III Ag test (antithrombin antigen) measures the quantity of protein, whereas the functional ATIII test (antithrombin activity) measures both its quantity and quality.
When the functional ATIII is normal, the Ag test is not required. A decrease in the level (Ag) and activity of ATIII suggests a type 1 deficiency. A normal Ag level combined with reduced activity suggests a Type 2 deficiency. Temporarily decreased levels can be observed in a series of conditions ranging from phlebitis to heparin therapy. High levels generally have no clinical significance.