What is the role of protein C and protein S testing in the workup of antithrombin III (ATIII) deficiency?

Updated: Mar 04, 2020
  • Author: James L Harper, MD; Chief Editor: Hassan M Yaish, MD  more...
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Protein C or protein S deficiencies are both associated with venous thrombosis and are important exclusions in evaluating congenital deficiency of antithrombin III.

In the newborn, protein S activity must be measured (in addition to total) because, whereas total antigen levels are lower in neonates than in adults, protein S activity is usually normal because of the lack of expression of C4-binding protein in the neonate. (C4 acts to bind protein S in children and adults.)

These tests may also be important in the acquired state to determine the extent of a given patient's thrombotic risk.

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