What is the role of antithrombin in hemostasis and how is it monitored?

• A. Inhibits thrombin and factor Xa; monitored by anti-Xa activity.
• B. Activates thrombin; monitored by PT
• C. Inhibits platelet aggregation; monitored by platelet count
• D. Activates factor VII; monitored by aPTT

Answer: (A)

Antithrombin acts as a natural brake on coagulation by neutralizing key enzyme players in the cascade, especially thrombin (factor IIa) and factor Xa. Its inhibition is dramatically accelerated by heparin, which binds antithrombin and enhances its ability to inactivate these proteases.

To assess how well antithrombin is functioning, the most direct and specific test is anti-Xa activity. In this assay, added factor Xa is inhibited in proportion to the amount of functional antithrombin present in the sample; the resulting signal (or lack of Xa activity) provides a quantitative measure of antithrombin activity. This is why anti-Xa activity is the standard monitoring approach for antithrombin function.

Antithrombin does not activate thrombin, does not inhibit platelet aggregation as its primary role, and does not activate factor VII. These points help explain why those options aren’t correct.