Thrombin time tests thrombin's action on fibrinogen.

• A. Activation of platelets
• B. Conversion of fibrinogen to fibrin
• C. Synthesis of fibrinogen
• D. Inhibition of thrombin

Answer: (B)

Thrombin time focuses on the last step of coagulation: thrombin’s action on fibrinogen to form fibrin. In the test, a known amount of thrombin is added to plasma and the time to clot formation is measured. This directly reflects how well fibrinogen is being converted into fibrin by thrombin. If fibrinogen is deficient or dysfunctional, or if thrombin’s activity is inhibited (by agents like heparin or direct thrombin inhibitors), the time is prolonged because the conversion process is impaired. Activation of platelets relates to primary hemostasis and isn’t what this test measures. Synthesis of fibrinogen is a hepatic production process and isn’t assessed by thrombin time. Inhibition of thrombin can affect the result, but the test’s purpose is to assess the conversion of fibrinogen to fibrin, not to quantify thrombin inhibition per se.