Which of the following will prolong the thrombin time (TT)?

• A. Factor VIII Deficiency
• B. Heparin
• C. von Willebrand disease
• D. Abnormal FV deficiency

Answer: (B)

Thrombin time measures how long it takes for fibrinogen to be converted to fibrin after thrombin is added to plasma. It reflects thrombin activity and fibrinogen quality, and it is sensitive to substances that inhibit thrombin.

Heparin prolongs the thrombin time because it boosts antithrombin III, which inhibits thrombin. With thrombin inhibited, the conversion of fibrinogen to fibrin is slowed, delaying clot formation.

Factor VIII deficiency and von Willebrand disease mainly affect the intrinsic pathway and platelet-related steps, not the direct action of thrombin on fibrinogen, so they typically don’t prolong TT. Abnormal factor V deficiency affects the common pathway, but TT uses exogenous thrombin, bypassing the need for factor V, so TT is not prolonged.

Thus, the test most likely to prolong TT is the presence of heparin.