Which laboratory test measures the time required for clot formation in whole blood?

The thrombin time (TT) is specifically designed to measure the time it takes for clot formation in the presence of thrombin, which is a key enzyme in the coagulation process. When thrombin is added to plasma, it converts fibrinogen to fibrin, leading to clot formation. Thus, this test assesses the final step of the coagulation cascade and can indicate the functional ability of fibrinogen and the presence of factors that may inhibit thrombin's activity.

On the other hand, both prothrombin time (PT) and activated partial thromboplastin time (aPTT) evaluate different phases of the coagulation cascade but not the final step involving thrombin directly in whole blood. The PT is primarily associated with the extrinsic pathway, while the aPTT evaluates the intrinsic pathway. The bleeding time test assesses vascular and platelet function in the initial stages of hemostasis rather than the clotting process itself. Therefore, the thrombin time is the most relevant test for directly measuring the time required for clot formation in whole blood.