Final answer:
Thromboplastin or tissue factor is added to a PT test to induce clotting by activating the extrinsic pathway of the clotting cascade.
Step-by-step explanation:
For a PT test, a substance called thromboplastin or tissue factor is added to induce clotting. Thromboplastin is a coagulation factor that plays a crucial role in the activation of the extrinsic pathway of the clotting cascade. When thromboplastin is added to the test, it activates factor VII, leading to the formation of thrombin and subsequent clotting of the blood sample.