What does the prothrombin time (PT) test evaluate?

The prothrombin time (PT) test is designed to evaluate the extrinsic and common pathways of coagulation. It measures the time it takes for blood to clot after the addition of a tissue factor (thromboplastin) to a blood sample. The extrinsic pathway is initiated by tissue damage, leading to the release of tissue factor, which then interacts with Factor VII to form a complex that activates Factor X.

The common pathway, which follows, involves further factors—specifically Factors X, V, prothrombin, and fibrinogen—leading to the formation of a fibrin clot. Thus, the PT test provides critical information about the integrity and functioning of these pathways, which is important for assessing bleeding disorders or monitoring patients on anticoagulant therapy, such as warfarin.

The other options do not pertain to the PT test's function. The intrinsic pathway is primarily evaluated by the activated partial thromboplastin time (aPTT) test, the function of platelets is assessed by platelet counts and other specific platelet function tests, and the speed of blood flow is not a function of the PT test but can be measured by different methods such as the use of Doppler ultrasound.