The prothrombin time will detect deficiencies in the ______ pathway(s) when calcium and a tissue factor source such as rabbit brain are added to plasma.

• A. Extrinsic
• B. Extrinsic and common
• C. Intrinsic
• D. Intrinsic and common

Answer: (B)

The prothrombin time measures the extrinsic and common pathways. When calcium and tissue factor are added to plasma, tissue factor (such as rabbit brain thromboplastin) activates the extrinsic pathway by converting Factor VII to VIIa. This then activates Factor X, and with Factor V in the common pathway, prothrombin is converted to thrombin to form a fibrin clot. So deficiencies in the extrinsic factors (notably Factor VII) or in the common pathway factors (X, V, II, I) will prolong the PT. The intrinsic pathway, tested by the aPTT, does not rely on tissue factor and is not assessed by this PT setup. Therefore the PT detects deficiencies in the extrinsic and common pathways.