Isolated prolonged PT that corrects with mixing is most consistent with a deficiency in which factor?

Isolated prolonged PT that corrects with mixing is most consistent with a deficiency in which factor?

• A. Factor II
• B. Factor VII
• C. Factor VIII
• D. Factor IX

Answer: (B)

Prothrombin time (PT) measures the extrinsic pathway, which hinges on Factor VII, as well as the common pathway factors. When a PT is prolonged but corrects with mixing, it signals a true deficiency of a clotting factor rather than an inhibitor: the normal plasma supplies the missing factor and the PT returns toward normal. Since only the PT is prolonged and the mixing corrects, the deficit is in the extrinsic pathway factor, namely Factor VII. Deficiencies of intrinsic pathway factors (like Factor VIII or IX) would prolong the aPTT rather than the PT, and a deficiency in a common-pathway factor such as Factor II would more often affect both tests rather than be isolated to the PT. Thus, Factor VII deficiency best fits the pattern.

Prothrombin time (PT) measures the extrinsic pathway, which hinges on Factor VII, as well as the common pathway factors. When a PT is prolonged but corrects with mixing, it signals a true deficiency of a clotting factor rather than an inhibitor: the normal plasma supplies the missing factor and the PT returns toward normal. Since only the PT is prolonged and the mixing corrects, the deficit is in the extrinsic pathway factor, namely Factor VII. Deficiencies of intrinsic pathway factors (like Factor VIII or IX) would prolong the aPTT rather than the PT, and a deficiency in a common-pathway factor such as Factor II would more often affect both tests rather than be isolated to the PT. Thus, Factor VII deficiency best fits the pattern.