Which factor deficiencies prolong PT but not always aPTT?

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

Answer: (A)

The test distinguishes between the extrinsic and intrinsic pathways of clotting. Prothrombin time measures the extrinsic pathway (tissue factor– VII complex) and the common pathway, while the aPTT measures the intrinsic pathway (contact activation) and the common pathway. A deficiency of Factor VII, which is part of the extrinsic pathway, slows the initial formation of the clot in response to tissue factor, so PT becomes prolonged. Because Factor VII has the shortest half-life among the vitamin K–dependent factors, PT tends to show prolongation earlier and more consistently, while aPTT can remain normal unless the deficiency is severe or other factors are involved. Deficiencies of Factor IX or Factor VIII primarily affect the intrinsic pathway, leading to prolonged aPTT rather than PT. Factor X feeds into the common pathway, so its deficiency typically prolongs both PT and aPTT.