What are common causes of a prolonged PT in a patient not on anticoagulants?

• A. Vitamin K deficiency, liver disease, DIC, or Factor VII deficiency
• B. Factor VIII deficiency
• C. Platelet aggregation disorders
• D. Vitamin C deficiency

Answer: (A)

Prothrombin time reflects the extrinsic pathway and the common pathway of coagulation, mainly sensitive to deficiencies or losses of factors II, V, VII, and X, with Factor VII playing a key role because it’s the vitamin K–dependent factor that initiates the extrinsic pathway and has the shortest half-life. When this pathway is impaired, clotting takes longer, so PT is prolonged.

Vitamin K deficiency reduces activity of II, VII, IX, and X; the extrinsic pathway is first affected, so PT prolongs. Liver disease lowers the production of these same factors, leading to a longer PT as well. Disseminated intravascular coagulation consumes clotting factors, including those in the extrinsic pathway, causing PT to lengthen. A deficiency specifically in Factor VII directly disrupts the tissue factor–VII complex that starts the extrinsic pathway, resulting in a prolonged PT.

In contrast, Factor VIII deficiency mainly affects the intrinsic pathway and is detected by an abnormal aPTT, not PT. Platelet aggregation disorders involve platelets rather than coagulation factors measured by PT, so PT is typically normal. Vitamin C deficiency doesn’t routinely cause a prolongation of PT through the coagulation factor pathways.

So, when a patient not on anticoagulants has a prolonged PT, the common suspects are vitamin K deficiency, liver disease, DIC, or Factor VII deficiency, all of which impair the extrinsic pathway or its initiation.