In a case of isolated prolonged PTT with normal PT, which factor deficiency might be suspected?

In cases of isolated prolonged activated partial thromboplastin time (APTT) with a normal prothrombin time (PT), a deficiency in a factor that is intrinsic to the clotting cascade is likely. The prothrombin time assesses factors that are part of the extrinsic and common pathways, while the activated partial thromboplastin time evaluates the intrinsic pathway and the common pathway.

Among the factors listed, Factor VIII is crucial in the intrinsic pathway, functioning as a co-factor for Factor IX in the activation of Factor X. A deficiency in Factor VIII leads to conditions such as Hemophilia A, which is characterized by prolonged APTT but a normal PT. In contrast, the other factors are not associated with such a specific presentation of isolated prolonged APTT alongside normal PT.

Factor II (prothrombin) and Factor VII are associated with the extrinsic pathway, and deficiencies in these factors typically cause both a prolonged PT and APTT. Factor IX, while also a part of the intrinsic pathway, when deficient, would also present with prolonged APTT but can be less common to identify than Factor VIII deficiencies, especially in clinical practice. Therefore, the most common clinical suspicion for isolated prolonged PTT with normal PT is