What could a prolonged aPTT combined with a normal PT indicate?

A prolonged activated partial thromboplastin time (aPTT) combined with a normal prothrombin time (PT) suggests an issue with the intrinsic pathway of the coagulation cascade. The intrinsic pathway involves factors such as factors VIII, IX, XI, and XII, which are critical for the aPTT test. When any of these factors are deficient, the aPTT will be prolonged, while the PT remains normal because the extrinsic pathway (measured by PT) is still functioning correctly.

This pattern is particularly indicative of conditions like hemophilia A (factor VIII deficiency) or hemophilia B (factor IX deficiency), where only the intrinsic pathway function is impaired, leaving the extrinsic pathway unaffected. Thus, the correct interpretation of a prolonged aPTT with a normal PT points directly to a deficiency in the factors associated with the intrinsic pathway.