Isolated elevation in the activated partial thromboplastin time (aPTT)

Isolated elevation in the activated partial thromboplastin time (aPTT). The prothrombin time (PT) measures the integrity of the extrinsic (and common) pathway, while the aPTT assays the integrity of the intrinsic (and common) pathway. Thus, an isolated elevation of the aPTT (meaning that the PT is normal) indicates a deficiency of, or inhibitor against one or more factors in the intrinsic pathway, namely factors XII, XI, IX and/or VIII. To keep things simple, we have used the term factor XII loosely to describe components of the contact pathway, which includes not only factor XII, but also high‐molecular‐weight kininogen and prekallikrein.