Which laboratory test is most commonly used to assess hemophilia?

Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) both affect the intrinsic coagulation pathway. The Partial Thromboplastin Time (PTT or aPTT) measures the intrinsic and common coagulation pathways and is prolonged in hemophilia. The Prothrombin Time (PT/INR) measures the extrinsic and common pathways and is normal in hemophilia. Bleeding time assesses platelet function. In hemophilia: PTT is prolonged, PT/INR is normal, platelet count is normal, and bleeding time is normal. This is a key distinguishing feature from platelet disorders and warfarin therapy.