This document defines requirements for in-vitro measuring systems used by patients to self-monitor vitamin-K antagonist oral anticoagulation therapy, ensuring accuracy, quality assurance, and user training. It applies to manufacturers, regulatory bodies, and conformity assessment organizations, focusing on prothrombin time systems that report results as international normalized ratios (INR). Excluded are systems used by health care providers, non-vitamin-K antagonist therapies, and medical aspects of anticoagulation management.