 Warfarin anticoagulation is prescribed to prevent stroke and venous thromboembolism. Optimal dosing requires achieving and maintaining a target range International Normalized Ratio INR. The INR itself is a prothrombin time that is standardized according to the type of thromboplastin reagent used by the coagulation laboratory. Each thromboplastin reagent has a designated International Sensitivity Index. The formula used to calculate the INR is shown below: 