A 64-year-old man is undergoing outpatient treatment for ischemic heart disease, diffuse cardiosclerosis, persistent atrial fibrillation, and functional class III heart failure. The pharmacotherapy consists of cordarone (amiodarone) twice a day, torasemide every other day, and trimetazidine twice a day. The family physician recommended taking warfarin (3 mg per day) to prevent thrombus formation. What parameter should be measured in this case for optimal control the effectiveness and safety of the anti-coagulant therapy?