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 amiodarone twice a day, torasemide every other day, and trimetazidine twice a day. The family physician recommended taking warfarin 3mg per day to prevent thrombus formation. What parameter should be measured in this case for the optimal control of the effectiveness and safety of the anticoagulant therapy?