Hi thanks for asking in Healthcare Magic. Sintrom is Acenocoumarol, an anticoagulant (drug which prevents clotting). This drug is given to you to prevent formation of thrombus or embolism near your prosthetic aortic valves. The dose of Sintrom is individualised (dose varies for each individual) by repeated measurement of your prothrombin time. INR or international normalized ratio values is a standardized system that determines the dose of anticoagulants based on the use of human thromboplastin (the values are different if rabbit thromboplastin is used). For prosthetic heart valves, the recommended INR values for Sintrom is 3 to 3.5. Less than that value will promote clotting. Your present treating doctor is right about insisting an increase in Sintrom dose as your INR value is only 1.5. It is much below the recommended value and you have the risk of developing thromboembolism if the dose is not increased. Hope my answer is useful to you.