Why do I need to have my blood tested when I'm taking warfarin?

If you take warfarin it is important to keep track of how well your blood clots in order to avoid excessive bleeding. Warfarin is unique in this regard because, unlike the other medications used to treat or prevent stroke, it directly decreases the ability of the blood to clot.

Your doctor will recommend how often you should have your blood clotting tested, either with tests in the doctor's office or with a home testing kit. Most health professionals calculate blood clotting rate based on the international normalized ratio (INR), a measure of how easily your blood clots. A level of 1 means the medication is providing no additional protection against blood clots. For most cardiovascular conditions, an INR of 2 to 3 is thought to provide fairly effective protection with an acceptable amount of bleeding risk. Higher INR readings would suggest that the warfarin is impeding blood-clotting mechanisms too much, raising the risk of accidental bleeding. In such cases, the doctor will reduce the warfarin dosage.
While you're taking warfarin, you need regular blood tests to see if your dose of warfarin needs to be adjusted. These blood tests, which are called protime (PT) tests or International Normalized Ratio (INR) tests, or PT/INR for short, measure how fast your blood clots. If your PT/INR test result is too high, your risk of bleeding increases; if it's too low, your risk of getting a blood clot increases. Your doctor and other healthcare providers will decide what range of numbers are best for you. Your dose of warfarin will be adjusted from time to time to keep your PT/INR within that range.