Anticoagulation therapy

There are a handful of reasons why you would need to take a "blood thinner" called warfarin. If you currently have a clot in one of your deeper veins, or have had multiple such clots in the past, if you have an artificial heart valve, or if your heart is beating in a disorganized way, you probably need to be taking warfarin at least temporarily.
If you do, you will need to have your blood drawn periodically to make sure it the drug isn't "thinning" your blood too much or too little. This test is called a prothrombin time (PT), and is often paired with a calculated value called the international normalized ratio (INR). When starting warfarin therapy, this test should be done more than once a week, until the right dose is found to maintain stabilize it. After stability is achieved, the test rarely needs to be performed more than once a month for the duration of therapy.