Therapy with coumadin (warfarin) is monitored by a blood test called the Prothrombin Time (PT), and for standardising the results amongst different laboratories, the result is expressed as a value called the INR (International Normalization Ratio). The INR is the value used to monitor anti-coagulation therapy. The test is done frequently when anti-coagulation therapy is first started and once the proper level of anti-coagulation is attained, the PT/INR test is done less frequently but regularly. As even small changes in diet, infections, other medicines etc., can alter the metabolism of the drug in a patient even after a patient has been taking warfarin for years, the blood levels still need to be checked often, usually on a weekly to monthly basis. If the PT/INR is too low (not anti-coagulated enough), the medicine will not work, and a problem from the underlying medical condition supposedly being treated will occur while too high a PT/INR (overly anti-coagulated), may cause bleeding. This is a simple test which is available globally and you need to check with your travel agent/medical insurer how to go about it in Spain.