Should you see a doctor if you are on Coumadin and have a trace of blood in urine?

Very likely. The blood thinner, Warfarin, is a strong blood thinner and any bleeding any place in the body is potentially a problem and should always be reported immediately to a health care professional. Best would be to contact the Doctor Who prescribed the medication in a situation like this, but if that doctor is not available on an urgent basis, then, if it is an emergency with a lot of blood, consider an urgent care center or emergency department and take along a sample of the bloody urine if possible. When calling to speak to the doctor, be ready to describe the color of the urine, when it started looking like there was blood in it, and if you have bleeding any place else (gums, nose, etc.).