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.).
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In healthy people, a trace of blood in the urine on urinalysis can mean a bladder infection or other urinary tract infection is present or recently was, but when just a trace, it is not a serious concern and if it is from a test ordered by a medical professional, they will contact the patient if there is a concern over the results of the urine test.
However, if the patient is taking the "blood thinner" Coumadin, even a trace of blood should be getting reported to the physician who prescribed the Coumadin. It can be a sign that the dosage needs to be adjusted. There are many tiny blood vessels in the urinary tract that could bleed from a Coumadin level that is too high and blood in the urine can be an early sign.