Blood may be visible in the urine because the stone has irritated the ureter. Blood in the urine, however, does not always mean a person has a kidney stone. There may be other reasons for the blood, including kidney and bladder infections, trauma, or tumors. Urinalysis with a microscope may detect blood even if it is not appreciated by the naked eye. Sometimes, if the stone causes complete obstruction, there may be no blood in the urine because it cannot get past the stone.