Red blood cells may show up in urine due to kidney stones and tumor, states MedicineNet. These cells can enter the urine from the trauma of bladder catheterization or from the vagina during menstruation.
Doctors can only detect the presence of red blood cells in urine in the lab, where they test urine samples with a dipstick and confirm the results with a microscopic examination, reports WebMD. A condition such as glomerulonephritis or kidney inflammation may cause red blood cells to look distorted when viewed under a microscope, notes Mayo Clinic. Vasculitis or other blood vessel diseases, viral disease, and immune problems such as IgA nephropathy can trigger glomerulonephritis, causing microscopic urinary bleeding.