Hi Tina - I have had microscopic hematuria since age 8 (I am 26 now). I have had just about every work-up under the sun to figure it out (24-hr urine, various blood tests, cystoscopy, multiple IVPs, multiple ultrasounds, nuclear renal scan, renal MRI, etc.) - I have seen urologists, nephrologists, etc. 99.9% of the time I have a urinalysis done they find red blood cells. I have never gotten a clear answer on why I have this. I do have a history of kidney stones but the red blood cells show up even w/o a stone and have been present for about 18 years now.
So I am in a similar situation as far as not knowing the cause - my kidney function is normal and they haven't found a reason for the red blood cells. I'm not sure whether or not an infection could cause this - I know every new doctor I go to gets concerned with the red blood cells and looks for a reason and can't find one!
Have you ever been tested for protein in your urine?