In a healthy bowel, there should be no reason for blood to appear in the stool. With some bowel conditions however, lesions can appear in the lining of the bowel, and these lesions can allow the passage of blood into the bowel. As digested matter is passed through the bowel, it may come in contact with the lesion(s) and this can result in blood being transferred to the digested matter.
Faecal immunochemical testing (FIT) looks for traces of blood in the stool, which may indicate that there is damage to the lining of the bowel. The FIT will not specify where in the bowel the bleeding has come from, or the nature of the affected area, but it will indicate to the clinician that there is a pathology in need of further investigation.