Diarrhea that wakes up the patient at night or is accompanied by weight loss or the presence of blood in the stool usually indicates an organic etiology, whereas the diarrhea of irritable bowel syndrome is rarely nocturnal.
Symptoms can usually indicate whether the diarrhea is small intestinal or colonic in origin.
Diseases of the small intestine cause large-volume, nonbloody diarrhea, often with diffuse abdominal pain and cramps, dehydration, and malabsorption. In contrast, diarrhea caused by colonic disease usually consists of frequent small-volume stools with gross or occult blood, often associated with lower abdominal pain and tenesmus.