To investigate whether the clinical history and basic laboratory test results can differentiate between an organic or functional cause of chronic diarrhea and thus avoid unnecessary hospital admissions and invasive procedures, we reviewed the charts of 58 adult patients admitted during 6 years because of chronic diarrhea who had normal stool and colonic examinations. The final diagnoses were irritable bowel syndrome in 34 patients, organic diarrhea in 21, and unknown cause in three. The following clinical data did not help in the differential diagnosis: age, sex, duration of diarrhea, presence of continuous diarrhea, abdominal pain, stool frequency or volume, and presence of stool mucus. Significant weight loss, nocturnal diarrhea, and the absence of tenesmus were associated with an organic cause. One or more laboratory alterations (increased erythrocyte sedimentation rate, anemia, hypokalemia, and low serum albumin level) were found in 62% of patients with organic diarrhea but in only 3% of those with functional disease; p < 0.001. In 20 of 21 patients with organic diarrhea, an syndromic diagnosis (fat malabsorption, n = 13; inflammatory bowel disease, n = 4; and secretory diarrhea, n = 3) could be obtained with three simple tests (stool fat, rectal biopsy, and fecal water osmolality and electrolyte determination, respectively). Our study confirms that a detailed history and a few simple laboratory data can help to distinguish between functional and organic diarrhea and so avoid extensive investigation. The syndromic diagnosis of organic diarrhea can also be approximated with relatively easy tests.
