Diarrhea is best defined as passage of loose stools often with more frequent bowel movements. For clinical purposes, the Bristol Stool Form Scale works well to distinguish stool form and to identify loose stools. Laboratory testing of stool consistency has lagged behind. Acute
diarrhea is likely to be due to
infection and to be self-limited. As
diarrhea becomes chronic, it is less likely to be due to
infection; duration of 1 month seems to work well as a cut-off for chronic
diarrhea, but detailed scientific knowledge is missing about the utility of this definition. In addition to duration of
diarrhea, classifications by presenting scenario, by pathophysiology, and by stool characteristics (e.g. watery, fatty, or inflammatory) may help the canny clinician refine the differential diagnosis of chronic
diarrhea. In this regard, a careful history remains the essential part of the evaluation of a patient with
diarrhea. Imaging the intestine with endoscopy and radiographic techniques is useful, and biopsy of the small intestine and colon for histological assessment provides key diagnostic information. Endomicroscopy and molecular pathology are only now being explored for the diagnosis of chronic
diarrhea. Interest in the microbiome of the gut is increasing; aside from a handful of well-described
infections because of pathogens, little is known about alterations in the microbiome in chronic
diarrhea. Serological tests have well-defined roles in the diagnosis of
celiac disease but have less clearly defined application in autoimmune enteropathies and
inflammatory bowel disease. Measurement of
peptide hormones is of value in the diagnosis and management of endocrine
tumors causing
diarrhea, but these are so rare that these tests are of little value in screening because there will be many more false-positives than true-positive results. Chemical analysis of stools is of use in classifying chronic
diarrhea and may limit the differential diagnosis that must be considered, but interpretation of the results is still evolving. Breath tests for assessment of
carbohydrate malabsorption, small bowel bacterial overgrowth, and intestinal transit are fraught with technical limitations that decrease sensitivity and specificity. Likewise, tests of
bile acid malabsorption have had limited utility beyond empirical trials of
bile acid sequestrants.