The cause of a chronic diarrhea may be difficult to identify if the cause is not immediately obvious. Initial analysis can separate the stool into 1 of 4 different types, which can help limit and guide further evaluation.


Types of chronic diarrhea:

(1) osmotic (increased nonabsorbed solute)

(2) secretory (increased water and electrolytes)

(3) inflammatory

(4) dysmotility (none of the above)


Workup for chronic diarrhea may include:

(1) fecal analysis for osmolality, sodium and potassium content

(2) fecal fat analysis

(3) stool culture for enteropathogens

(4) stool examination for ova and parasites

(5) effect of fasting for 48-72 hours on stool excretion

(6) evaluation for lactose intolerance

(7) breath tests for sugar absorption

(8) evaluation of laxative intake

(9) endoscopy with bowel biopsy

(10) stool smear for white blood cells

(11) evaluation of hormone producing neuroendocrine tumors

(12) complete history with attention to diabetes, medications, travel, radiation therapy, and previous surgery


Osmotic diarrhea features:

(1) fecal osmotic gap > 50

(2) < 500 g of stool with fasting


Secretory diarrhea features:

(1) fecal osmotic gap < 50

(2) > 500 g of stool with fasting


Inflammatory diarrhea features:

(1) neutrophils in stool

(2) colonic ulcerations


Dysmotility diarrhea features:

(1) positive glucose breath test

(2) negative stool examination for bacteria, ova and parasites


Causes for the different types:

(1) osmotic: lactase deficiency; nonabsorbed sugars (sorbitol, mannitol, lactose); sulfates; phosphates; magnesium salts (citrate, hydroxide, oxide); intestinal malabsorption; pancreatic insufficiency; glucose-galactose malabsorption

(2) secretory: enterotoxin-mediated infectious diarrhea (cholera, E. coli); hormone-mediated (calcitonin, serotonin, VIP, gastrin); secreting villous adenoma; bile salt malabsorption; fat malabsorption; collagenous colitis; lymphocytic colitis; infectious colitis (cryptosporidiosis, others)

(3) inflammatory: inflammatory bowel disease; acute radiation colitis; diversion colitis; enteroinvasive infectious colitis (shigellosis, yersiniosis, amebiasis, etc.)

(4) dysmotility: irritable bowel syndrome; diabetes; scleroderma; bacterial overgrowth


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