What is Osmolality?
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In most cases of diarrhea, the disease is due to an infection or to a
bacterial toxin, and the diarrhea resolves in a short period of time.
Stool osmolality is of little use in such patients. If diarrhea persists
for more than a week and if cultures are negative, the cause of diarrhea
may be more difficult to discern. Gastroenterologists have described two
major forms of diarrhea: osmotic and secretory. In osmotic diarrhea, there
is some unabsorbed substance in the stool, which prevents normal absorption
of water (for example, this is how most laxatives work). In such patients,
the osmotic gap is typically over 50 mOsm/kg. An osmotic diarrhea is seen
in patients who ingest excess laxatives, and also in cases of malabsorption
If there is intrinsic damage to the intestinal mucosa such that it cannot
absorb water and electrolytes, then a "secretory" diarrhea occurs.
This is common in patients with mechanical disease of the intestine, such
as inflammation, tumors, and decreased blood flow.
An important consideration in measuring stool osmolality is that bacterial metabolism produces osmotically active substances; measurements must be made within 30 minutes of collection of a specimen, or the specimen should be refrigerated until analysis is to be performed.