What is Osmolality?

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Clinical Uses of Osmolality:

Stool Osmolality
Measurement of osmolality of stool samples is helpful in evaluating patients with diarrhea. In normal stool, most small molecular weight substances are totally absorbed (except for electrolytes); thus, most of the osmotic activity of stool comes from electrolytes. A stool osmotic gap has been defined as the difference between the measured osmolality and a calculated osmolality (determined as two times the sum of Na and K ions in stool) or, more accurately, serum osmolality.
Separation of Secretary from Osmotic Diarrhea

Osmotic Diarrhea

  • Presence of unabsorbed solutes
  • Due to laxative abuse, malabsorption

Secretary Diarrhea
* Due to agents damaging mucosa (inflammation, infections, drugs such as phenolphthalein)

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 of nutrients.

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.