Osmotic Gap (Stool)
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Stool Sodium (mOsm/kg)
Stool Potassium (mOsm/kg)
R e s u l t s
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Stool osmotic gap
 
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Osmotic Gap (Stool)

Osmotic Gap (Stool) = 290 mOsm/kg - 2 × (Stool Na + + Stool K + )

Normal stool analysis:

  • Osmolality: ~290 mOsm/kg
  • Na + : ~30 mmol/L
  • K + : ~75 mmol/L

The osmotic gap in stool can be used to evaluate diarrheal illness. The major mechanisms of diarrhea are:

  • Osmotic
  • Secretory

8-9 liters of fluid enters the intestines daily, with 6-7 liters being from endogenous sources (salivary, gastric, pancreatic, biliary and intestinal secretions). Absorption is primarily in the small bowel, with 1-2 liters of content presenting to the colon daily and only 100-200 grams of this eventually being passed as stool.

Very high or very low stool osmolarity are suggestive of factitious diarrhea (e.g. dilution with other substances - most commonly water)

Osmotic diarrhea:

  • Osmotic gap >50 mOsm/kg (mmol/kg) [usually >100 mOsm/kg]
  • This type of diarrhea occurs due to a nonabsorbable or poorly absorbable solute exerting osmotic pressure across the intestinal mucosa; with resultant excessive water remaining in the bowel.
  • With osmotic diarrhea, a nonabsorbed solute is present and results in a lower concentration of electrolytes; with resulting increased osmotic gap.

Causes include:

  • Antacids (Magnesium containing)
  • Lactose intolerance
  • Laxatives
  • Malabsorption
  • Pancreatic insufficiency
  • Poorly absorbed sugars (lactulose, mannitol, sorbitol, xylitol)
  • Whipple's disease

Secretory Diarrhea

Osmotic gap <50 mOsm/kg (mmol/kg)

This type of diarrhea is the result of increased secretion, decreased absorption, or both. Infection, inflammation or drugs are the most common causes.

Causes include:

  • Bacterial gastroenteritis (C. difficile, cholera, E. coli, Shigella)
  • Bile salt enteropathy
  • Carcinoid tumor
  • Celiac sprue
  • Colitis (collagenous, lymphocytic)
  • Drugs (colchicines, SSRI's, etc)
  • Hyperthyroidism
  • Laxatives (stimulant; e.g. phenolphthalein)
  • Rectal villous adenoma
  • VIPoma

References:

  • Fine KD, Schiller LR. AGA technical review on the evaluation and management of chronic diarrhea. Gastroenterology. 1999;116(6):1464-86.
  • Phillips S, Donaldson L, Geisler K, Pera A, Kochar R. Stool composition in factitial diarrhea: a 6-year experience with stool analysis. Ann Intern Med. 1995;123(2):97-100.
  • Schiller LR. Diarrhea. Med Clin North Am. 2000;84(5):1259-74, x.
  • Topazian M, Binder HJ. Brief report: factitious diarrhea detected by measurement of stool osmolality. N Engl J Med. 1994;330(20):1418-9.