stool Osmotic Gap in diagnosing chronic diarrhea.pdf

stool osmotic gap

1. STOOL OG
Mahmoud aboud
supervised by prof.Dr.Ahmed Foad

2. WHEN TO USE?
•Use with persistent (>4 weeks) watery diarrhea (>3 episodes/day) to
guide diagnosis and treatment.
•The calculator relies on the assumption that stool osmolarity is
fairly constant and similar to serum osmolarity (290-300 mmol/L).
supervised by prof.Dr.Ahmed Foad

3. FORMULA
•Stool Osmolal Gap = Stool Osm - (2 * (Na + K))
•Stool Osmolal Gap = 290[mOsm/kg] - (2 * (Na + K))
supervised by prof.Dr.Ahmed Foad

4. supervised by prof.Dr.Ahmed Foad

5. WHY TO USE?
•Distinguishing between major classes of diarrhea can be tricky.
•Osmotic diarrhea usually ceases with fasting and during the night whereas
secretory diarrhea often persists at night and continues despite fasting.
•The stool gap is most helpful when reported symptoms are atypical and can
guide the clinician toward a category, decreasing the cost of the diagnostic
workup.
•The test is cheap and straightforward
supervised by prof.Dr.Ahmed Foad

6. PEARLS/PITFALLS
•Useful to determine whether chronic watery diarrhea is from osmotic or secretory
causes.
•Not indicated in evaluation of acute diarrhea.
•Most useful in the outpatient setting, very rarely in hospital or ED.
•Initially validated in adults but used in children as well. (Eherer AJ 1992, Castro-
Rodriguez JA 1997)
•A thorough history is the most helpful tool in determining the cause of chronic diarrhea.
supervised by prof.Dr.Ahmed Foad

7. PEARLS/PITFALLS
•Osmotic diarrhea is caused when water remains in the gut lumen due to a poorly
absorbed, non-electrolyte substance (Gap >100, specificity increases with higher cutoff
values of 125, 160 etc). (Binder HJ 2006)
•Secretory diarrhea is due to disorders in intestinal electrolyte transport (either true
secretion or poor absorption). Gap is usually <50.
•Interpret with caution: variability in measured electrolytes has been reported in
individual stool samples, and changes if the sample is left out at room temperature.
(Duncan A 1992)
supervised by prof.Dr.Ahmed Foad

8. Preferred Specimen(s)
•10 mL random watery liquid fecal collected , or 24-hour, 48-hour or 72-hour watery
liquid feces collected.
Minimum Volume: 2 mL
Collection Instructions
•Submit a frozen specimen of watery liquid feces in a plastic screw-cap container.
Keep feces refrigerated during collection and transport frozen. Only watery liquid
feces are an acceptable specimen. In the event a formed fecal specimen is
submitted, the test will not be performed and will be cancelled.
•Note: Specimen must be shipped frozen to reduce the odor during shipping and to
minimize the risk of the container rupturing due to gas accumulation.
•This test only has clinical utility if performed on a watery fecal specimen.
supervised by prof.Dr.Ahmed Foad

9. •This test only has clinical utility if performed on a watery fecal specimen.
Transport Container
•Plastic leak-proof feces container
Transport Temperature
•Frozen
Specimen Stability
•Room temperature: Unacceptable
•Refrigerated: 7 days
•Frozen: 60 days
Reject Criteria
•Specimens in paint cans • Formed stool • Specimens received refrigerated
supervised by prof.Dr.Ahmed Foad

10. FACTS & FIGURES
osmotic diarrhoea:
Excess amount of poorly absorbed substances that exert
osmotic effect = water is drawn into the bowels = diarrhea.
•Stool output is usually not massive.
•Fasting improves the condition.
•Stool OG is high: loss of hypotonic fluid.
supervised by prof.Dr.Ahmed Foad

11. FACTS & FIGURES
osmotic diarrhoea:
Excess amount of poorly absorbed
substances that exert osmotic effect =
water is drawn into the bowels = diarrhea.
Stool output is usually not massive.
Fasting improves the condition.
Stool OG is high: loss of hypotonic fluid.
supervised by prof.Dr.Ahmed Foad

12. FACTS & FIGURES
Causes of osmotic diarrhoea include:
•Carbohydrate malabsorption (Lactose, Fructose)
•Dietary (high FODMAP : Fermentable Oligo-Di-Monosaccharides and
Polyols)
•Enteric feeding
•Osmotic laxatives: Mg+ based. PEG. Lactulose.
•Sorbitol ingestion
•Short gut syndrome
supervised by prof.Dr.Ahmed Foad

13. FACTS & FIGURES
Secretory Diarrhea
•There is an increase in the active secretion of water.
•High stool output.
•Lack of response to fasting.
•The most common cause of this type is a bacterial
toxin (E.coli, cholera) which stimulates the secretion
of anions.
supervised by prof.Dr.Ahmed Foad

14. FACTS & FIGURES
Secretory Diarrhea
•There is an increase in the active secretion of water.
•High stool output.
•Lack of response to fasting.
•The most common cause of this type is a bacterial
toxin (E.coli, cholera) which stimulates the secretion
of anions.
supervised by prof.Dr.Ahmed Foad

15. FACTS & FIGURES
Causes of Secretory Diarrhea include:
•Addison’s disease
•Bile acid malabsorption
•Congenital electrolyte transport disorders
•Hyperthyroidism
•Stimulant laxatives (bisocodyl, senna)
•Small intestinal bacterial overgrowth
•Tumors (rare): Carcinoid. Lymphoma. VIPoma, Gastrinoma (Zollinger Ellison syndrome)
•Whipple’s disease.
supervised by prof.Dr.Ahmed Foad

16. •Fatty diarrhea / Malabsorption / steatorrhea usually has an
osmolar gap >50, but not invariably.
•Inflammatory and infectious diarrhea often have a low gap (<50)
but should generally be considered as distinct clinical entities.
•Inflammatory/Infectious diarrhea are characterized by positive fecal
leukocytes or other markers of inflammation in the gut like fecal
calprotectin or stool lactoferrin. Very few causes in the secretory list
should cause (+) fecal leukocytes.
•As an aside, infectious diarrhea is more often acute but exceptions
include giardia, c.dif, e.histolytica, and parasites. These should all
give (+) fecal leukocytes
•Overlap between disorders with both osmotic and secretory
components is common (ex. infection, celiac disease, motility
disorders, and functional disorders like irritable bowel syndrome)
supervised by prof.Dr.Ahmed Foad

17. •ADVICE
•Osmotic diarrhea is generally treated by avoidance of the offending agent.
•Treatment for secretory diarrhea is targeted at and specific to the underlying cause.
•MANAGEMENT
•Symptom management is also dependent on the underlying cause.
•Some options include:
•Increasing dietary fiber
•Low FODMAP (Fermentable Oligo-Di-Monosaccharides and Polyols) diet
•Active culture yogurt and probiotics
•Bile acid resins (cholestyramine)
•Bismuth
•Anticholinergics
•Empiric antibiotics (metronidazole, rifaximin)
•Antimotility agents (loperamide, diphenoxylate/atropine, codeine); must rule out
infection first!
supervised by prof.Dr.Ahmed Foad

18. About the creator
John S. Fordtran, MD, is the director of
gastrointestinal physiology at Baylor
University Medical Center (BUMC), Texas.
Previously, he was chief of internal
medicine at BUMC and president of the
Baylor Research Institute. Dr. Fordtran is an
accomplished author of over 180 peer-
reviewed articles on gastrointestinal
pathophysiology, diagnosis and
management and won the Janssen Award
in Gastroenterology for Lifetime
Achievement in Digestive Sciences
supervised by prof.Dr.Ahmed Foad

19. supervised by prof.Dr.Ahmed Foad

20. References
supervised by prof.Dr.Ahmed Foad
❑ Medstudy 11
th gastroenterology
❑ https://www.mdcalc.com/calc/101/stool-osmolar-
osmotic-gap
❑ https://pubmed.ncbi.nlm.nih.gov/?term=Fordtran
❑ Dr ahmed foad lectures.
❑
Thank you