Stool Osmolar/Osmotic Gap

Why 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.

When to Use

Use with persistent (>4 weeks) watery diarrhea (>3 episodes/day) to guide diagnosis and treatment.

Formula

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. 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. The calculator relies on the assumption that stool osmolarity is fairly constant and similar to serum osmolarity (290-300 mmol/L). 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 )

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!

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.

More Information

Causes of osmotic diarrhea include: Carbohydrate malabsorption Lactose Fructose Dietary (high FODMAP) Enteric feeding Osmotic laxatives Mg+ based PEG Lactulose Sorbitol ingestion Short gut syndrome 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 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)