Test Catalog

Test ID: EFPO    
Electrolyte and Osmolality Panel, Feces

Useful For Suggests clinical disorders or settings where the test may be helpful

Workup of cases of chronic diarrhea


Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea)

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

The concentration of electrolytes in fecal water and their rate of excretion are dependent upon 3 factors:

-Normal daily dietary intake of electrolytes

-Passive transport from serum and other vascular spaces to equilibrate fecal osmotic pressure with vascular osmotic pressure

-Electrolyte transport into fecal water due to exogenous substances and rare toxins (eg, cholera toxin)


Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major affector of this equilibrium. Fecal osmolality is normally 2 x (sodium + potassium) unless there are exogenous factors inducing a change in composition, such as the presence of other osmotic agents (magnesium sulfate, saccharides) or drugs inducing secretions, such as phenolphthalein or bisacodyl.


Osmotic diarrhea is caused by ingestion of poorly absorbed ions or sugars and can be characterized by the following:

-Stool volume typically decreased by fasting

-Fecal fluid usually has an elevated osmotic gap

-Osmotic agents such as magnesium, sorbitol, or polyethylene glycol may be the cause through the intentional or inadvertent use of laxatives

-Carbohydrate malabsorption due most commonly to lactose intolerance

-Carbohydrate malabsorption can be differentiated from other osmotic causes by a low stool pH (<6)


Secretory diarrhea is caused by disruption of epithelial electrolyte transport and can be characterized by the following:

-Stool volume is usually unaffected by fasting

-Fecal fluid usually has elevated electrolytes (primarily sodium and chloride) and a low osmotic gap (<50 mOsm/kg)

-Common causes include bile acid malabsorption, inflammatory bowel disease, endocrine tumors, and neoplasia

-Secretory agents such as anthraquinones, phenolphthalein, bisacodyl, or cholera toxin should also be considered

-Infection is a common secretory process; however, it does not typically cause chronic diarrhea (defined as symptoms >4 weeks)

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

No established reference values

Interpretation Provides information to assist in interpretation of the test results

Osmotic Gap:

-Osmotic gap is calculated as 290 mOsm/kg-(2[Na]+2[K]). Typically, stool osmolality is similar to serum since the gastrointestinal (GI) tract does not secrete water.(1)

-An osmotic gap >50 mOsm/kg is suggestive of an osmotic component contributing to the symptoms of diarrhea.(1-3)

-Magnesium-induced diarrhea should be considered if the osmotic gap is >75 mOsm/kg and is likely if the magnesium concentration is >110 mg/dL.(1)

-An osmotic gap < or =50 mOsm/kg is suggestive of secretory causes of diarrhea.(1-3)

-A highly negative osmotic gap or a fecal sodium concentration greater than plasma or serum suggests the possibility of either sodium phosphate or sodium sulfate ingestion by the patient.(4)



-Phosphorus elevation >102 mg/dL is suggestive of phosphate-induced diarrhea.(4)



-Sodium is typically found at lower concentrations (mean 30 +/- 5 mmol/L) in patients with osmotic diarrhea caused by magnesium-containing laxatives, while typically at higher concentrations (mean 104 +/- 5 mmol/L) in patients known to be taking secretory laxatives.(5)



-Stool osmolality <220 mOsm/kg indicates dilution with a hypotonic fluid.(1)

-Stool osmolality >330 mOsm/kg in the absence of increased serum osmolality indicates improper storage.(1)


Sodium and Potassium:

-High sodium and potassium in the absence of an osmotic gap indicate active electrolyte transport in the GI tract that might be induced by agents such as cholera toxin or hypersecretion of vasointestinal peptide.(1)



-Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.(6)

-Fecal chloride may be elevated (>35 mmol/L) in phenolphthalein- or phenolphthalein plus magnesium hydroxide-induced diarrhea.(3)

-Fecal chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(3)

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Steffer KJ, Santa Ana CA, Cole JA, Fordtran JS: The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea. Gastroenterol Clin North Am 2012;41:539-560

2. Sweetser S: Evaluating the patient with diarrhea: A case-based approach. Mayo Clin Proc 2012;87:596-602

3. Eherer AJ, Fordtran JS: Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology 1992;103:545-551

4. Fine KD, Ogunji F, Florio R, et al: Investigation and diagnosis of diarrhea caused by sodium phosphate. Dig Dis Sci 1998;43(12):2708-2714

5. Phillips S, Donaldson L, Geisler K, et al: Stool composition in factitial diarrhea: a 6-year experience with stool analysis. Ann Intern Med 1995;123:97-100

6. Casprary WF: Diarrhea associated with carbohydrate malabsorption. Clin Gastroenterol 1986;15:631-655

7. Ho J, Moyer TP, Phillips SF: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092

8. Fine KD, Santa Ana CA, Fordtran JS: Diagnosis of magnesium-induced diarrhea. N Engl J Med 1991;324:1012-1017