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Test Code LABEPF Electrolyte and Osmolality Panel, Feces

Additional Codes

Test Name in EPIC EPIC Test Code Mnemonic Mayo Test ID


Reporting Name

Electrolyte and Osmolality Panel, F

Useful For

Workup of cases of chronic diarrhea


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

Profile Information

Test ID Reporting Name Available Separately Always Performed
NA_F Sodium, F No Yes
K_F Potassium, F No Yes
CL_F Chloride, F No Yes
OSMOF Osmolality, F No Yes
MG_F Magnesium, F No Yes
OG_F Osmotic Gap, F No Yes
POU_F Phosphorus, F No Yes

Method Name

OG_F: Calculation

NA_F, K_F, CL_F: Indirect Ion-Selective Electrode (ISE) Potentiometry

OSMOF: Freezing Point Depression

POU_F: Photometric, Ammonium Molybdate

MG_F: Colorimetric Titration

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type


Specimen Required

Collection Container/Tube: Stool container (T291)

Specimen Volume: 10 g

Collection Instructions: Collect a very liquid stool specimen.

Additional Information:

1. Do not send formed stool. In the event a formed stool is submitted, the test will not be performed. The report will indicate "A formed stool specimen was submitted for analysis. This test was not performed because it only has clinical value if performed on a watery stool specimen."

2. Osmolality results will be reported as mOsm/kg regardless of collection duration.

3. Sodium, chloride, and potassium will be reported as mmol/L

4. Magnesium and phosphorus will be reported as mg/dL

Specimen Minimum Volume

5 g

Specimen Stability Information

Specimen Type Temperature Time
Fecal Frozen (preferred) 14 days
  Refrigerated  7 days
  Ambient  48 hours

Reject Due To








Formed stool

Reference Values

No established reference values

Day(s) and Time(s) Performed

Monday, Wednesday, Friday; Evening

CPT Code Information





84999 x 2-Osmolality, Potassium

LOINC Code Information

Test ID Test Order Name Order LOINC Value
EFPO Electrolyte and Osmolality Panel, F 88697-8


Result ID Test Result Name Result LOINC Value
NA_F Sodium, F 15207-4
K_F Potassium, F 15202-5
CL_F Chloride, F 15158-9
MG_F Magnesium, F 29911-5
OG_F Osmotic Gap, F 73571-2
POU_F Phosphorus, F 88713-3
OSMOF Osmolality, F 2693-0

Specimen Retention Time

7 days

Analytic Time

Same day/1 day

Method Description

Osmotic Gap:

Calculated result=290 mOsm/kg - 2(stool Na [mmol/L] + stool K [mmol/L])



The depression of the freezing point of serum or other fluid is used to measure osmolality using an Advanced Instruments osmometer. The extent of lowering below 0° C (the freezing point of water) is a function of the concentration of substances dissolved in the serum. By definition, 1 milliosmole per kilogram lowers the freezing point 0.001858° C.(Murphy JE, Henry JB: Evaluation of renal function, and water, and electrolyte, and acid base balance. In Todd-Sanford-Davidsohn Clinical Diagnosis and Management by Laboratory Methods. 16th edition. Edited by JB Henry. Philadelphia, WB Saunders Company, 1979, pp 135-152)


Sodium, Potassium, and Chloride:

The Roche Cobas c 501 analyzer dilutes samples 1:31 and makes use of the unique properties of certain membrane materials to develop an electrical potential (electromotive force: EMF) for the measurements of ions in solution. The electrode has a selective membrane in contact with both the test solution and an internal filling solution. The internal filling solution contains the test ion at a fixed concentration. The membrane EMF is determined by the difference in concentration of the test ion in the test solution and the internal filling solution. The EMF develops according to the Nernst equation for a specific ion in solution.(Package insert: Roche ISE reagent. Roche Diagnostics Corp, 2009-09, V8)



In the presence of sulfuric acid, inorganic phosphate and ammonium molybdate form an ammonium phosphomolybdate complex. The concentration of phosphomolybdate formed is measured photometrically and is directly proportional to the inorganic phosphate concentration.(Package insert: Roche Phosphorus reagent. Roche Diagnostics Corp., Indianapolis, IN, 2010-10, V5)



In an alkaline solution, magnesium forms a purple complex with xylidyl blue, a diazonium salt. The magnesium concentration is measured photometrically via the decrease in the xylidyl blue absorbance.(Package insert: Magnesium reagent. Roche Diagnostics Corp., Indianapolis, IN, 2012-04, V2)

Clinical Information

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)


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 ≤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)

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.