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26193 Electrolyte and Osmolality, Feces (EFPO)

Electrolyte and Osmolality, Feces (EFPO)
Test Code: EFPOSO
Useful For

​Workup of cases of chronic diarrhea. Diagnosis of factitious diarrhea.

Specimen Requirements
Specimen TypePreferred Container/TubeAcceptable Container/TubeSpecimen VolumeSpecimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Stool​Sage Container​Sterile Container​10 g​5 g
Collection Processing Instructions

​Collect a very liquid stool specimen.  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."

Acceptable Specimen Types

​Stool

Specimen Stability Information
Specimen TypeTemperatureTime
​Fecal​Ambient​48 hours
​Refrigerated​7 days
​Frozen (preferred)​14 days
Rejection Criteria
Formed Stool
Performing Laboratory Information
Performing LocationDay(s) Test PerformedAnalytical TimeMethodology/Instrumentation
​Mayo Clinic Laboratories​Monday, Wednesday, Friday; Evening​2 days​Freezing Point Depression; Roche Cobas c501 analyzer
Reference Lab
Reference Range Information
Performing LocationReference Range
​Mayo Clinic Laboratories​No establsihed reference range
Interpretation

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.

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

-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.

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

-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.

Phosphorus:
-Phosphorus elevation >102 mg/dL is suggestive of phosphate-induced diarrhea.

Sodium:
-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.

Osmolality:
-Stool osmolality <220 mOsm/kg indicates dilution with a hypotonic fluid.

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

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.

Chloride:
-Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.
-Fecal chloride may be elevated (>35 mmol/L) in phenolphthalein- or phenolphthalein plus magnesium hydroxideinduced diarrhea.
-Fecal chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(3)

Outreach CPTs
CPTModifier
(if needed)
QuantityDescriptionComments
​82438​1​Chloride
​83735​1​Magnesium
​84302​1​Sodium
​84100​1​Phosphorus
​84999​2​Osmolality, Potassium
Ordering Applications
Ordering ApplicationDescription
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Specimen TypePreferred Container/TubeAcceptable Container/TubeSpecimen VolumeSpecimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Stool​Sage Container​Sterile Container​10 g​5 g
Collection Processing

​Collect a very liquid stool specimen.  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."

Acceptable Specimen Types

​Stool

Specimen Stability Information
Specimen TypeTemperatureTime
​Fecal​Ambient​48 hours
​Refrigerated​7 days
​Frozen (preferred)​14 days
Rejection Criteria
Formed Stool
Useful For

​Workup of cases of chronic diarrhea. Diagnosis of factitious diarrhea.

Reference Range Information
Performing LocationReference Range
​Mayo Clinic Laboratories​No establsihed reference range
Interpretation

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.

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

-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.

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

-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.

Phosphorus:
-Phosphorus elevation >102 mg/dL is suggestive of phosphate-induced diarrhea.

Sodium:
-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.

Osmolality:
-Stool osmolality <220 mOsm/kg indicates dilution with a hypotonic fluid.

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

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.

Chloride:
-Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.
-Fecal chloride may be elevated (>35 mmol/L) in phenolphthalein- or phenolphthalein plus magnesium hydroxideinduced diarrhea.
-Fecal chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(3)

For more information visit:
Performing Laboratory Information
Performing LocationDay(s) Test PerformedAnalytical TimeMethodology/Instrumentation
​Mayo Clinic Laboratories​Monday, Wednesday, Friday; Evening​2 days​Freezing Point Depression; Roche Cobas c501 analyzer
Reference Lab
For billing questions, see Contacts
Outreach CPTs
CPTModifier
(if needed)
QuantityDescriptionComments
​82438​1​Chloride
​83735​1​Magnesium
​84302​1​Sodium
​84100​1​Phosphorus
​84999​2​Osmolality, Potassium
For most current information refer to the Marshfield Laboratory online reference manual.