Lab Test

Electrolytes and Osmolality Profile, Fecal

Electrolyte and Osmolality Profile, Fecal

Test Codes

EPIC: LAB7098, FECPA

Specimen Collection Criteria

Collect: Random stool sample in a sterile collection cup.

  • Stool must be liquid in consistency. Formed (solid) stool will NOT be accepted.
  • Refrigerate the specimen immediately after collection.

Physician Office/Draw Specimen Preparation

Refrigerate (2-8°C or 36-46°F) the specimen immediately after collection. Freeze (-20°C/-4°F or below) specimens that will not be received in the Laboratory within two hours of collection.

Preparation for Courier Transport

Transport: Stool specimen, refrigerated (2-8°C or 36-46°F) or frozen (-20°C/-4°F or below).

Rejection Criteria

Formed (solid) stool specimens.

Specimens not collected and processed as indicated.

In-Lab Processing

The specimen should be frozen (-20°C/-4°F or below) if not analyzed within two hours of collection.

For more information on this test please see procedures at 

Storage

Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): Unacceptable
Refrigerated (2-8°C or 36-46°F): 2 hours
Frozen (-20°C/-4°F or below): Undetermined

Specimen Storage in Department Prior to Disposal:

Specimens are disposed within 48 hours of testing.

Laboratory

Royal Oak Automated Chemistry Laboratory

Performed

Sunday – Saturday, 24 hours a day.
Results available within 24 hours.

Reference Range

Sodium (mmol/L), Fecal: Not established
Potassium (mmol/L), Fecal: Not established

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

Osmolality (mOsmol/kg), Fecal:  Stool osmolality should be similar to serum osmolality.  Marked increases (>330 mOsmol/kg) in the absence of increased serum osmolality indicate improper storage.  Marked decreases (<220 mOsmol/kg) may indicate dilution with hypotonic fluid, e.g., factitious diarrhea.  The test should be integrated into the clinical context for interpretation. 

Osmotic Gap (mOsmol/kg, calculated), Fecal:  An Osmotic gap of >125 mOsmol/kg and fecal sodium <60 mmol/L suggests an osmotic cause of diarrhea.  An Osmotic gap <= 50 mOsmol/kg and fecal sodium > 90 mmol/L suggests a secretory cause of diarrhea.  The test result should be integrated into the clinical context for interpretation.

Osmotic Gap = Measured Osmolality (mOsmol/kg) – Calculated Osmolality (mOsmol/kg)

Calculated Osmolality = 2 x (fecal Na + fecal K)

Test Methodology

Ion Selective Electrode, Freezing Point Depression

Clinical Utility

Fecal osmolality is useful in cases of chronic diarrhea.  It may be helpful to the physician to know whether the diarrhea is:

  1. A secretory type caused by either an organism or an abnormality of water or electrolyte transport across the cell wall of the gut.
  2. An osmotic type caused by malabsorption of non-electrolyte substances, most commonly carbohydrates or certain laxatives (e.g., magnesium).

Fecal osmolality should be similar to serum osmolality.  If the fecal osmolality is significantly lower than the serum or plasma osmolality (< 220 mOsmol/kg), factitious diarrhea (i.e., addition of water or liquid to stool by patient) should be suspected.  If the fecal sample was not refrigerated immediately after collection, and if necessary frozen, the measured osmolality may be inappropriately elevated (> 330 mOsmol/kg).  This change is due to bacterial metabolism which results in production of osmotically active substances. 

The Osmotic Gap is equal to the measured osmolality (mOsmol/kg) minus the calculated osmolality (in mOsmol/kg, equal to 2 times the fecal sodium plus fecal potassium).  An Osmotic Gap > 125 mOsmol/kg with a fecal sodium < 60 mmol/L suggests an osmotic cause of the diarrhea.  An Osmotic Gap < or = 50 mOsmol/kg with a fecal sodium > 90 mmol/L suggests a secretory cause of the diarrhea.  The test results should be integrated into the clinical context for interpretation.

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

CPT Codes

82438, 84302, 84999.

Contacts

Last Updated

4/9/2021

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