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Electrolytes and Osmolality Profile, Fecal

Electrolyte and Osmolality Profile, Fecal

Test Codes

EPIC: LAB7098, Beaker: XELOSF, ARUP: 20699

Department

Chemistry

Specimen Collection Criteria

Collect: 24-hour or random liquid stool.

24-hour Collection: Freeze partially filled containers during the collection period. Continue to add stool to the container until it reaches the fill line or the defined collection duration is completed. Collect using the Kit, Fecal Fat, Stool. Please call storeroom (248-551-2946) for this kit.

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.

24-hour Collection: Transport the entire collection in the original collection kit. (Minimum: 5 g) Do not add saline or water to liquefy specimen. Indicate time and volume.

Random Collection: Transfer 5 g stool to unpreserved stool transport vial. (Minimum: 5 g) Do not add saline or water to liquefy specimen. Indicate time and volume.

CRITICAL FROZEN. Separate specimens must be submitted when multiple tests are ordered.

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): 1 month

Laboratory

Sent to ARUP laboratories, Salt Lake City, UT.

Performed

Sunday – Saturday.
Results available within 48 hours.

Reference Range

By report.

Test Methodology

Quantitative Ion-Selective Electrode/Freezing Point.

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

84999 x2, 84302, 83735.

Contacts

Last Updated

9/29/2023