Ordering Recommendation




Quantitative Ion-Selective Electrode/Freezing Point




Within 48 hours

New York DOH Approval Status

This test is New York DOH approved.

Specimen Required

Patient Preparation

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 (ARUP Supply #28077) available online through eSupply using ARUP Connect™ or contact ARUP Client Services at (800) 522-2787. Multiple collection containers may be required.

Specimen Preparation

24-hour Collection: Transport the entire collection in the original collection kit. (Min: 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 (ARUP Supply #40910) available online through eSupply using ARUP Connect™ or contact
ARUP Client Services at (800) 522-2787. (Min: 5 g) Do not add saline or water to liquefy specimen. Indicate time and volume.

Storage/Transport Temperature

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

Unacceptable Conditions

Formed or viscous stools.


Ambient: Unacceptable; Refrigerated: Unacceptable; Frozen: 1 month

Reference Interval

Component Reference Interval
Sodium, Fecal Not Established
Potassium, Fecal Not Established
Magnesium, Fecal 0-110 mg/dL
Calculated Osmolality, Fecal By report
Osmolality, Fecal (measured) 0-16 years: 271-296 mOsm/kg
17 years and older: 280-303 mOsm/kg
Osmolal gap By report

Interpretive Data

Normal fecal fluid has an osmolality close to that of plasma (i.e., 290 mOsm/kg), a sodium concentration of about 30 mmol/L, a potassium concentration of about 75 mmol/L, and a magnesium concentration between 12 and 240 mg/dL (depending on diet) although it is usually less than 110 mg/dL. A fecal sodium greater than 90 mmol/L and an osmolal gap less than 50 mOsm/kg suggest secretory diarrhea or osmotic diarrhea due to sodium-containing laxatives. A fecal sodium less than 60 mmol/L and an osmolal gap greater than 100 mOsm/kg suggest osmotic diarrhea. Fecal sodium greater than 150 mmol/L and osmolality greater than 400 mOsm/kg suggest contamination with concentrated urine. Fecal osmolality less than 250 mOsm/kg suggests contamination with hypoosmotic urine or water. 

Compliance Category



Hotline History


CPT Codes

84999 x2; 84302; 83735


Component Test Code* Component Chart Name LOINC
0020377 Fecal Total Weight 30078-0
0020379 Sodium, Stool 15207-4
0020380 Potassium, Stool 15202-5
0020388 Magnesium, Fecal - per volume 29911-5
0020389 Magnesium, Fecal - per 24h 29912-3
0020392 Collection Time for Fecal Spec 13363-7
0020586 Calculated Osmolality, Fecal 2693-0
0020587 Osmolal Gap 73571-2
0060717 Collection Time for Fecal Specimen 13363-7
0097131 Fecal Weight 30078-0
0098122 Osmolality Fecal 2693-0
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.


Electrolyte and Osmolality Profile, Fecal