Electrolyte and Osmolality Profile, Fecal
Ordering Recommendation
Quantitative Ion-Selective Electrode/Freezing Point
Within 48 hours
New York DOH Approval Status
This test is New York DOH approved.
Submit With Order
ARUP Consult®
Disease Topics
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
ComponentReference Interval
Sodium, FecalNot Established
Potassium, FecalNot Established
Magnesium, Fecal0-110 mg/dL
Calculated Osmolality, FecalBy report
Osmolality, Fecal (measured)0-16 years: 271-296 mOsm/kg
17 years and older: 280-303 mOsm/kg
Osmolal gapBy 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.

Hotline History
CPT Code(s)
Component Test Code*Component Chart NameLOINC
0020377Fecal Total Weight30078-0
0020379Sodium, Stool15207-4
0020380Potassium, Stool15202-5
0020388Magnesium, Fecal - per volume29911-5
0020389Magnesium, Fecal - per 24h29912-3
0020392Collection Time for Fecal Spec13363-7
0020586Calculated Osmolality, Fecal
0020587Osmolal Gap73571-2
0060717Collection Time for Fecal Specimen13363-7
0097131Fecal Weight30078-0
0098122Osmolality Fecal2693-0
* 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.