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Fecal Osmotic Gap and Electrolyte Content

Purpose: To evaluate fecal osmolality and electrolyte content as an aid in the diagnosis of diarrhea.

Description

Stool contains water, electrolytes and solutes. The fecal osmotic (or osmolar) gap measures the differences between measured fecal osmolality and concentrations of electrolytes in the stool, and it indicates the concentration of poorly absorbed solutes in the feces. This can provide useful information in patients with diarrhea.


Fecal Parameter

Normal Findings

osmolality

290 mOsm per kg water, similar to plasma

sodium

30 mmol/L

potassium

75 mmol/L

magnesium

often < 45 mmol/L, varies with diet

 

Fecal osmolality is usually close to that of plasma, or 290 mOsm/kg. If the stool or plasma osmolality cannot be measured, then 290 (or 280 in MKSAP) may be used as an estimate for fecal osmolality.

 

The osmolality may be abnormal if the stool specimen is contaminated during collection.

 

Fecal Osmolality

Fecal Sodium

Contamination

> 375 mOsm/kg

> 150 mmol/L

with concentrated urine

< 200 mOsm/kg

NA

with diluted urine or water

 

fecal osmotic gap =

= (measured osmolality) – (2 * (sodium concentration in mmol/L)) – (2 * (potassium concentration in mmol/L))

 

Fecal Osmotic Gap

Fecal Sodium

Interpretation

< 50 mOsm/kg

> 90 mmol/L

secretory diarrhea, or osmotic diarrhea due to sodium-containing laxative

> 50 mOsm/kg (usually > 100)

< 60 mmol/L

osmotic diarrhea

 

daily loss of sodium in stool in mmol per day =

= (daily volume of stool in liters) * (fecal sodium concentration in mmol/L)

 

daily loss of potassium in stool in mmol per day =

= (daily volume of stool in liters) * (fecal potassium concentration in mmol/L)


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Specialty

Objective

ICD-10