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


290 mOsm per kg water, similar to plasma


30 mmol/L


75 mmol/L


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


> 375 mOsm/kg

> 150 mmol/L

with concentrated urine

< 200 mOsm/kg


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


< 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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