Osmolality, Stool

Measurement of osmolality of stool samples is helpful in evaluating patients with diarrhea. In normal stool, most small molecular weight substances are totally absorbed (except for electrolytes); thus, most of the osmotic activity of stool comes from electrolytes. A stool osmotic gap has been defined as the difference between the measured osmolality and a calculated osmolality (determined as two times the sum of Na and K ions in stool)

Normal range:

0 " “16 years: 271 " “296 mOsm/kg

17 years and older: 280 " “303 mOsm/kg

Use

Determines if chronic diarrhea is osmotic or secretory in nature.

Interpretation

A fecal sodium >90 mmol/L and an osmolal gap <50 mOsm/kg suggests secretory diarrhea or osmotic diarrhea due to sodium-containing laxatives.

A fecal sodium <60 mmol/L and an osmolal gap >100 mOsm/kg suggests osmotic diarrhea.

Fecal sodium >150 mmol/L and osmolality >400 mOsm/kg suggests contamination with concentrated urine.

Fecal osmolality <250 mOsm/kg suggests contamination with hypoosmotic urine or water.