Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1098 page)

TABLE 16–60. The Relationship Between Serum and Urine Osmolality and the Clinical Significance of Laboratory Values

OSMOLALITY, STOOL

   Definition

   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.