Interpretation

Osmotic Gap:

Osmotic gap is calculated as 290 mOsm/kg-(2[Na]+2[K]). Typically, stool osmolality is similar that seen in serum since the gastrointestinal (GI) tract does not secrete water.(1)

An osmotic gap above 50 mOsm/kg is suggestive of an osmotic component contributing to the symptoms of diarrhea.(1,5,7)

Magnesium-induced diarrhea should be considered if the osmotic gap is above 75 mOsm/kg and is likely if the magnesium concentration is above110 mg/dL.(1)

An osmotic gap of50 mOsm/kg or less is suggestive of secretory causes of diarrhea.(1,5,7)

A highly negative osmotic gap or a fecal sodium concentration greater than plasma or serum suggests the possibility of either sodium phosphate or sodium sulfate ingestion by the patient.(4)

Phosphorus:

Phosphorus elevation above 102 mg/dL is suggestive of phosphate-induced diarrhea.(4)

Osmolality:

Osmolality below 220 mOsm/kg indicates dilution with a hypotonic fluid.(1)

Sodium and Potassium:

High sodium and potassium in the absence of an osmotic gap indicate active electrolyte transport in the GI tract that might be induced by agents such as cholera toxin or hypersecretion of vasoactive intestinal peptide.(1)

Sodium:

Sodium is typically found at lower concentrations (mean 30 ± 5 mmol/L) in patients with osmotic diarrhea caused by magnesium-containing laxatives, while typically at higher concentrations (mean 104 ± 5 mmol/L) in patients known to be taking secretory laxatives.(8)

Chloride:

Chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(5)

Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.(6)