Workup of cases of chronic diarrhea. Diagnosis of factitious diarrhea.
Collect a very liquid stool specimen. Do not send formed stool. In the event a formed stool is submitted, the test will not be performed. The report willindicate "A formed stool specimen was submitted for analysis. This test was not performed because it only has clinical value if performed on a watery stool specimen."
Osmotic Gap:-Osmotic gap is calculated as 290 mOsm/kg-(2[Na]+2[K]). Typically, stool osmolality is similar to serum since the gastrointestinal (GI) tract does not secrete water.
-An osmotic gap >50 mOsm/kg is suggestive of an osmotic component contributing to the symptoms ofdiarrhea.
-Magnesium-induced diarrhea should be considered if the osmotic gap is >75 mOsm/kg and is likely if themagnesium concentration is >110 mg/dL.
-An osmotic gap < or =50 mOsm/kg is suggestive of secretory causes of diarrhea.
-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.
Phosphorus:-Phosphorus elevation >102 mg/dL is suggestive of phosphate-induced diarrhea.
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.
Osmolality:-Stool osmolality <220 mOsm/kg indicates dilution with a hypotonic fluid.
-Stool osmolality >330 mOsm/kg in the absence of increased serum osmolality indicates improper storage.
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 vasointestinal peptide.
Chloride:-Markedly elevated fecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhea.-Fecal chloride may be elevated (>35 mmol/L) in phenolphthalein- or phenolphthalein plus magnesium hydroxideinduced diarrhea.-Fecal chloride may be low (<20 mmol/L) in sodium sulfate-induced diarrhea.(3)