Sodium, Potassium, Chloride, Osmolality, Magnesium, Osmotic Gap, Phosphorus
Workup of cases of chronic diarrhea.
Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea).
This test is only clinically valid if performed on watery specimens. In the event a formed fecal specimen is submitted, the test will not be performed.
Patient Preparation: No barium, laxatives, or enemas may be used for 96 hours prior to start of, or during, collection.
Prolonged storage at incorrect temperatures may cause osmolality to increase due to bacterial metabolism generating osmotically active products.
In very rare cases, gammopathy, in particular type IgM (Waldenstrom macroglobulinemia), may cause unreliable results.
Phospholipids contained in liposomal drug formulations (eg AmBisome) may be hydrolyzed in the test due to the acidic reaction pH and thus lead to elevated phosphate results.(1,2)
Falsely high chloride values have been reported from patients receiving perchlorate medication. This is due to an interference of perchlorate ions with chloride ion-selective electrode determination.
NA_F, K_F, CL_F: Indirect Ion-Selective Electrode (ISE) Potentiometry
OSMOF: Freezing Point Depression
POU_F: Photometric, Ammonium Molybdate
MG_F: Colorimetric Titration
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 elevation above 102 mg/dL is suggestive of phosphate-induced diarrhea.(4)
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 vasointestinal peptide.(1)
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 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)