Fecal osmotic gap and pH in experimental diarrhea of various causes

Secretory diarrhea

“…As mentioned above, analysis of stool electrolytes allows the clinician to categorize diarrhea as osmotic or secretory [3]. Whereas finding an osmotic diarrhea produces a short list of suspect conditions, finding a secretory diarrhea leaves many possibilities.…”

“…Examples of osmotic diarrhea include lactose malabsorption and diarrhea caused by ingestion of magnesium laxatives (Table 1). Electrolyte absorption is unaffected by these osmotically active substances, and, typically, stool water contains very little unabsorbed sodium or potassium [3]. This is the basis for calculation of the "fecal osmotic gap."…”

“…The contribution of fecal electrolytes is calculated as twice the sum of sodium and potassium ions to account for the anions that accompany these cations. Recent work indicates that a fecal osmotic gap of greater than 50 mOsm/kg suggests osmotic diarrhea [3]. For example, an individual with diarrhea caused by ingestion of magnesium hydroxide might have the following stool water analysis: [Na + ]=7 mmol/L, [K + ]=10 mmol/L, and [Mg +2 ]=80 mmol/L (equivalent to 194 mg/dL, almost twice the upper limit of normal, which is 45.2 mmol/L [110 mg/dL]).…”

“…Stool sodium and potassium assays and stool pH can be used to further classify watery diarrhea as osmotic or secretory (Fig. 1) [3]. These tests can be done on "spot" samples or on a quantitative stool collection.…”

Fecal electrolyte testing for evaluation of unexplained diarrhea: Validation of body fluid test accuracy in the absence of a reference method

“…Sodium and potassium concentrations are used to calculate an osmotic gap where a gap N50 mOsm/kg is suggestive of an osmotic etiology while b50 mOsm/kg suggests secretory [6]. Our laboratory recently validated a fecal electrolyte panel which includes measurements of sodium (Na + ), potassium (K + ), magnesium (Mg 2+ ), chloride (Cl − ), and phosphorus (PO 4 3 − ) performed on an automated chemistry analyzer.…”