Electrolyte and Osmolality Profile, Fecal

Normal fecal fluid has an osmolality close to that of plasma (i.e., 290 mOsm/kg), a sodium concentration of about 30 mmol/L, a potassium concentration of about 75 mmol/L, and a magnesium concentration between 12 and 240 mg/dL (depending on diet) although it is usually less than 110 mg/dL. A fecal sodium greater than 90 mmol/L and an osmolal gap less than 50 mOsm/kg suggest secretory diarrhea or osmotic diarrhea due to sodium-containing laxatives. A fecal sodium less than 60 mmol/L and an osmolal gap greater than 100 mOsm/kg suggest osmotic diarrhea. Fecal sodium greater than 150 mmol/L and osmolality greater than 400 mOsm/kg suggest contamination with concentrated urine. Fecal osmolality less than 250 mOsm/kg suggests contamination with hypoosmotic urine or water.