How is the fecal osmotic gap calculated?
Fecal osmotic gap represents the osmotic activity in stool water not due to electrolytes.
The sum of the concentrations of sodium and potassium in stool water is multiplied by 2 to account for the anions that are also present and this product is subtracted from 290 mOsm/kg, the approximate osmolality of luminal contents within the intestine. (This number is a constant in this calculation because the relatively high permeability of the intestinal mucosa beyond the stomach means that osmotic equilibration with plasma will have taken place by the time that luminal contents reach the rectum.)
As an example, let us assume that a patient with watery diarrhea has a sodium concentration of 75 mmol/L and a potassium concentration of 65 mmol/L in stool water. Adding these together yields a concentration of 140 mmol/L.
Doubling this to account for anions means that electrolytes account for 280 mOsm/kg of stool water osmolality.
Subtracting this from 290 mOsm/kg yields an osmotic gap of 10 mOsm/kg. In contrast, if stool sodium was 10 mmol/L and potassium concentration was 20 mmol/L, the combined contribution of cations and anions in stool water would be only 60 mOsm/kg, yielding a fecal osmotic gap of 230 mOsm/kg.
This represents the amount of some unmeasured substance that is contributing to fecal osmolality, presumably some poorly absorbed substance that is being ingested but not absorbed.
How is the fecal osmotic gap interpreted?
Fecal osmotic gaps less than 50 mOsm/kg correlate well with secretory diarrheas caused by incomplete electrolyte absorption. Fecal osmotic gaps greater than 50 mOsm/kg are associated with osmotic diarrheas.
What precautions are necessary when measuring fecal osmotic gaps?
Be certain that the stool has not been contaminated with either water or urine. Dilution by water or hypotonic urine will falsely lower fecal electrolyte concentrations and will elevate the calculated osmotic gap.
This can be detected by actually measuring fecal osmolality; values that are substantially less than 290 mOsm/kg indicate dilution.
Contamination with hypertonic urine may also affect fecal electrolyte concentrations, but is harder to detect unless the concentration of creatinine in stool water is measured or the sum of measured cations and assumed anions is much greater than 290 mmol/L.