How secretory and osmotic watery diarrhea distinguished?
The most useful way to differentiate secretory and osmotic types of watery diarrhea is to measure fecal electrolytes and calculate the fecal osmotic gap. In many diarrheal conditions, sodium and potassium along with their accompanying anions are the dominant electrolytes in stool water. In secretory diarrhea, there is a failure to completely absorb electrolytes or actual electrolyte secretion by the intestine; sodium, potassium, and their accompanying anions are responsible for the bulk of osmotic activity in stool water and the retention of water within the gut lumen. In contrast, in osmotic diarrhea, ingestion of poorly absorbed, osmotically active substances is responsible for holding water within the gut lumen; electrolyte absorption is normal and thus sodium and potassium concentrations can become quite low. The fecal osmotic gap calculation takes advantage of these distinctions to differentiate the two conditions.