Stool Osmotic Gap Formula = Stool Osmolality - 2 * (Na +K). The stool osmotic gap is used to determine the etiology of diarrhea. If the stool osmotic gap is greater than 100, the diarrhea is osmolar. If the stool osmotic gap is less than 100, the diarrhea is secretory.
Source: EMedicine: Diarrhea.