Fecal electrolyte concentrations in secretory diarrhea (left column) and in osmotic diarrhea (right column). Note that most of the fecal osmolality can be attributed to fecal electrolytes in secretory diarrhea, whereas most of the osmolality in osmotic diarrhea results from the unmeasured (nonelectrolyte) osmoles. Calculation of the fecal osmotic gap allows an estimate of the contribution of unmeasured osmoles to fecal osmolality.⁵⁹