American Board of Internal Medicine (ABIM) Certification Practice Exam

What stool osmotic gap value is indicative of osmotic diarrhea?

1. 50 mOsm/kg

2. 100 mOsm/kg

3. 150 mOsm/kg

4. 200 mOsm/kg

The correct answer is: 100 mOsm/kg

The stool osmotic gap is a useful clinical tool for distinguishing between osmotic and secretory diarrhea. It is calculated using the formula: Stool Osmotic Gap = 290 - 2 x (stool sodium + stool potassium). In general, an osmotic gap of less than 50 mOsm/kg suggests secretory diarrhea, where the primary issue is related to the secretion of electrolytes and water, and not the presence of osmotically active substances in the intestinal lumen. Conversely, an osmotic gap greater than 100 mOsm/kg indicates the presence of osmotically active substances in the gut, which is characteristic of osmotic diarrhea. Osmotic diarrhea occurs when non-absorbable substances in the intestines draw water into the lumen, leading to watery stools. A stool osmotic gap of 100 mOsm/kg, therefore, represents a threshold indicating the presence of such active substances. Values above this cutoff strengthen the evidence that osmotic diarrhea is occurring, as these substances contribute to a higher osmolarity in the stool. Values like 150 mOsm/kg or 200 mOsm/kg further demonstrate this phenomenon, but a gap starting from 100 mOsm/kg already strongly suggests