American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the normal stool osmotic gap value?

1. Less than 25

2. Less than 50

3. Less than 75

4. Less than 100

The correct answer is: Less than 50

The normal stool osmotic gap value is indeed less than 50. The stool osmotic gap is a calculation that helps differentiate between osmotic diarrhea and secretory diarrhea. It is derived from the formula: osmotic gap = 290 - (2 × stool sodium + 2 × stool potassium). In healthy individuals, the osmotic gap typically falls below 50, suggesting that the osmotic forces in the stool are within normal limits and that there is no excess unmeasured osmotically active substances present that would indicate osmotic diarrhea. A stool osmotic gap greater than 50 generally suggests an increased concentration of osmotically active particles, which may be seen in conditions leading to osmotic diarrhea, such as lactose intolerance or malabsorption syndromes. Therefore, recognizing the normal range is crucial for passing the certification exam and understanding the underlying pathophysiology associated with various gastrointestinal disorders.