American Board of Internal Medicine (ABIM) Certification Practice Exam

What does the stool osmotic gap calculation involve?

1. Stool volume measured by weight

2. Stool Na and stool K levels

3. Dietary intake of fiber

4. Stool appearance and texture

The correct answer is: Stool Na and stool K levels

The calculation of the stool osmotic gap primarily involves measuring the concentrations of sodium (Na) and potassium (K) in the stool. The stool osmotic gap is a useful diagnostic tool for differentiating between osmotic and secretory diarrhea. In this context, the osmotic gap is calculated using the formula: Osmotic gap = 290 - (2 × (stool Na + stool K)) Here, the normal osmotic gap is typically less than 50 mOsm/kg. When the osmotic gap is high, it indicates that the diarrhea is likely due to an osmotic cause, which can be due to unabsorbed solutes such as lactose in lactose intolerance or certain carbohydrate malabsorption syndromes. On the other hand, a low osmotic gap suggests a secretory diarrhea process, such as infections or certain toxins. The other options do not directly pertain to the stool osmotic gap calculation. Measuring stool volume would not help in this specific calculation, dietary fiber intake is more relevant to stool bulk than osmotic forces, and stool appearance and texture provide subjective information that does not impact the scientific calculation of the osmotic gap. Thus, the measurement of stool sodium and potassium is indeed central to understanding and calculating