American Board of Internal Medicine (ABIM) Certification Practice Exam

What osmotic gap range is considered equivocal in diagnosing diarrhea?

1. 0 to 50 mOsm/kg

2. 50 to 100 mOsm/kg

3. 100 to 150 mOsm/kg

4. 150 to 200 mOsm/kg

The correct answer is: 50 to 100 mOsm/kg

The osmotic gap is a clinical measure used to help differentiate between osmotic and secretory diarrhea. It is calculated using the formula: osmotic gap = measured osmolality - (2 x [Na+ + K+]). A higher osmotic gap indicates the presence of unmeasured osmoles, which can occur in osmotic diarrhea due to substances like lactose or magnesium in the stool. An osmotic gap in the range of 50 to 100 mOsm/kg is considered equivocal because it is not definitively indicative of osmotic or secretory diarrhea. Values below 50 typically suggest secretory diarrhea, while values above 100 lean toward osmotic diarrhea. Therefore, a value within the 50 to 100 mOsm/kg range indicates uncertainty and requires further investigation to assess the underlying cause of the diarrhea. It suggests that other factors may be influencing the diarrhea, making it challenging to categorize confidently. This understanding of the osmotic gap and its implications is crucial for accurate diagnosis and management of patients presenting with diarrhea.