American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the osmotic gap formula for measuring stool osmotic gap?

1. 290 - (2 × [stool sodium + stool potassium])

2. 300 - (2 × [stool sodium + stool potassium])

3. 290 + (2 × [stool sodium - stool potassium])

4. 250 - (2 × [stool sodium + stool potassium])

The correct answer is: 290 - (2 × [stool sodium + stool potassium])

The formula for measuring the stool osmotic gap is determined by the calculation that reflects the concentration of solutes in the stool. The correct formula is: Osmotic Gap = 290 - (2 × [stool sodium + stool potassium]). This formula works by estimating the osmolarity of stool, which is typically around 290 mOsm/L. The term (2 × [stool sodium + stool potassium]) effectively accounts for the major constituents of osmotic activity due to these electrolytes. Sodium and potassium are present in stool and contribute significantly to the stool's osmotic pressure. The osmotic gap is useful in distinguishing between causes of diarrhea: a high osmotic gap (typically > 50) suggests an osmotic diarrhea due to unmeasured solutes (like certain sugars or laxatives), while a low osmotic gap suggests secretory diarrhea, where the osmotic activity is mainly from the measured sodium and potassium. Therefore, using this formula allows clinicians to derive meaningful insights into the underlying mechanisms of diarrhea based on the calculated osmotic gap, making the choice of the first option the appropriate one.