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Difficulty: MediumInterpreting Laboratory Results

A 28-year-old woman comes to the clinic complaining of chronic, watery diarrhea for the past 4 months. She states she has 3 to 5 loose, large-volume bowel movements daily, accompanied by significant flatulence and abdominal bloating. The patient notes that her symptoms improve significantly when she fasts or restricts her diet. Her past medical history is unremarkable, and she takes no medications. Vital signs are within normal limits. Physical examination reveals a non-tender abdomen with hyperactive bowel sounds.

A fresh stool sample is sent for chemical and electrolyte analysis, and the results are as follows:

Measured stool osmolality: 295 mOsm/kg295\text{ mOsm/kg}
Stool sodium: 30 mEq/L30\text{ mEq/L}
Stool potassium: 45 mEq/L45\text{ mEq/L}
Stool chloride: 20 mEq/L20\text{ mEq/L}
Stool pH: 5.45.4

Based on the laboratory results provided, what is the calculated stool osmotic gap in mOsm/kg?

Answer: 145 mOsm/kg

Answer

145
The stool osmotic gap is correctly calculated by subtracting twice the sum of stool sodium and potassium from the measured stool osmolality (295−2×(30+45)=145295 - 2 \times (30 + 45) = 145). A gap greater than 50 mOsm/kg is characteristic of osmotic diarrhea, consistent with this patient's history of symptom improvement with fasting and consumption of a dairy-rich diet.

Step-by-Step Solution

1
Identify the formula for calculating the stool osmotic gap.
Stool Osmotic Gap = Measured Stool Osmolality - 2 × (Stool Na + Stool K)
This formula estimates the contribution of unmeasured osmoles (such as unabsorbed carbohydrates) in the stool.
2
Extract the relevant laboratory values from the clinical vignette.
Measured stool osmolality = 295 mOsm/kg, Stool Na = 30 mEq/L, Stool K = 45 mEq/L.
Only sodium, potassium, and measured osmolality are needed; chloride and pH are distractors for the calculation.
3
Calculate the sum of the stool cations (sodium and potassium).
30 + 45 = 75 mEq/L
Sodium and potassium are the primary measured cations in the stool fluid.
4
Multiply the sum of the cations by 2.
2 × 75 = 150 mOsm/kg
Multiplying by 2 accounts for the osmotic contribution of the unmeasured accompanying anions.
5
Subtract the calculated value from the measured stool osmolality.
295 - 150 = 145 mOsm/kg
This yields the final stool osmotic gap, which is greater than 50 mOsm/kg, confirming an osmotic etiology for the diarrhea.

Key Concept

Calculating the stool osmotic gap helps differentiate between osmotic and secretory diarrhea. A gap > 50 mOsm/kg (and specifically > 100 mOsm/kg) indicates osmotic diarrhea, while a gap < 50 mOsm/kg indicates secretory diarrhea.
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Interpreting Laboratory Results

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