Normal stool water osmolality

Q: Normal stool water osmolality is approximately equal to serum osmolality.

A) Yes

B) No

Answer: A

It is of clinical importance to understand the normal composition of stool. A normal human excretes only 100 mL of water each day. Due to this small amount, despite stool's high electrolyte concentrations, very little sodium i.e., 4 mEq, and potassium i.e., 9 mEq are lost in normal stool each day. Similarly, the total daily loss of bicarbonate plus organic acid anion salts is not significant.

Stool water osmolality is similar to the osmolality of serum i.e., 300 mOsm/kg.

Paying attention to stool water osmolality and electrolyte abnormalities may guide toward the underlying cause of diarrhea (see references.)

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References:

1. Gennari FJ, Weise WJ. Acid-base disturbances in gastrointestinal disease. Clin J Am Soc Nephrol 2008; 3:1861.

2. Steffer KJ, Santa Ana CA, Cole JA, Fordtran JS. The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea. Gastroenterol Clin North Am 2012; 41:539.