Osmolality, Fecal

Laboratory assay overview

Distinction of osmotic from nonosmotic secretory diarrhea. A large deficit (>50 mOsm/kg) in the calculated osmolality (twice the sum of the stool sodium and stool potassium) from the measured stool osmolality indicates osmotic diarrhea.

Laboratory assay overview

Direct measurement of osmolality in liquid stool helps determine whether a specimen has been diluted with a hypotonic fluid (eg, water) [1]. Because bacterial metabolism in a specimen often increases stool osmolality after collection, this test may not be suitable for estimating osmotic gap [2]. Depending on how the water content of stool is drawn in the intestine lumen, diarrhea can be categorized as secretory or osmotic. Osmotic gap-the difference between the stool osmolality and twice the sum of the stool sodium and potassium levels-is often calculated to differentiate between secretory and osmotic diarrhea [1]. The estimation of osmotic gap usually uses theoretical stool osmolality, which is approximately the same as plasma osmolality (290 mOsm/kg). However, when a stool specimen is deliberately diluted to simulate diarrhea, the increased osmotic gap is indifferentiable from the one in osmotic diarrhea. Directly measured osmolality in diluted stool specimen may show a decreased value as opposed to a normal value in osmotic diarrhea [3]. Because bacterial metabolism generates osmotically active substances during specimen storage and results in falsely elevated stool osmolality, this test may not be suitable for estimating osmotic gap [2]. The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings. References 1. Block DR, et al. Body fluids. In: Rifai R, et al. eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier Inc; 2022. 2. Schiller LR. Pract Res Clin Gastroenterol. 2012;26(5):551-562. 3. Shiau YF, et al. Ann Intern Med. 1985;102(6):773-775.