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Osmolality, Feces
MessageSpecimen must be shipped frozen to reduce the odor during shipping and to minimize the risk of the container rupturing due to gas accumulation.
Note: This test only has clinical utility if performed on a watery fecal specimen.
Test Code
CPT Codes
84999
Preferred Specimen
1 mL random watery liquid fecal collected in a plastic screw-cap container or 24-hour or 48-hour watery liquid fecal collected in a plastic screw-cap container
Minimum Volume
0.2 mL
Instructions
Note: This test only has clinical utility if performed on a watery fecal specimen.
Stable up to 3 freeze-thaw cycles
Transport Temperature
Frozen
Specimen Stability
Room temperature: Unacceptable Refrigerated: Unacceptable Frozen: Not established
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Specimens in paint cans • Formed stool • Specimens received room temperature • Specimens received refrigerated
Methodology
Freezing Point Depression
Setup Schedule
Set up: Sun-Fri; Report available: 3 days
Reference Range
No age/sex specific ranges for this test
Clinical Significance
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.
Performing Laboratory
Quest Diagnostics Nichols Institute 14225 Newbrook Drive Chantilly, VA 20153
Last Updated: February 23, 2012