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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.
Transport Container
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
FDA Status
This test was developed and its analytical performance characteristics have been determined by Quest Diagnostics. It has not been cleared or approved by the FDA. This assay has been validated pursuant to the CLIA regulations and is used for clinical purposes.
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