Osmolality, Feces
Test Summary
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.
Aliases
- N/A
Specimen Collection
Special Instructions
Preferred Specimen
Minimum Volume
Instructions
Note: This test only has clinical utility if performed on a watery fecal specimen.
Stable up to 3 freeze-thaw cycles
Patient Preparation
Storage
Transport Temperature
Specimen Stability
- Room temperature: Unacceptable
- Refrigerated: 48 hours
- Frozen: 60 days
Limitations
Other Acceptable Specimens
Unacceptable Specimens
Order Code
EPIC (Premier) Code
Includes
CPT Code
- 84999
Billing Code
- 671160
CPT Statement
Methodology
FDA Status
Physician Attestation of Informed Consent
Testing Laboratory
14225 Newbrook Drive
Chantilly VA, 20153
Department
Reference Range
Setup Schedule / Expected Turnaround Time
Specimen Collection
Special Instructions
Preferred Specimen
Minimum Volume
Instructions
Note: This test only has clinical utility if performed on a watery fecal specimen.
Stable up to 3 freeze-thaw cycles
Patient Preparation
Storage
Transport Temperature
Specimen Stability
- Room temperature: Unacceptable
- Refrigerated: 48 hours
- Frozen: 60 days