Osmolality, Feces

Test Code

968

CPT Codes
84999

Preferred Specimen

1 mL random watery liquid fecal collected in a plastic screw-cap container, or 24-hour, 48-hour or 72-hour watery liquid fecal collected in a plastic screw-cap container

Minimum Volume

0.2 mL

Instructions

Specimen 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.
Stable up to 3 freeze-thaw cycles

Transport Temperature

Frozen

Specimen Stability

Room temperature: Unacceptable
Refrigerated: 48 hours
Frozen: 60 days

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

Not established

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.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.