Osmolality, Feces

Test Codes

EPIC: LAB1231974, Beaker: Osmolality, Quest: 968
NOTE: This test only has clinical utility if performed on a watery fecal specimen.

Department

Send Outs

Specimen Collection Criteria

Collect: 1.0 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.

Collect for 24-hour, 48-hour or 72-hour using the Kit, Fecal Fat, Stool. Please call the Storeroom 248-551-2946, for this kit.

Physician Office/Draw Specimen Preparation

Refrigerate (2-8°C or 36-46°F) the specimen immediately after collection.
Freeze (-20°C/-4°F or below) specimens that will not be received in the Laboratory within two hours of collection.

Preparation for Courier Transport

Transport: Fecal specimen frozen (-20°C/-4°F or below).

Rejection Criteria

• Formed (solid) stool specimens.
• Received in paint cans.
• Specimens not collected and processed as indicated.

In-Lab Processing

The specimen should be frozen (-20°C/-4°F or below) if not analyzed within two hours of collection.

24-Hour Collection: Transport the entire collection in the original collection kit. (Minimum: 0.2 mL) Do not add saline or water to liquefy specimen. Indicate time and volume.

Random Collection: Transfer 1.0 mL feces to unpreserved stool transport vial. (Minimum: 0.2 mL) Do not add saline or water to liquefy specimen. Indicate time and volume.

CRITICAL FROZEN. Separate specimens must be submitted when multiple tests are ordered.

Storage

Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): Unacceptable
Refrigerated (2-8°C or 36-46°F): 48 hours
Frozen (-20°C/-4°F or below): 60 days

Laboratory

Sent to Quest Diagnostics, Wood Dale, IL.

Performed

Monday – Friday.
Results available within 2-3 days.

Reference Range

Not established.

Test Methodology

Freezing Point Depression

Clinical Utility

Direct measurement of osmolality in liquid stool helps determine whether a specimen has been diluted with a hypotonic fluid (e.g., 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.

Reference

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.

CPT Codes

84999

Contacts

Last Updated

11/25/2025