Osmolality, Feces

Casandra

Casandra Test Code MC76418Version 1 (DRAFT)

Performing Lab

Lab Mayo Clinic LabLab Test ID OSMOF

Clinical Use

Use

Measurement of osmolality is used in the workup of chronic diarrhea and to diagnose factitious diarrhea, where a patient may adulterate stool with fluid to simulate diarrhea. Fecal osmolality normally approximates twice the sum of fecal sodium and potassium unless exogenous osmotic agents or secretagogues are present ([mayocliniclabs.com](https://www.mayocliniclabs.com/test-catalog/overview/606758?utm_source=openai)).

Special Instructions

This test must be performed only on watery, liquid stool specimens—formed fecal specimens will not be performed. Patient preparation requires avoidance of barium, laxatives, or enemas for 96 hours prior to and during collection. Supplies include stool containers (24, 48, 72 Hour Kit T291) ([mayocliniclabs.com](https://www.mayocliniclabs.com/test-catalog/overview/606758?utm_source=openai)).

Limitations

Specimen suitability is evaluated upon receipt. Inappropriate forms such as formed stool will not be tested. Prolonged storage at incorrect temperatures can alter osmolality due to bacterial metabolism. Stool osmolality below 220 mOsm/kg may indicate dilution with hypotonic fluid. Interpretive context—and occasionally calculation of osmotic gap—is required for accurate interpretation ([mayocliniclabs.com](https://www.mayocliniclabs.com/test-catalog/overview/606758?utm_source=openai)).

Test Details

New York Approved