Osmotic Gap, Feces
Use
This test is useful for the workup of cases of chronic diarrhea, particularly for differentiating osmotic from non‑osmotic causes by calculating an osmotic gap using fecal electrolyte measurements (sodium and potassium). A high osmotic gap suggests osmotic diarrhea, whereas a low gap is consistent with secretory diarrhea. Magnesium‑induced diarrhea may be considered if the gap is particularly elevated, and a highly negative gap or fecal sodium exceeding serum can suggest ingestion of phosphate or sulfate salts.
Special Instructions
Patient preparation requires avoidance of barium, laxatives, or enemas for 96 hours before and during collection. Only watery (liquid) fecal specimens are valid; formed specimens are not acceptable. Supplies include stool containers via the 24, 48, 72 Hour Kit (T291).
Limitations
Specimens that are formed or otherwise not sufficiently liquid will not be analyzed. All specimens are evaluated for suitability; unsuitable specimens are rejected. Osmotic gap calculations assume normal stool osmolality and can be affected by exogenous substances or toxins. Interpretation may be limited by unusual electrolyte content or interfering substances.
New York Approved
Methodology
Automated Analyzer (Clinical Chemistry)
Biomarkers
LOINC Codes
- 88697-8
- 15207-4
- 15202-5
- 73571-2
Result Turnaround Time
1-3 days
Related Documents
For more information, please review the documents below
Specimen
Stool
Volume
10 g
Minimum Volume
5 g
Container
Stool container
Collection Instructions
Collect a very liquid fecal specimen.
Patient Preparation
No barium, laxatives, or enemas may be used for 96 hours prior to start of, or during, collection.
Causes for Rejection
Formed specimens or those not sufficiently liquid will not be performed; all specimens evaluated for suitability.
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 48 hours |
| Refrigerated | 7 days |
| Frozen | 14 days |