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HelpOrder Code OSG_F Osmotic Gap, Feces
Ordering Guidance
This test is only clinically valid if performed on watery specimens. In the event a formed fecal specimen is submitted, the test will not be performed.
Specimen Required
Patient Preparation: No barium, laxatives, or enemas may be used for 96 hours prior to start of, or during, collection.
Supplies: Stool containers - 24, 48, 72 Hour Kit (T291)
Collection Container/Tube: Stool container
Specimen Volume: 10 g
Collection Instructions: Collect a very liquid fecal specimen.
Useful For
Workup of cases of chronic diarrhea
Differentiating osmotic from non-osmotic causes of chronic diarrhea.
Profile Information
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| NA_F | Sodium, F | No | Yes |
| K_F | Potassium, F | No | Yes |
| OG_F | Osmotic Gap, F | No | Yes |
Method Name
OG_F: Calculation
NA_F, K_F: Indirect Ion-Selective Electrode (ISE) Potentiometry
Reporting Name
Osmotic Gap, FSpecimen Type
FecalSpecimen Minimum Volume
5 g
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Fecal | Frozen (preferred) | 14 days | |
| Refrigerated | 7 days | ||
| Ambient | 48 hours |
Reject Due To
All specimens will be evaluated at Mayo Clinic Laboratories for test suitability.Reference Values
An interpretive report will be provided
Method Description
Osmotic Gap:
Calculated result=290 mOsm/kg - 2(stool Na [mmol/L] + stool K [mmol/L])
Sodium and Potassium:
The Roche cobas c 501 analyzer makes use of the unique properties of certain membrane materials to develop an electrical potential (electromotive force: EMF) for the measurements of ions in solution. The electrode has a selective membrane in contact with both the test solution and internal filling solution. The internal filling solution contains the test ion at a fixed concentration. The membrane EMF is determined by the difference in concentration of the test ion in the test solution and the internal filling solution. The EMF develops according to the Nernst equation for a specific ion in solution.(Package insert: Roche ISE reagent. Roche Diagnostics; V14 02/2018)
Performing Laboratory
Mayo Clinic Laboratories in Rochester
CPT Code Information
84302-Sodium
84999-Potassium
Day(s) Performed
Monday, Thursday