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HelpTest Code MG_F Magnesium, 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)
Container/Tube: Stool container
Specimen Volume: 10 g
Collection Instructions: Collect a very liquid stool specimen.
Useful For
Workup of cases of chronic diarrhea
Identifying the use of magnesium-containing laxatives contributing to osmotic diarrhea
Method Name
Colorimetric Titration
Reporting Name
Magnesium, 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.Clinical Information
The concentration of electrolytes in fecal water and their rate of excretion are dependent upon 3 factors:
-Normal daily dietary intake of electrolytes
-Passive transport from serum and other vascular spaces to equilibrate fecal osmotic pressure with vascular osmotic pressure
-Electrolyte transport into fecal water due to exogenous substances and rare toxins (eg, cholera toxin)
Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major effector of this equilibrium. Fecal osmolality is normally 2 x (sodium + potassium) unless there are exogenous factors inducing a change in composition, such as the presence of other osmotic agents (magnesium sulfate, saccharides) or drugs inducing secretions, such as phenolphthalein or bisacodyl (1).
Osmotic diarrhea is caused by ingestion of poorly absorbed ions or sugars.(1) There are multiple potential causes of osmotic diarrhea. Measurement of magnesium in liquid stool can assist in identifying intentional or inadvertent use of magnesium and/or phosphate containing laxatives as the cause.(2-4) The other causes of osmotic diarrhea include ingestion of osmotic agents such as sorbitol or polyethylene glycol laxatives, or carbohydrate malabsorption due most commonly to lactose intolerance. Carbohydrate malabsorption can be differentiated from other osmotic causes by a low stool pH (<6).(5,6)
Reference Values
An interpretive report will be provided
Cautions
In very rare cases, gammopathy, in particular type IgM (Waldenstrom macroglobulinemia), may cause unreliable results.
Method Description
In an alkaline solution, magnesium forms a purple complex with xylidyl blue, a diazonium salt. The magnesium concentration is measured photometrically via the decrease in the xylidyl blue absorbance.(Package insert: Magnesium reagent. Roche Diagnostics; V8.0 01/2020)
Performing Laboratory
Mayo Clinic Laboratories in Rochester
Test Classification
This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.CPT Code Information
83735
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| MG_F | Magnesium, F | 29911-5 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| MG_F | Magnesium, F | 29911-5 |
Day(s) Performed
Monday, Thursday