Magnesium, 24 Hour, Feces
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Magnesium is a normal constituent in the diet and is normally present in fecal material. Magnesium is also a commonly used laxative that can be the cause of chronic diarrhea.
Sodium, potassium, chloride, and sulfate are usually the major contributors to fecal osmolality. Fecal osmolality is normally 2 x (sodium + potassium) unless there are exogenous factors inducing a change in this ratio, such as the presence of other osmotic agents (magnesium sulfate, saccharides), or drugs inducing secretions, such as phenolphthalein or bisacodyl.
If magnesium is excreted at a high rate (>30 mEq/24 hour), with an osmotic gap >100 mOsm/kg, magnesium is likely to be the principle contributor to osmotic diarrhea. Use of magnesium salts in the form of over-the-counter stool softeners is the most likely source of such excess excretion.
Diagnosis of osmotic diarrhea due to excessive ingestion of magnesium
Ruling out excessive ingestion of magnesium in the work-up of chronic diarrhea
The osmotic theory of diarrhea is explained by the equation: 2x (stool sodium + stool potassium) = stool Osmolality + or - 30 mOsm.
Normal fecal sodium and potassium in the presence of an osmotic gap (>30 mOsm/kg) suggests osmotic diarrhea.
Ingestion of more than usual dietary magnesium leading to increased excretion of fecal magnesium contributes to osmotic diarrhea.
Magnesium excretion at >30 mEq/24 hour is frequently associated with decreased fecal sodium and potassium, and is an indicator of excessive consumption of magnesium that is likely the cause of diarrhea.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test will only be performed on watery stool (diarrhea).
Formed stools are highly unlikely to contain excessive concentrations of magnesium. Formed stools indicate normal osmolality, ruling out excessive magnesium concentration. In the event a formed stool is submitted, the test will not be performed, and the report will indicate: "A formed stool specimen was submitted for analysis. This test was not performed because it only has clinical value if performed on a naturally occurring watery stool specimen."
High concentrations of gadolinium and iodine are known to interfere with most metals tests. If either gadolinium- or iodine-containing contrast media has been administered, a specimen must not be collected for 96 hours.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
0-15 years: not established
> or =16 years: 0-29 mEq/24 hour
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Phillips S, Donaldson L, Geisler K, et al: Stool composition in factitial diarrhea: a 6-year experience with stool analysis. Ann Intern Med 1995;123:97-100
2. Ho J, Moyer T, Phillips S: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092
3. Fine KD, Santa Ana CA, Fordtran JS : Diagnosis of magnesium-induced diarrhea. N Engl J Med 1991;324:1012-1017