Interpretive Handbook
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Test 60031:
Potassium, 24 Hour, Feces
Clinical Information 
Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Potassium is an intracellular cation normally present in fecal water at a concentration approximately 20 times higher than the serum concentration.
The fecal water potassium content is comprised of 2 fractions-approximately 15% of the normal dietary intake of potassium (80 mEq/day) passes through the gastrointestinal (GI) tract to be deposited directly in fecal water, and a small fraction of potassium crosses the epithelial barrier of the GI tract from extra cellular fluids based on osmotic pressure.
In adults, the median daily excretion of potassium is 9 mEq/day, ranging from 0 mEq/day to 30 mEq/day. The median concentration of potassium in fecal water is 40 mEq/kg, ranging from 0 mEq/day to 200 mEq/kg. Potassium excretion is race-related; excretion in blacks is less than in Caucasians, usually by a factor of 2.
The fecal water potassium concentration and daily excretion rate will be normal if the cause of diarrhea is bacteria or due to ingestion of osmotic agents such as magnesium, phenolphthalein, and sulfate. The fecal water potassium daily excretion rate will be normal, but the measured concentration will be increased in patients with contracted colon volume.
Both the daily excretion rate of potassium and potassium concentration will be elevated in ulcerative colitis or other diseases where there is bleeding into the GI tract, exposure to cholera toxin, and in patients with islet cell tumors, increased secretion of vasointestinal peptide (vipoma syndrome), primary aldosteronism, ingestion of mineralocorticoids, and due to bacterial metabolism of unabsorbed carbohydrates passing through the GI tract.
Useful For Suggests clinical disorders or settings where the test may be helpful
Work-up of a patient with chronic diarrhea
Interpretation Provides information to assist in interpretation of the test results
Typically, stool potassium is 20 times serum potassium. A useful formula is 2x (stool sodium + stool potassium) = stool osmolality + or - 30 mOsm.
Fecal potassium concentration and daily excretion rate are usually below the median level in patients with osmotic diarrhea. Normal fecal sodium and potassium in the presence of an osmotic gap (>30 mOsm/kg) suggests osmotic diarrhea.
Increased fecal sodium content or daily excretion rate with normal fecal potassium and no osmotic gap indicates secretory diarrhea.
High fecal potassium in association with normal or low fecal sodium suggests deterioration of the epithelial membrane or a bleeding lesion
High sodium and potassium (3 times normal) in the absence of an osmotic gap indicate active electrolyte transport in the gastrointestinal tract that might be induced by agents such as cholera toxin, hypersecretion of vasointestinal peptide, or islet cell tumor.
For very low stool osmolality, consider factitial diarrhea.
The fecal potassium concentration and excretion rate are increased 2-fold to 3-fold with ulcerative colitis, or bleeding into the GI tract, when exposed to cholera toxin, with ingestion of mineralocorticoids, in primary aldosteronism, and due to bacterial metabolism of unabsorbed carbohydrates.
The fecal water potassium concentration and daily excretion rate exceeds 3 times normal in association with islet cell tumors and increased secretion of vasointestinal peptide.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test will be performed on watery stools (diarrhea) only.
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: Stool composition in factitial diarrhea: a 6-year experience with stool analysis. Ann Intern Med 1995;123:97-100
2. Agarwal R, Afzalpurkar R, Fordtran JS: Pathophysiology of potassium absorption and secretion by the human intestine. Gastroenterology 1994;107:548-571
3. Ho J, Moyer T, Phillips S: Chronic diarrhea: the role of magnesium. Mayo Clin Proc 1995;70:1091-1092
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