Sorted By Test Name - Mayo Medical Laboratories

KFT
60031
Reference values have not been established for patients that are less than 12 months of age.
SODIUM
> or =12 months: 135-145 mmol/L
Reference values have not been established for patients that are less than 12 months of age.
Clinical References: See Individual Unit Codes
Potassium, 24 Hour, Feces
Clinical Information: 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: Work-up of a patient with chronic diarrhea
Interpretation: 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.
Reference Values:
0-15 years: not established
> or =16 years: 0-29 mEq/24 hour
Clinical References: 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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