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EFP24
60035
Reference Values:
See individual test listings.
Clinical References: See Individual Unit Codes
Electrolytes, 24 Hour, Feces
Clinical Information: The concentration of electrolytes in fecal water and their rate of excretion are
dependent upon 3 factors: -The normal daily dietary intake of electrolytes. -Electrolytes are passively
transported from serum and other vascular spaces to equilibrate fecal osmotic pressure with vascular
osmotic pressure -Exogenous substance and rare toxins (cholera toxin) cause electrolyte transport into
fecal water Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major
affector of this equilibrium. 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.
Useful For: The work-up of cases of chronic diarrhea Making the diagnosis of factitial diarrhea The
relationship, osmolality equals 2 x (sodium + potassium), is the basis for this evaluation.
Interpretation: A useful formula is 2x (stool sodium + stool potassium)=stool osmolality + or - 30
mOsm. Typically, stool osmolality is similar to serum since the gastrointestinal (GI) tract does not secrete
water. Osmotic Diarrhea: If the measured osmolality exceeds the calculation of 2 x (sodium + potassium)
by >30 mEq/kg, an osmotic gap exists indicating osmotic diarrhea. Fecal potassium concentration and
daily excretion rate are usually below the median level in patients with osmotic diarrhea. Modest
increases (2x) in fecal chloride concentration and excretion rate may be observed when fecal sodium is
elevated in association with osmotic diarrhea. Osmotic agents such as magnesium, sorbitol, or
polyethylene glycol may be the cause of this. Magnesium concentration of >200 mEq/kg is frequently
associated with decreased fecal sodium and potassium and is an indicator of excessive consumption of
magnesium, a common cause of osmotic diarrhea. Secretory Diarrhea: Increased fecal sodium and
chloride content or daily excretion rate with normal fecal potassium and no osmotic gap indicates
secretory diarrhea. If sodium concentration or 24-hour sodium excretion rate is 2 to 3 times normal and
osmotic gap >30 mOsm/kg, secretory diarrhea is also indicated. Agents such as phenolphthalein,
bisacodyl, or cholera toxin should be suspected. An osmotic gap >100 mOsm/kg indicates factitial
diarrhea, likely due to magnesium or phenolphthalein consumption. For very low stool osmolality,
consider factitial diarrhea. Normal or low fecal sodium in association with high fecal potassium suggests
deterioration of the epithelial membrane or a bleeding lesion High sodium and potassium in the absence
of an osmotic gap indicate active electrolyte transport in the GI tract that might be induced by agents such
as cholera toxin or hypersecretion of vasointestinal peptide. Fecal chloride concentration or daily
excretion rate are markedly elevated (7-10 times normal) in association with congenital hypochloremic
alkalosis with chloridorrhea.
Reference Values:
CHLORIDE
0-15 years: not established
> or =16 years: 0-29 mEq/24 hour
MAGNESIUM
0-15 years: not established
> or =16 years: 0-29 mEq/24 hour
OSMOLALITY
0-15 years: not established
> or =16 years: 220-280 mOsm/kg
POTASSIUM
0-15 years: not established
> or =16 years: 0-29 mEq/24 hour
Current as of January 4, 2013 7:15 pm CST 800-533-1710 or 507-266-5700 or MayoMedicalLaboratories.com Page 659