Interpretive Handbook

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.

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.

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.

Correct interpretation requires that the stool specimen submitted be a watery stool.

In the event a soft or 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 watery stool specimen."

Bacterial action on insoluble carbohydrates in feces will cause an artifactual increase in the fecal water content of electrolytes. The specimen should be stored and shipped frozen to avoid this artifactual increase.  

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.

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

SODIUM

0-15 years: not established

> or =16 years: 0-19 mEq/24 hour

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 TP, Phillips SF: 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