Test ID: UOSMF
Osmolality, Feces
Useful For 
Suggests clinical disorders or settings where the test may be helpful
In conjunction with a serum osmolality to calculate an osmotic gap
Diagnosis of factitious diarrhea (where patient adds water to stool to simulate diarrhea)
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Fecal osmolality is normally in equilibrium with vascular osmolality, and sodium is the major affecter of this equilibrium.
Stool osmolality is normally 2x (stool sodium + stool potassium) unless there are exogenous factors inducing a change in this ratio (demonstrated by the presence of an osmotic gap), such as the presence of other osmotic agents (eg, magnesium, sulfate, saccharides), or drugs (eg, bisacodyl, phenolphthalein) inducing secretions.
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: 220-280 mOsm/kg
Interpretation Provides information to assist in interpretation of the test results
Typically, stool osmolality is similar to serum since the gastrointestinal tract does not secrete water. A useful formula is 2x (stool sodium + stool potassium) = stool osmolality + or - 30 mOsm.
If sodium concentration or 24-hour sodium excretion rate is 2 to 3 times normal and osmotic gap >30 mOsm/kg, secretory diarrhea may be the cause. 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.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test should be performed on watery stools (diarrhea).
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."
Clinical Reference 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
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