I could find no formal procedure written to cover fecal osmolality,
either in a text, or in our procedure manual. However, I was informed by
the supervisor that the fecal sample is treated in the same way as a
urine osmolality: that is, it is first centrifuged as a urine would be,
and the supernatant is assayed by our freezing point depression method
(Advanced Osmometer).
I found in a monograph entitled "Osmometry" written by D. Robert Dufour,
M.D. one important consideration for the assay: "bacterial metabolism
produces osmotically active substances; measurements must be made within
30 minutes of collection of a specimen, or the specimen should be
refrigerated until analysis is performed."
Stool osmolalities are used to help sort out the causes of diarrhea.
Calculate the osmolal gap as 2 x (Na + K) in the fecal supernatant.
Practically all the osmotic effect in stool should be from
electrolytes. A higher than expected osmolal gap indicates the presence
of either unabsorbed solutes, laxative abuse, malabsorption (Osmotic
diarrhea). A low osmolal gap points to intestinal mucosal damaging
agents, including, but not limited to infections, inflammations, or
certain drugs (Secretory diarrhea).
Typically, the cutoff in the osmolal gap is 50 mOsm/kG, as mentioned by
Dr. Dufour in his monograph, (which was originally presented as an
industry workshop at A.A.C.C in New York in 1993), and in Tietz,
Textbook of Clinical Chemistry, 2nd Ed.1986.
I hope this helps.
Lucie Fritz
Graduate Student
Medical College of Virginia, / Virginia Commonwealth University
Richmond, Va.
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