What is the role of an anion gap test in the workup of diarrhea?

Updated: Jan 31, 2020
  • Author: Stefano Guandalini, MD; Chief Editor: Carmen Cuffari, MD  more...
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Because the pathogenesis of diarrhea can be either osmolar (due to the presence of an excess of unabsorbed substrates in the gut lumen) or secretory (due to active anion secretion from the enterocytes), the anion gap in the stools is occasionally used to ascertain the nature of the diarrhea. The stool anion gap is calculated according to the formula: 290 - [(Na+K) X 2]. If the value is more than 100, osmolar diarrhea can be assumed to be present. If the value is less than 100, the diarrhea has a secretory origin.

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