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Stool Osmolar/Osmotic Gap
Distinguishes osmotic from secretory causes of chronic diarrhoea.
Stool Osmolar/Osmotic Gap
Stool sodium
mEq/L
Stool potassium
mEq/L
Complete the fields above to see the result.
When to use
- Working up chronic watery diarrhoea to distinguish osmotic from secretory mechanisms once stool electrolytes are available.
Formula
Stool osmotic gap = 290 - 2 x (stool Na+ + stool K+), assuming a fixed stool osmolality of 290 mOsm/kg. Gap >125 suggests osmotic diarrhoea; <50 suggests secretory diarrhoea.
Pearls and pitfalls
- A very low or negative gap in a patient with watery diarrhoea should also raise suspicion for surreptitious ingestion of a poorly absorbed salt (e.g. magnesium).