Formula
Stool osmotic gap
An estimate derived from stool sodium and potassium concentrations that helps distinguish osmotic from secretory diarrhea.
ResultmOsm/kg
Enter all values
Formula
290 − 2 × (stool Na + stool K)
Used whenUsed to help distinguish osmotic from secretory diarrhea.
Inputs
- mmol/L
- mmol/L
Worked example
Stool sodium: 40 mmol/L, Stool potassium: 20 mmol/L → 170 mOsm/kg
Educational use only. Source: Advances in Evaluation of Chronic Diarrhea in Infants, Thiagarajah JR, Kamin DS, Acra S, et al., Gastroenterology, 2018, https://pubmed.ncbi.nlm.nih.gov/29654747/.
The gap estimates how much of stool osmolality is not accounted for by sodium and potassium, which are doubled to stand for their anions. It is used when chronic diarrhea is being sorted by mechanism, and a larger gap fits an osmotic process, in which unabsorbed solutes hold water in the bowel. The fixed number in the formula is a presumed stool osmolality; the original definition subtracts twice the electrolytes from the measured osmolality. Sodium and potassium are entered in mmol/L, and the result is given in mOsm/kg.