Diagnostic Use
Used in the calculation for faecal osmolar gap.
Faecal osmotic gap = 290 – 2(Na + K)
- DO NOT use measured faecal osmolality to calculate faecal osmotic gap.
- This test is used to distinguish between secretory and osmotic diarrhoea.
Osmolar gap traditionally has been used to differentiate osmotic, secretory and factitious diarrhoea.
In secretory diarrhoea, unabsorbed electrolytes retain water in the lumen while in osmotic diarrhoea non-electrolyte osmotically active compounds cause water retention in the intestinal lumen. Thus typically osmotic gap is large (>100mosm/kg) in osmotic diarrhoea and small (<50mosml/kg) in secretory diarrhoea. Further differentiation of osmotic and secretory diarrhoea may be provided by a trial of 48-72 fasting (usually as an inpatient under supervision). Continuation of diarrhoea despite fasting implies a secretory or factitious cause while cessation of diarrhoea is highly suggestive of osmotic diarrhoea.
Measured faecal osmolality should not be used to calculate faecal osmolar gap. Faecal osmolality is normally around 290mosm/kg or similar to serum even in patients taking laxatives or those with osmotic or secretory diarrhoea. However, stool osmolality naturally starts to rise inside the colon and continues post-defaecation due to bacterial fermentation on unabsorbed carbohydrates. Assigning faecal osmolality as 290mosm/kg is the correct way to calculate osmolar gap.
Measuring stool osmolality is only be of value in detecting samples that have been contaminated by water or dilute urine. Such samples have an osmolality of <290mosm/kg. Measurement of stool creatinine may be able to assess urine contamination in non-renal failure patients. Raised stool osmolality can be due to contamination by concentrated urine – in this case, the urine sodium concentration can be >150mmol/L.
Presence of faecal reducing substance AND faecal pH of <5.6 in a sample with raised faecal osmolar gap are suggestive of carbohydrate malabsorption in causing the osmotic diarrhoea.
Reference Intervals
Faecal Potassium: approximately 5 mmol/L on a normal diet.
Faecal osmolar gap: between 50-100 (around 80) in normal individuals.
Test Method
Refer to Potassium (Blood)