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.
References:
1. Schiller LR, Sellin JH. Diarrhoea (chapter 9) in Sliesenger and Fordtran's Gastrointestinal and Liver disease - pathophysiology / Diagnosis/ management . 8th edition (2006) Feldman M, Friedman LS, Brandt LJ (eds). Saunders Elsevier, Philadelphia
2. Duncan A et al The fecal osmotic gap: technical aspects regarding its calculation. J Lab Clin Med 1992; 119:359-63
3. Thomas PD et al. Guidelines for the investigation of chronic diarrhoea. Gut 2003; 52(suppl V): V1-5
4. Castro-Rodriguez JA et al Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements. Archives of disease in Childhood 1997; 77:201-5
5. Eherer AJ Fecal osmotic gap and pH in experimental diarrhoea of various causes. Gastroenterology 1992; 103:545-551
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