Wednesday, April 29, 2009

23 - Faecal osmolar gap

Faecal osmolality is used to determine the faecal osmolar gap. This is defined as the difference between the measured osmolality and an osmolality calculated from 2 x (Na+K).

 If the gap is greater than 100 mmol/L this is consistent with an osmotic diarrhoea (eg carbohydrate (poor absorption, eg mannitol, sorbitol, lactulose); monosaccharides; short chain fatty acids; magnesium as used in antacids).

 A normal faecal osmolar gap indicates a secretory diarrhoea indicating damage or irritation of the gastro-intestinal mucosa. This test can only be applied to faecal samples which contain a high fluid content.

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