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.

19 - Osmolality clinical uses

Serum osmolality is used in two main circumstances: investigation of hyponatraemia and identification of an osmolar gap. Urine osmolality is an important test of renal concentrating ability, for identifying disorders of the ADH mechanism, and identifying causes of hyper-or hyponatraemia. Faecal osmolality can be used to assist with diagnosis of the cause of diarrhoea.