Stool Electrolytes
Diarrhoea
K (normal <5mmol/L) high in ?
- used to screen for causes of hypokalaemia
Na high (normal <45mmol/L) in secretory diarrhoea
- used to differentiate secretory from osmotic diarrhoea
- (Secretory: pancreatic islet cell tumour, cathartics, enterotoxins eg, cholera)
Mg
- screen for laxative abuse (Mg containing laxative)
- screen for cause of hypoMg
Osmolality (normal is same as serum ie 290)
<250 mOsm/kg suggests addition of water
>400 mOsm/kg suggests addition of urine (bowel cannot concentrate higher than serum)
gap >75 mOsmol/kg osmotic: magnesium, sorbitol, lactose, lactulose, or polyethylene glycol
<75 mOsmol/kg secretory: senna, bisacodyl
magnesium >90 mEq/L Mg-induced
phosphate >33 mmol/L PO4-induced
Scope melanosis coli ?anthraquinone eg senna but not specific
Reducing substances
- screen for sugar intolerance (lactose, sucrose or glucose/galactose)
- NB only positive if offending sugar has been ingested recently
- pH doesn't add much
pH (normal 5-9)
- undigested sugars are fermented in the gut to produce acids lowering pH
- doesn't add much to reducing substances
fat globules
- fat breakdown problem
fatty acid crystals
- fat absorption problem
Faecal Fat
- screen for malabsorption, not routine
Faecal elastase
- screen for pancreatic exocrine failure more common screen than fat
Stool alpha1AT (not secreted, absorbed or digested can only come from leak)
- for protein losing enteropathy
K (normal <5mmol/L) high in ?
- used to screen for causes of hypokalaemia
Na high (normal <45mmol/L) in secretory diarrhoea
- used to differentiate secretory from osmotic diarrhoea
- (Secretory: pancreatic islet cell tumour, cathartics, enterotoxins eg, cholera)
Mg
- screen for laxative abuse (Mg containing laxative)
- screen for cause of hypoMg
Osmolality (normal is same as serum ie 290)
<250 mOsm/kg suggests addition of water
>400 mOsm/kg suggests addition of urine (bowel cannot concentrate higher than serum)
gap >75 mOsmol/kg osmotic: magnesium, sorbitol, lactose, lactulose, or polyethylene glycol
<75 mOsmol/kg secretory: senna, bisacodyl
magnesium >90 mEq/L Mg-induced
phosphate >33 mmol/L PO4-induced
Scope melanosis coli ?anthraquinone eg senna but not specific
Reducing substances
- screen for sugar intolerance (lactose, sucrose or glucose/galactose)
- NB only positive if offending sugar has been ingested recently
- pH doesn't add much
pH (normal 5-9)
- undigested sugars are fermented in the gut to produce acids lowering pH
- doesn't add much to reducing substances
fat globules
- fat breakdown problem
fatty acid crystals
- fat absorption problem
Faecal Fat
- screen for malabsorption, not routine
Faecal elastase
- screen for pancreatic exocrine failure more common screen than fat
Stool alpha1AT (not secreted, absorbed or digested can only come from leak)
- for protein losing enteropathy