Faecal electrolytes

Clinical background:

For the investigation of chronic diarrhoea (liquid stools lasting longer than 4 weeks), including factitious diarrhoea.

Specimen container paediatric:

Faeces in universal container

Specimen container adult:

Faeces in universal container

Minimum volume paediatric:

5 mL liquid faeces

Minimum volume adult:

5 mL liquid faeces

Special requirements:

Faeces must be liquid

Sample stability:

After centrifugation:at +2° to +8°C  48hr
at -15° to -20°C 14 days

Interpretation:

• Osmolality
The faecal osmolality is in equilibrium with serum/plasma osmolality. A measured stool osmolality of < 290 mOsm/kg or > 600 mOsm/kg is a potential clue to factitious diarrhoea.A low osmolality (< 290 mOsm/kg) can only result by adding a hypotonic solution, such as water or urine, to stool. A very high stool osmolality (> 600 mOsm/kg) may be a clue to stool diluted with hypertonic solutions, such as tomato juice or blood.It is important to note that faecal osmolality may be increased factitiously because carbohydrates may be metabolised by colonic bacteria causing an increase faecal osmolality. This can be minimised by analysing samples with 24 hours of receipt, or freezing samples until analysis.
• Osmolar gap
The osmolar gap is calculated as follow:    290   –    [2 x (faecal sodium + faecal potassium)]Note: the measured stool osmolality should not be used to calculate the osmolar gap because it largely reflects bacterial metabolism in vitro, not intraluminal osmolality.An osmotic gap indicates the presence of poorly absorbable solute(s). In osmotic diarrhoea the osmolar gap is typically >50 mOsm/kg and secretory diarrhoea the osmolar gap is <50 mOsm/kg.Osmotic diarrhoea:Usually due to ingestion of poorly absorbed cations (i.e. magnesium) or anions (i.e. phosphate or sulphate), which are often contained in laxatives and antacids, or to carbohydrate malabsorption from ingestion of poorly absorbed sugars or sugar alcohols (i.e. sorbitol or xylitol).Lactose intolerance is the most common type of carbohydrate malabsorption.Measuring a stool pH can help distinguish between osmotic diarrhoea due to poorly absorbed ions and that due to poorly absorbed sugars. Carbohydrate malabsorption will result in a stool pH < 6.Secretory diarrhoea:Major causes are infection, bile acid malabsorption, non-osmotic laxatives, inflammatory bowel disease, peptide-secreting endocrine tumours (i.e. carcinoid and gastrinoma), and neoplasia.An essential characteristic of osmotic diarrhoea is that stool volume decreases with fasting, whereas secretory diarrhoea typically continues unabated with fasting.A highly negative osmotic gap or a faecal sodium concentration greater than plasma or serum suggests the possibility of either sodium phosphate or sodium sulphate ingestion by the patient.
• Sodium and potassium
Faecal sodium of >150 mmol/L and an osmolality of >375-400 mOsm/kg suggests contamination with concentrated urine.High sodium and potassium in the absence of an osmotic gap indicate active electrolyte transport in the GI tract that might be induced by agents such as cholera toxin or hypersecretion of vasoactive intestinal peptide (VIP).Sodium is typically found at lower concentrations (mean 30 ± 5 mmol/L) in patients with osmotic diarrhoea caused by magnesium-containing laxatives, while typically at higher concentrations (mean 104 ± 5 mmol/L) in patients known to be taking secretory laxatives.
• Chloride
Chloride may be low (<20 mmol/L) in sodium sulphate-induced diarrhoea.Markedly elevated faecal chloride concentration in infants (>60 mmol/L) and adults (>100 mmol/L) is associated with congenital and secondary chloridorrhoea.