Raised fecal osmolar gap

HOSP #	MRN123486438	WARD	Victoria Hospital Pediatric Ward
CONSULTANT	Dr Jody Rusch	DOB/AGE	2y male

Abnormal Result

Fecal osmolar gap 152 mmol/L (<100 mmol/L)

Presenting Complaint

The patient presented with chronic diarrhoea upon which fecal chemistry was requested.

History

The full history is not known.

He presented with an increased anion gap metabolic acidosis a week prior to the stool investigations being requested.

The most common explanation thereof is likely two-fold in the acute setting when the child presented:

• A fasting response with lipolysis and generation of ketone bodies leading to the acidosis
• Dehydration leading to hypoperfusion of tissues with a lactic acidemia
  • Uremia and the increased creatinine in the child was indicative thereof

Examination

Not available

Laboratory Investigations

Stool chemistry:
Faecal sodium 22 mmol/L
Faecal potassium 39.4 mmol/L
Faecal osmolality 275 mOsm/kg
Faecal osmolarity (calculated) 123 mmol/L
Faecal osmolar gap 152 mmol/L
A faecal osmolar gap (the difference between measured and calculated osmolarity) of > 100 mmol/L suggests the presence of poorly absorbed solutes.

Other Investigations

Stool reducing substances: 1+

If a positive stool reducing substances is found, characterization of the reducing substance follows. In our laboratory, this is done by thin layer chromatography.

Test Item	23/04/2021 (6 days later)	17/04/2021 (Presentation)
Na	138	135 L
K	4,2	4
Cl	106	102
Bicarb	δ+   20 L	13 L
Anion gap	16	24 H
Urea	δ-  2,2	6,8 H
Creat	25	32 H
CRP		4

Final Diagnosis

Chronic (or persistent) diarrhoea: In this case the stool microscopy also showed species of Cryptosporidium, likely the cause of the prolonged diarrhoea. Also the relative lactase deficiency after episodes of enteritis is common due to the lactase enzyme being primarily located at the apical surface of the microvili of the enterocytes, also the common site of infection of various infective organisms, such as cryptosporidium.

Take Home Message

A faecal osmolar gap (the difference between measured and calculated osmolarity) of >100 mmol/L suggests the presence of poorly absorbed solutes. A “quick and dirty” way of excluding this is by doing a dipstick of fecal fluid. In cases where diarrhea has persisted for more than two weeks, testing the stool for glucose and pH can be helpful in identifying those children with severe villous atrophy. This can be done easily at the bedside with a urine dipstick if available. Glucose test tape, nitrazine paper, and Clinitest tablets also have been used. A stool glucose of greater than 2+ or a pH of less than 5.0 suggests substantial villous atrophy.

A low stool osmotic gap suggests secretory diarrhea, wherein the digestive tract is hyperpermeable and losing electrolytes, while a high gap suggests osmotic diarrhea, wherein the digestive tract is unable to absorb solutes from the chyme, either because the digestive tract is hypopermeable (e.g. inflammation), or non-absorbable compounds (e.g. Epsom Salts, Lactose) are present. The reason for this is that secreted sodium and potassium ions make up a greater percentage of the stool osmolality in secretory diarrhea, whereas in osmotic diarrhea, other molecules such as unabsorbed carbohydrates are more significant contributors to stool osmolality.