Stool Osmotic Gap Calculator

How does this stool osmotic gap calculator work?

This is a health tool that determines the nature of chronic diarrhea and indicates based on stool osmotic gap whether the diarrhea is secretory or osmotic.

The stool osmotic gap calculator provides a quick and simple calculation mean with the user having to input just the Na, K and stool osmolarity values, the first two being available to use in either mEq/L or mmol/L.

Most of the osmotic activity of stool comes from electrolytes because in normal stool, all other substances are in theory properly absorbed. Therefore the stool osmotic gap that results is the difference between the measured osmolarity, similar to serum osmolarity (290 – 300 mOsm/kg) and the calculated osmolarity (difference between the Na⁺ and K⁺ ions in stool).

The formula to calculate stool osmotic or osmolal gap is: Stool osmolarity - 2 * (stool Na + stool K).

The stool osmolality/osmolarity is not directly measured but given a constant in the range of 290 to 300 mOsm/kg.

It is important to consider the effect of bacterial metabolism and how its production of osmotically active substances can affect measurements, therefore all determinations should be made within 30 minutes of collection.

The other normal fecal fluid values are: Na⁺ = 30 mEq/L and stool K⁺ = 75 mEq/L.

The interpretation of the stool gap is as follows:

■ low osmolar gap – values below 50 mOsm/kg – indicates secretory diarrhea;

■ normal osmolar gap – values between 50 and 100 mOsm/kg;

■ high osmolar gap – values above 100 mOsm/kg – indicates osmotic diarrhea.

The difference between the two types of diarrhea stands between the secreted sodium and potassium ions which make a big percentage of stool osm in secretory diarrhea while in the osmotic one they make up a small percentage of stool osm because of the other unabsorbed molecules being present, such as carbohydrates.

Stool osmolarity is just one of the determinations that look at ion concentration in the body, often urine anion gap analysis being used to differentiate metabolic acidosis diagnoses.

Types of diarrhea

■ Chronic diarrhea is defined as diarrhea persisting for more than 4 weeks with diarrheal stool, with watery or loose consistency. This is most commonly caused by malabsorption or as a persistent symptom of inflammatory bowel disease (IBD).

■ Osmotic diarrheas have as causes: celiac sprue, bile salt deficiency, pancreatic insufficiency, lactase deficiency, intestinal lymphoma and Whipple’s disease. These are consistent with the presence of unabsorbed solutes, usually because the absorbing mucosa has been damaged.

■ Secretory diarrhea cases are most often caused by toxins such as cholera, enterotoxigenic strains of E. coli or by secretagogues or endocrine tumors. This is also the type induced by laxative abuse. In most cases, after the cause is being treated, meaning that the intestinal mucosa reestablishes its function of proper absorption, the diarrhea resolves.

References

1) Shiau YF, Feldman GM, Resnick MA, Coff PM. (1985) Stool electrolyte and osmolality measurements in the evaluation of diarrheal disorders. Ann Intern Med; 102(6):773-5.

2) Steffer K, Santa Ana CA, Cole JA, Fordtran JS. (2012) The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea. Gastroenterol Clin North Am; 41(3):539-60.

3) Castro-Rodríguez JA, Salazar-Lindo E, León-Barúa R. (1997) Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements. Arch Dis Child; 77(3):201-5.