Stool Osmotic Gap Calculator

Checks whether diarrhea is of a secretory or osmotic cause and provides the osmolar gap in mOsm/Kg.

You can read more about the variables and formula used and about the original study, in the text below the form.

This stool osmotic gap calculator retrieves the osmolar gap which is one of the determinations that look at ion concentration in the body.

It can provide information on the type of diarrhea the patient may be suffering from: osmotic or secretory.

The formula to calculate stool osmotic (osmolal gap) is:

Stoop osmotic = Stool osmolarity - 2 x (stool Na + stool K)

The normal fecal fluid values are: stool Na = 30 mEq/L, stool K = 75 mEq/L and stool osmolality/osmolarity = 290 - 300 mOsm/kg.

The table below shows the result interpretation:

Osmolarity Osmolar gap Type of diarrhea
<50 mOsm/Kg Low Secretory
50 – 100 mOsm/Kg Normal -
>100 mOsm/Kg High Osmotic

Stool sodium
Stool potassium
Stool osmolarity
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Stool osmolal gap formula

The above tool determines the nature of chronic diarrhea based on the stool osmotic gap. In order to find that, three variables are employed:

■ Stool sodium: normal value of 30 mEq/L.

■ Stool potassium: normal value of 75 mEq/L.

■ Stool osmolarity: this is not directly measured but given a constant in the range of 290 to 300 mOsm/kg.

Most of the osmotic activity of stool comes from electrolytes. Therefore, the stool osmotic gap that results is the difference between the measured osmolarity and the calculated osmolarity, which is basically the difference between the Na+ and K+ ions in stool.

The formula used is:

Stool osmotic gap = Stool osmolarity - 2 x (stool Na + stool K)

The following table introduces the possible results:

Osmolarity Osmolar gap Type of diarrhea
<50 mOsm/Kg Low Secretory
50 – 100 mOsm/Kg Normal -
>100 mOsm/Kg High Osmotic

In secretory diarrhea, the sodium and potassium ions make a big percentage of stool osmolarity.

In osmotic diarrhea on the other hand, the two types of ions makes up only a small percentage because there are other unabsorbed molecules, such as carbohydrates, which contribute to the osmotic activity.

All determinations (stool sodium and potassium) need to be done within 30 minutes of stool collection because bacterial metabolism may impact on the result as it produces osmotically active substances.

Urine anion gap analysis is another type of determination that evaluates ion concentrations in the body.


About the study

The study conducted by Eherer and Fordtran in 1992 was aimed at proving scientifically that measurement of fecal fluid osmotic gap and pH can distinguish between diarrhea causes.

During the study, normal subjects were induced diarrhea by different mechanisms:

■ Secretory diarrhea was induced by phenolphthalein;

■ Osmotic diarrhea was induced by sodium-sulphate.

The fecal fluid osmotic gap was measured in all cases.

A similar method was applied for patients with diarrhea by carbohydrate malabsorption (lactulose or sorbitol), where pH was measured.


Diarrhea types

■ Chronic diarrhea – is characterized by persisting (more than 4 weeks) diarrheal stools of watery or loose consistency. This is a symptom of inflammatory bowel disease (IBD) or caused by malabsorption.

■ Osmotic diarrhea – is characterized by the presence of unabsorbed solutes. Some of the causes include carbohydrate malabsorption, bile salt deficiency, pancreatic insufficiency, intestinal lymphoma or sorbitol ingestion.

■ Secretory diarrhea – can be resolved after the underlying cause is addressed and the intestinal mucosa re-establishes proper absorption. It is caused by Addison’s disease, stimulant laxatives, small intestine bacterial overgrowth or electrolyte transport disorders.


Original source

Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992; 103(2):545-51.

Other references

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

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

Specialty: Gastroenterology

System: Digestive

Objective: Diagnosis

Type: Calculator

No. Of Variables: 3

Year Of Study: 1992

Article By: Denise Nedea

Published On: May 30, 2017 · 07:52 AM

Last Checked: May 30, 2017

Next Review: May 30, 2023