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Osmolality Stool Test

Last updated May 9, 2019

Approved by: Maulik P. Purohit MD, MPH

The Osmolality Stool Test measures the osmolality of stool to determine abnormalities. It is often performed to detect alcohol poisoning and kidney disease and to determine the cause of diarrhea.

What are the other Names for this Test? (Equivalent Terms)

  • Osmo Stool Test
  • Stool Osmolality

What is Osmolality Stool Test? (Background Information)

  • Osmolality refers to the number of dissolved solute particles in a solvent, in this case stool. Osmolality is reported in osmoles (Osm) per kilogram of stool
  • Osmolality is similar to osmolarity, which measures the concentration of a compound in osmoles (Osm) per liter of total solution
  • An osmole is a quantity of dissolved particles that contributes to the osmotic pressure of a solution. The greater the difference in osmolarity between two sides of a semipermeable membrane, the greater the osmotic pressure
  • Osmotic pressure causes a cell placed in salt water to shrink. This is because the higher osmolarity outside the cell relative to the inside creates osmotic pressure that is directed towards the cell outside
  • Importantly, the cell’s membrane is semi-permeable. It allows water to pass, but does not allow other molecules that are charged, polar, or too large, to pass
  • Solutes in the body include electrolytes and non-electrolytes, such as glucose
  • Electrolytes are chemicals that dissociate in solution and conduct an electrical current. They are important to the body’s acid-base, electrical, and hydrostatic stability
  • Electrolytes are mainly acquired through the diet. Major electrolytes include sodium (Na+), potassium (K+), chloride (Cl-), bicarbonate (HCO3-), and phosphate (HPO42-)
  • Electrolytes are necessary because they allow cells to regulate the flow of water across their membranes. This is called osmotic regulation. Osmotic regulation protects cells from bursting due to an influx of water if their surroundings lack salt
  • When osmolality is measured, it is compared to an expected result that has been calculated. The difference is the osmotic gap. If the osmotic gap is too great, toxins or diseases may be present
  • The Osmolality Stool Test measures the osmolality of stool to determine abnormalities. It is often performed to detect alcohol poisoning and kidney disease and to determine the cause of diarrhea

What are the Clinical Indications for performing the Osmolality Stool Test?

Following are the clinical indications for performing an Osmolality Stool Test:

  • Acute diarrhea or chronic diarrhea: Generally, acute diarrhea gets better on its own within a few days. However, if it lasts more than 4 weeks, then it is considered chronic diarrhea
  • Excessive thirst
  • Altered mental status
  • Headache
  • Vision problems
  • Fatigue

Disorders causing chronic diarrhea for which Osmolality Stool Test may be performed include:

  • Bacterial infection
  • Viral infection
  • Parasitic infection
  • Diarrhea due to inflammatory bowel syndrome and inflammatory bowel disease, such as Crohn’s disease and ulcerative colitis

Other cases when the Osmolality Stool Test may be performed are:

  • Individuals with a weakened immune system, AIDS/HIV infection, cancer, use of chemotherapy agents, or allergic disorders such as celiac disease and other food allergies
  • Diarrhea due to antibiotic therapy
  • Past history of abdominal surgery
  • Past history of radiation therapy to abdomen
  • Laxatives abuse causing factitious diarrhea
  • Diabetes
  • Thyroid disorder

How is the Specimen Collected for Osmolality Stool Test?

Following is the specimen collection process for Osmolality Stool Test:

Sample required: Stool (collected using any of the following methods)


  • Random stool test; when the stool sample is collected using a sterile container, at any random time
  • Timed stool test; when the stool is collected using a sterile container and tested over a specific time interval such as 24, 48, or 72 hours

Preparation required:

  • No special preparation is needed prior to the test
  • However, the healthcare provider shall give specific instructions on how to collect the stool sample
  • It is important to note that the stool sample must not be collected from the toilet bowl

What is the Significance of the Osmolality Stool Test Result?

Decreased osmolality levels in the stool may indicate:

  • Factitious diarrhea that may be due to abuse of laxatives or decreased nutrient absorption in the GI tract (malabsorption)

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. 

Additional and Relevant Useful Information:

  • In general, the Osmolality Test measures the osmolality of blood, urine, or stool, to determine abnormalities
  • Certain factors interfere with the results of the Osmolality Test. These include hydration status, pregnancy, and consumption of electrolyte-rich foods

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: June 2, 2015
Last updated: May 9, 2019