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Osmolality (Stool)

Does this test have other names?

Stool osmolality

What is this test?

This test measures the concentration, or osmolality, of certain particles in a sample of your watery stool. The amount of sodium, potassium, and other substances in your stool can affect its consistency. The test is used to find out why your stool isn't solid.

Acute diarrhea often clears up on its own within a few days. But it's considered chronic diarrhea when the loose, watery bowel movements last more than four weeks. A large number of disorders and medications can cause chronic diarrhea:

  • Infections from parasites, bacteria or virusesIntestinal diseases like Crohn's disease, ulcerative colitis, and celiac disease

  • Irritable bowel syndrome

  • Endocrine diseases like diabetes and thyroid disease

  • Weakened immune system from cancer or HIV/AIDS

  • Previous abdominal surgery or radiation to the abdomen

  • Allergies

  • Medications such as antibiotics, chemotherapy drugs, and laxatives

Some people – mostly women – secretly abuse laxatives. This can cause chronic, watery diarrhea. Watery diarrhea caused by laxative abuse is called factitious diarrhea. This condition may have a mental health component.

This test also is used to find out whether your diarrhea is osmotic or secretory. Osmotic diarrhea results from something drawing water into your bowel. Secretory diarrhea happens when your body releases water into the bowel when it shouldn't.

Why do I need this test?

You may need this test if you have chronic watery diarrhea for no apparent reason. This test may be helpful when the cause of diarrhea is unclear or if your doctor suspects that you are abusing laxatives.

What other tests might I have along with this test?

Your doctor may also order other tests to help determine the cause of your diarrhea. Which tests you have depend on your symptoms and what your health care provider suspects may be causing your diarrhea. These tests may include:

  • Complete blood count, or CBC, and differential

  • Thyroid function

  • Serum electrolytes

  • Total protein

  • Albumin

  • Stool occult blood

  • Stool test to look for the active ingredients in laxatives

  • Sigmoidoscopy or colonoscopy

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

If your stool osmolality is very low, you may have factitious diarrhea.

How is this test done?

This test requires a stool sample. You will be asked to give a random or a timed stool sample. A timed stool sample is collected over a period of time, such as 24, 48, or 72 hours.

Your doctor will instruct you how to collect a sample into a disposable specimen container with a lid. Do not collect fecal material from the toilet bowl or put toilet paper into the specimen container.

In some cases, your doctor may need to collect the sample using a rectal swab.

Does this test pose any risks?

This test poses no known risks. If a rectal swab is used to collect your sample, you may feel some pressure or discomfort when it is inserted into your rectum.

What might affect my test results?

A sample contaminated with urine or toilet paper will not be accurate. Certain medications can also affect your results.

How do I prepare for this test?

If your doctor suspects that a certain food is causing your diarrhea, you may need to fast before the test. In addition, be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.