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Osmolality (Urine)

Does this test have other names?

Osmotic gap, osmolal gap test

What is this test?

This test measures the concentration, or osmolality, of particles in your urine. It finds out whether your electrolyte balance is normal and whether your kidneys are working properly.

Why do I need this test?

You may have this test if your doctor needs to look at the concentration of your urine, as well as at your fluid and electrolyte balance. This may be necessary if your doctor suspects that you have:

  • Kidney disease or disorder

  • Eaten of a toxic substance

  • Diabetes insipidus

You may also need this test if you have:

  • Chronic diarrhea

  • Severe prolonged vomiting

  • Excessive urination

  • High or low blood sodium

  • Dehydration

What other tests might I have along with this test?

You may also have some of these tests:

  • Plasma electrolytes

  • Creatinine

  • Blood urea nitrogen, or BUN

  • Blood glucose test to rule out diabetes mellitus

  • Blood osmolality test

  • Blood calcium and albumin

You may some of these tests if the concentration of sodium in your blood is too high or too low.

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.

Results of this test are given in millimoles per kilogram (mmol/kg). A range of 50 to 1,200 mmol/kg is considered normal.

If your results are higher than normal, you may have one of these conditions:

  • Dehydration

  • Glycosuria, or too much sugar in your urine

  • Adrenal problems

  • Heart failure, if you also have low urine sodium

  • Liver cirrhosis, if you also have low urine sodium

  • High-protein diet

Results that are lower than normal could mean you have:

  • Diabetes insipidus

  • Kidney failure

  • Glomerulonephritis, a type of kidney disease

  • Excessive water intake

How is this test done?

This test requires a urine sample.

Does this test pose any risks?

This test poses no known risks.

What might affect my test results?

A high-protein diet could increase your osmolality levels. Drinking large amounts of water could lower them.

How do I get ready for this test?

You don't need to prepare for this test.