Does this test have other names?
Osmotic gap, osmolal gap test
What is this test?
This test measures the concentration (osmolality) of particles in your urine. It finds
out whether your electrolyte balance is normal and whether your kidneys are working
as they should.
Why do I need this test?
You may have this test if your healthcare provider needs to look at the concentration
of your urine, as well as at your fluid and electrolyte balance. This may be necessary
if your provider suspects that you have:
You may also need this test if you have:
What other tests might I have along with this test?
You may also have some of these tests:
Blood urea nitrogen, or BUN
Blood glucose test to rule out diabetes mellitus
Blood osmolality test
Blood calcium and albumin
You may need 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 healthcare
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:
Too much sugar in your urine (glycosuria)
Heart failure, if you also have low urine sodium
Liver cirrhosis, if you also have low urine sodium
Results that are lower than normal could mean you have:
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. But be sure your healthcare provider 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.