Osmotic gap, osmolal gap 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.
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:
Kidney disease or disorder
Eaten a toxic substance
You may also need this test if you have:
Severe, prolonged vomiting
High or low blood sodium
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.
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 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:
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:
Glomerulonephritis, a type of kidney disease
Excessive water intake
This test requires a urine sample.
This test poses no known risks.
A high-protein diet could increase your osmolality levels. Drinking large amounts of water could lower them.
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.