What is Urine Osmolality Test

Urine Osmolality test checks the balance of fluid and dissolved particles (electrolytes) in your urine. Osmolality refers to how many particles are in a fluid.

Your kidneys filter certain particles out of your blood, and then those particles leave your body through urine. Electrolytes that leave the body through urine include sugar (glucose), potassium, calcium, and salt (sodium). If there are too many or too few of these electrolytes in your urine, this may be a sign that your kidneys are not functioning properly.

You may have this test:

• If you have symptoms such as:
  • Excessive urination.
  • Excessive thirst.
  • High or low blood sodium levels.
  • Long-term (chronic) diarrhea.
  • Dehydration.
• To evaluate kidney function.
• To monitor your treatment if you are taking medicines that may affect electrolyte levels.

What is being tested?

This test measures urine osmolality, which is the concentration of particles in urine. Urine osmolality may be measured:

• Regardless of when you last ate (random urine osmolality).
• After you follow eating and drinking restrictions before the test (fasting urine osmolality).

When there is a small amount of particles in the urine, osmolality is low. When there are a lot of particles in urine, osmolality is high.

What kind of sample is taken?

A urine sample is required for this test.

How do I collect samples at home?

When collecting a urine sample at home, make sure you:

• Use supplies and instructions that you received from the lab.
• Collect urine in the cup that you received from the lab.
• Do not let any toilet paper or stool (feces) get into the cup.
• Refrigerate the sample until you can return it to the lab.
• Return the sample to the lab as instructed.

How do I prepare for this test?

For a random urine osmolality test, no specific preparation is needed.

For a fasting urine osmolality test, follow instructions from your health care provider about eating and drinking restrictions, which may include:

• Eat a high-protein diet starting 3 days before your test.
• Eat a dry meal the night before your test. This means not having liquid foods, like soup, and not drinking fluids with your meal.
• Do not drink any fluids, including water, starting the night before or the morning of your test.

Tell a health care provider about:

• Any allergies you have.
• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any blood disorders you have.
• Any surgeries you have had.
• Any medical conditions you have.
• Whether you are pregnant or may be pregnant.

How are the results reported?

Your test results will be reported as a value that indicates your urine osmolality. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

• Random urine osmolality: 50–1200 mOsm/kg H2O or 50–1200 mmol/kg, depending on fluid intake.
• Fasting urine osmolality: greater than 850 mOsm/kg H2O.

What do the results mean?

A result within the reference range is considered normal. This means that there is a normal concentration of particles in your urine.

Results that are higher than the reference range mean that there are too many particles in your urine. This may indicate:

• Problems with your antidiuretic hormone (ADH). ADH acts on your kidneys to control how much fluid is kept in your bloodstream or is lost through urination.
• Complications from cancer.
• Shock.
• Long-term scarring of the liver (cirrhosis).
• Heart failure.
• Elevated sodium levels.

Results that are lower than the reference range mean that there are too few particles in your urine. This may mean that you:

• Have diabetes insipidus. This is a rare condition that causes the body to produce more urine than normal. This is not related to type 1 or type 2 diabetes mellitus, which affect insulin and blood sugar.
• Drink too much fluid (excess fluid intake).
• Have kidney disease or infection.
• Have abnormal levels of some electrolytes, such as potassium or calcium, in your blood.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• The urine osmolality test checks the balance of fluid and dissolved particles (electrolytes) in your urine. Too many or too few electrolytes may indicate that your kidneys are not functioning properly.
• Various actions and conditions can lead to results that are higher or lower than the reference range.
• Talk with your health care provider about what your results mean.