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Osmolality - urine

Definition

The osmolality urine test the concentration of particles in urine. Osmolality (particles/kg water) and osmolarity (particles/liter of solution) are sometimes confused, but for dilute fluids such as urine they are essentially the same.

A blood test may also be done to measure osmolality. See: Serum osmolality

How the test is performed

Collect a "clean-catch" (midstream) urine sample.

To obtain a clean-catch urine sample, boys and men should wipe the head of the penis clean. Girls and women need to wipe between the vagina "lips" (labia) with soapy water and rinse well. Your doctor may give you a special clean-catch kit that contains a cleansing solution and sterile wipes.

Urinate a small amount into the toilet bowl to clear the urethra of any contaminants. Then, collect a sample of urine in a clean or sterile container. About 1 - 2 ounces of urine is needed for a test. Remove the container from the urine stream without stopping the flow. You may finish urinating into the toilet bowl. Take the sample to the lab.

For infants, the genital area is cleaned and dried, and then a collection device is attached to collect the urine. If you are asked to collect the urine, be sure the collection device is attached securely to prevent leakage. After your baby has urinated, the urine (at least 20 cc) is placed in a sterile container.

How to prepare for the test

No special preparation is necessary for this test, but if the collection is being taken from an infant, a couple of extra collection bags may be necessary.

How the test will feel

The test involves normal urination, and there is no discomfort.

Why the test is performed

This test helps evaluate your body's water balance and urine concentration.

Osmolality is a more exact measurement of urine concentration than the urine specific gravity test.

Normal Values

Normal values are as follows:

• Random specimen: 50 to 1400 milliosmoles per kilogram (mOsm/kg)
• 12 to 14 hour fluid restriction: Greater than 850 mOsm/kg

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Abnormal results are indicated as follows:

Greater-than-normal measurements may indicate:

• Addison's disease (rare)
• Congestive heart failure
• Shock
• Syndrome of inappropriate ADH secretion

Lower-than-normal measurements may indicate:

• Aldosteronism (very rare)
• Diabetes insipidus (rare)
• Excess fluid intake
• Renal tubular necrosis
• Severe pyelonephritis

Additional conditions under which the test may be performed:

• Complicated UTI (pyelonephritis)
• Dilutional hyponatremia (SIADH)

Review Date:10/22/2007
Reviewed By:Robert Mushnick, M.D., Clinical Assistant Professor, Department of Nephrology, SUNY Downstate Health Center, Brooklyn, NY. Review provided by VeriMed Healthcare Network.

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