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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:

A "clean-catch" (midstream) urine sample is needed. For information on how to collect the sample, see clean catch urine culture.

How to prepare for the test:

Your health care provider will tell you if you need to stop taking any drugs that may interfere with the test. Drugs that can increase specific gravity measurements include dextran and sucrose.

Receiving intravenous dye (contrast medium) for an x-ray exam up to 3 days before the test can also interfere with results.

Eat a normal, balanced diet for several days before the test.

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 1200 milliosmoles per kilogram (mOsm/kg)
  • 12 to 14 hour fluid restriction: Greater than 850 mOsm/kg

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. 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:

Lower-than-normal measurements may indicate:

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

Additional conditions under which the test may be performed:

  • Complicated UTI (pyelonephritis)
  • High blood sodium level
  • Low blood sodium level
  • Excessive urination
References:

Bazari H. Approach to the patient with renal disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 115.


Review Date: 8/21/2011
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. Copyright 2002 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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