Osmolality - urine

Osmolality test

An osmolality urine test is performed to measure the concentration of particles in urine. Greater than normal results may indicate conditions such as Addison’s disease, congestive heart failure or shock. Lower-than-normal measurements may indicate aldosteronism, diabetes insipidus, excess fluid intake, renal tubular necrosis or severe pyelonephritis.

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 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:

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.

See also:

  • Test

    Osmolality - blood

    Osmality is a test that measures the concentration of all chemical particles found in the fluid part of blood.

  • Disease

    Pyelonephritis

    Tuberculosis in the kidney

    Pyelonephritis is an infection of the kidney and the ureters, the ducts that carry urine away from the kidney.

  • Test

    ADH

    ADH is a test that measures the amount of antidiuretic hormone (ADH) in blood. ADH is found in the body, and it may be given as a medication.

  • Injury

    Shock

    Shock

    Shock is a life-threatening condition that occurs when the body is not getting enough blood flow. This can damage multiple organs. Shock requires immediate medical treatment and can get worse very rapidly.

  • Disease

    Hyponatremia

    Hyponatremia is a metabolic condition in which there is not enough sodium (salt) in the body fluids outside the cells.

  • Disease

    Heart failure

    Heart, section through the middle

    Heart failure, also called congestive heart failure, is a condition in which the heart can no longer pump enough blood to the rest of the body.

  • Disease

    Acute tubular necrosis

    Kidney anatomy

    Acute tubular necrosis is a kidney disorder involving damage to the tubule cells of the kidneys, resulting in .

Review date:
August 7, 2009
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.
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