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Osmolality - urine

Definition

The osmolality urine test measures the concentration of particles in urine.

Osmolality (particles per kilogram of water) and osmolarity (particles per liter of solution) are sometimes confused. But for dilute fluids such as urine, they are essentially the same.

Osmolality can also be measured using a blood test.

How the Test is Performed

A clean-catch urine sample is needed. The clean-catch method is used to prevent germs from the penis or vagina from getting into a urine sample. To collect your urine, the health care provider may give you a special clean-catch kit that contains a cleansing solution and sterile wipes. Follow instructions exactly so that the results are accurate.

How to Prepare for the Test

Your health care provider will ask you to temporarily stop any medicines that may affect the test results. Be sure to tell your provider about all the medicines you take, including dextran and sucrose. Do not stop taking any medicine before talking to your doctor.

Also tell your provider if you recently received intravenous dye (contrast medium) for an x-ray. The dye can also affect test results.

How the Test will Feel

The test involves normal urination. There is no discomfort.

Why the Test is Performed

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

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

Normal Results

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

Normal values vary from lab to lab. 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:

  • Heart failure
  • Loss of body fluids (dehydration)
  • Narrowing of the kidney artery (renal artery stenosis)
  • Shock
  • Sugar, or glucose, in the urine   
  • Syndrome of inappropriate ADH secretion (SIADH)

Lower-than-normal measurements may indicate:

  • Damage to kidney tubule cells (renal tubular necrosis
  • Drinking too much fluid
  • Kidney failure 
  • Severe kidney infection (pyelonephritis)

Risks

There are no risks with this test.

References

Gerber GS, Brendler CB. Evaluation of the urologic patient: history, physical examination, and urinalysis. In: Wein AJ, Kavoussi LR, Novick AC, et al., eds. Campbell-Walsh Urology. 10th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 3.

McPherson RA, Ben-Ezra J. Basic examination of urine. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 28.


Review Date: 8/25/2013
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.
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