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 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
• Dehydration
• Glycosuria
• Renal artery stenosis
• Shock
• Syndrome of inappropriate ADH secretion

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.