Urine concentration test

Definition

A urine concentration test measures the ability of the kidneys to conserve or excrete water.

Alternative Names

Water loading test; Water deprivation test

How the Test is Performed

For this test, the specific gravity of urine, urine electrolytes, and/or urine osmolality are measured before and after one or more of the following:

• Water loading. Drinking large amounts of water or receiving fluids through a vein.
• Water deprivation. Not drinking fluids for a certain amount of time.
• ADH administration. Receiving antidiuretic hormone (ADH), which should cause the urine to become concentrated.

After you provide a urine sample, it is tested right away. For urine specific gravity, the health care provider uses a dipstick made with a color-sensitive pad. The dipstick color changes and tells the provider the specific gravity of your urine. The dipstick test gives only a rough result. For a more accurate specific gravity result or measurement of urine electrolytes or osmolality, your provider will send your urine sample to a lab.

If needed, your provider will ask you to collect your urine at home over 24 hours. Your provider will tell you how to do this. Follow instructions exactly.

How to Prepare for the Test

Eat a normal, balanced diet for several days before the test. Your provider will give you instructions for water loading or water deprivation.

Your 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 provider.

Also tell your provider if you recently received intravenous dye (contrast medium) for an imaging test such as a CT or MRI scan. The dye can also affect test results.

How the Test will Feel

The test involves only normal urination. There is no discomfort.

Why the Test is Performed

This test is most often done if your doctor suspects central diabetes insipidus. The test can help tell that disease from nephrogenic diabetes insipidus.

This test may also be done if you have signs of syndrome of inappropriate ADH (SIADH).

Normal Results

In general, normal values for specific gravity are as follows:

• 1.005 to 1.030 (normal specific gravity)
• 1.001 after drinking excessive amounts of water
• More than 1.030 after avoiding fluids
• Concentrated after receiving ADH

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

Increased urine concentration may be due to different conditions, such as:

• Heart failure
• Loss of body fluids (dehydration) from diarrhea or excessive sweating
• Narrowing of the kidney artery (renal arterial stenosis)
• Sugar, or glucose, in the urine
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
• Vomiting

Decreased urine concentration may indicate:

• Diabetes insipidus
• Drinking too much fluid
• Kidney failure (loss of ability to reabsorb water)
• Severe kidney infection (pyelonephritis)

Risks

There are no risks with this test.

References

Fogazzi GB, Garigali G. Urinalysis. In: Feehally J, Floege J, Tonelli M, Johnson RJ, eds. Comprehensive Clinical Nephrology. 6th ed. Philadelphia, PA: Elsevier; 2019:chap 4.

Riley RS, McPherson RA. Basic examination of urine. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 28.