Definition of Urine concentration test:

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

See also:

• Urine osmolality test
• Urine specific gravity

How the test is performed:

For this test, the specific gravity of urine is measured before and after one or more of the following:

• Water loading -- drinking excessive amounts of water or receiving fluids through a vein
• Water deprivation -- avoiding fluids for a certain amount of time
• ADH administration -- receiving antidiuretic hormone (ADH), which should cause the urine to become concentrated

The test requires a clean-catch urine sample.

To obtain a clean-catch sample, men or boys should clean the head of the penis. Women or girls need to wash the area between the lips of the vagina with soapy water and rinse well.

As you start to urinate, allow a small amount to fall into the toilet bowl to clear the urethra of contaminants. Then, put a clean container under your urine stream and catch 1 to 2 ounces of urine. Remove the container from the urine stream. Cap and mark the container and give it to the health care provider or assistant.

For infants, thoroughly wash the area around the urethra. Open a urine collection bag (a plastic bag with an adhesive paper on one end), and place it on the infant. For boys, the entire penis can be placed in the bag and the adhesive attached to the skin. For girls, the bag is placed over the labia. Diaper as usual over the secured bag.

This procedure may take a couple of attempts -- lively infants can displace the bag. The infant should be checked frequently and the bag changed after the infant has urinated into the bag. The urine is drained into the container for transport to the laboratory.

How to prepare for the test:

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

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test. Drugs that can affect measurements include dextran and sucrose, and x-ray contrast medium (dye) received within 3 days.

How the test will feel:

The test involves only normal urination, and 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 differentiate it from nephrogenic diabetes insipidus.

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