A urine concentration test measures the ability of the kidneys to conserve or excrete water appropriately.
- Urine osmolality test
- Urine specific gravity
Water loading test; Water deprivation test
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).
In general, normal values are as follows:
- 1.0000 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
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
Increased urine concentration may indicate:
- Diarrhea that leads to dehydration
- Excessive sweating that leads to dehydration
- Heart failure (related to decreased blood flow to the kidneys)
- Renal arterial stenosis
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Water restriction
Decreased urine concentration may indicate:
- Excessive fluid intake
- Diabetes insipidus
- Renal failure (loss of ability to reabsorb water)
Additional conditions under which the test may be performed:
- Complicated UTI (pyelonephritis)
- Nephrogenic diabetes insipidus
What the risks are
Landry DW, Bazari H. Approach to the patient with renal disease. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 116.
Shorecki K, Ausiello D. Disorders of sodium and water homeostasis. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 118.