Water loading test; Urine concentration test
A urine concentration test measures the ability of the kidneys to conserve or excrete water appropriately. (See also urine specific gravity.)
How the test is performed
Collect a “clean-catch” (midstream) 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 males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, 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, causing the specimen to be absorbed by the diaper. 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.
For this test, the specific gravity of urine is measured before and after water loading (urine should become dilute), water deprivation (urine should become concentrated), or administration of ADH (anti-diuretic hormone, which should cause the urine to become concentrated).
Water loading means you drink excessive amounts of water or you receive intravenous solutions. Water deprivation means you do not drink fluids for a period of time.
How to prepare for the test
Eat a normal, balanced diet for several days before the test. Submit a urine sample to the laboratory. If contamination of the specimen (by vaginal discharge or bleeding) is likely, collect a clean-catch sample.
Instructions will be given for water loading or water deprivation. The health care provider may advise you to withhold drugs that can affect the test.
If the collection is being taken from an infant, a couple of extra collection bags may be necessary.
How the test will feel
The test involves only normal urination, and there is no discomfort.
Why the test is performed
Most often, a urine concentration test is performed when diabetes insipidus is suspected and to help differentiate it from nephrogenic diabetes insipidus.
SIADH (syndrome of inappropriate ADH ) - a condition where urine concentrations are excessively high (water is conserved) given the state of an individual’s degree of hydration - is another condition where urine concentration values are helpful for diagnostic and therapeutic purposes.
Normally, the kidneys regulate the concentration of urine by adjusting the excretion of water. Water excretion is regulated, in part, by ADH (anti-diuretic hormone).
Normal values are indicated as follows (normal value ranges may vary slightly among different laboratories):
- 1.010 to 1.025 (normal specific gravity)
- 1.001 after water loading
- More than 1.025 after water deprivation
- Concentrated (increased specific gravity) after ADH administration
What abnormal results mean
Increased urine specific gravity may indicate:
- Excessive sweating
- Heart failure (related to decreased blood flow to the kidneys)
- Renal arterial stenosis
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Water restriction
Decreased urine specific gravity 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
Drugs that can increase specific gravity measurements include dextran and sucrose, and radiographic contrast medium within 3 days.
by Dave R. Roger, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.