| Subject | Contents |
| Definition | A test that measures the ability of the kidneys to conserve or excrete water appropriately (see also Urine specific gravity ). |
| Alternative Names | Water loading test; Water deprivation test |
| How the test is performed | Child or adult: Collect a "clean-catch" (midstream) urine sample. To obtain a clean-catch sample, men or boys should wipe 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 (this clears the urethra of contaminants). Then, in a clean container, catch about 1 to 2 ounces of urine and remove the container from the urine stream. Give the container to the health care provider or assistant. Infant: 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 your 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. Place a diaper over the infant (bag and all). Check your baby frequently and remove the bag after the infant has urinated into it. For active infants, this procedure may take a couple of attempts -- lively infants can displace the bag, causing an inability to obtain the specimen. The urine is drained into a container for transport back to the health care provider. For the urine concentration test, the specific gravity (the concentration of particles in a solution) of urine is measured before (sometimes) and after water loading (urine should become dilute), water deprivation (urine should become concentrated), and/or administration of ADH (anti-diuretic hormone, a hormone that should cause the urine to become concentrated). Water loading involves drinking excessive amounts of water or infusion of a quantity of intravenous solutions. Water deprivation involves consuming no 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 the contamination of the specimen (by vaginal discharge or bleeding ) is likely, then 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 central diabetes insipidus 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 individuals 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 | Normal values are indicated as follows: 1.010 to 1.025 (normal specific gravity) 1.001 after water loading More than 1.025 after water deprivation Concentrated (increase in specific gravity) after ADH administration |
| What abnormal results mean | Abnormal results are indicated as follows: Increased urine specific gravity may indicate:DehydrationDiarrheaExcessive sweatingGlucosuriaHeart failure (related to decreased blood flow to the kidneys) ProteinuriaRenal arterial stenosisSyndrome of inappropriate antidiuretic hormone secretion ( SIADH )Vomiting Water restriction Decreased urine specific gravity may indicate: Excessive fluid intake Diabetes insipidus - centralRenal failure (i.e., loss of ability to re-absorb water) GlomerulonephritisPyelonephritis Additional conditions under which the test may be performed: Complicated UTI (pyelonephritis)Diabetes insipidus - nephrogenic |
| What the risks are | |
| Special considerations | Drugs that can increase specific gravity measurements include dextran and sucrose; and radiographic contrast medium within 3 days. |
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