Urine specific gravity; Urine s.g.
Urine specific gravity measures the concentration of particles in the urine.
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 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.
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.
How to prepare for the test
The health care provider may advise you to withhold drugs that can affect the test (see “special considerations”).
Eat a normal, balanced diet for several days before 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
Specific gravity measures the concentration of particles in a solution (grams/ml). Osmolality is a more exact measurement of urine concentration than specific gravity because specific gravity depends on the precise nature of the molecules present in the urine.
Specific gravity also requires correction for the presence of glucose or protein. But the specific gravity measurement is easier and more convenient to test. It frequently makes the osmolality measurement unnecessary.
Normal values are between 1.002 to 1.028. Normal value ranges may vary slightly among different laboratories.
What abnormal results mean
Abnormal results indicate the following:
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 - central
- Diabetes insipidus - nephrogenic
- Renal failure (that is, loss of ability to reabsorb water)
Additional conditions under which the test may be performed:
- Acute tubular necrosis
- Complicated UTI (pyelonephritis)
- Hepatorenal syndrome
- Interstitial nephritis
What the risks are
There are no risks.
Drugs that can increase specific gravity measurements include dextran and sucrose, and radiographic contrast medium within 3 days.
by Simon D. Mitin, M.D.