A urine specific gravity test compares the density of urine with the density of water. It allows doctors to see the concentration of all particles that are present in the urine.
The test involves collecting a urine sample that will help give a doctor an idea of a person's kidney function and hydration status.
A urine specific gravity test gives information about how well the kidneys concentrate the urine and a person's hydration status.
The primary function of the kidneys is to maintain a healthy fluid and electrolyte balance. The kidneys excrete excess water and minerals from the body in the form of urine. When the body needs more water, the kidneys retain fluid.
A doctor will use a urine specific gravity test if they suspect that a person is dehydrated, has impaired kidney function, or has a condition that can alter the body's fluid status, such as diabetes insipidus.
The test should not be uncomfortable and is usually brief. The doctor might request multiple urine specific gravity tests in one day to see how well the kidneys compensate for a possible problem.
A person may be asked to collect a "clean catch" specimen so as not to contaminate the sample with bacteria from the skin.
To collect a clean catch specimen, a person must clean the urethral area before urinating, then collect the urine sample midstream in a specimen cup.
The doctor will send the urine sample to a laboratory for testing. A laboratory technician performs the test using a refractometer, which projects light into the sample and helps determine the density of the urine.
Less commonly, a technician may use dipstick method. The dipstick will change color according to the content of the urine.
Preparation for a urine specific gravity test is typically simple and straightforward. A person may need to stop taking certain medications before the test, particularly those that contain sucrose.
People should avoid any foods that can change the color of the urine for a few days leading up to the test. These foods include:
- fava beans
People should also let their doctor know if they have recently received intravenous contrast dye for a medical test.
Although it varies slightly between different laboratories, the normal urine specific gravity values are between 1.002 and 1.035.
A doctor will explain a person's results to them, especially if the value is outside of the normal range.
Urine tends to be darker and has higher specific gravity when a person does not consume enough fluids. Urine is lighter and has lower specific gravity when a person is well-hydrated.
Urine specific gravity tests can give a more precise measurement of the urine's concentration than just looking at the color alone.
Causes of high and low readings
In cases of high specific gravity, a person may be mildly dehydrated and should drink more clear fluids.
In cases of low specific gravity, a person may be drinking too much fluid or have a condition that makes them thirsty.
Further testing is often needed to determine if a heart condition, kidney problem, or metabolic disorder is the cause of the abnormal result.
Low specific gravity suggests that urine is too diluted.
Conditions that cause low specific gravity include:
- diabetes insipidus
- kidney failure
- drinking too much fluid due to increased thirst
- damage to kidney tubular cells
High specific gravity suggests that the urine is too concentrated.
Conditions that cause high specific gravity include:
Excess substances in the urine can also cause high specific gravity.
- red or white blood cells
The doctor may perform additional urine testing, including urinalysis, urine culture, urine osmolality, urine pH, or ketone testing. Results from these other tests will help the doctor make an accurate diagnosis.
If a person has high specific gravity due to dehydration, then the doctor will probably advise them to drink more fluids.
If a person is severely dehydrated or unable to drink, doctors can provide intravenous fluids.
A urine specific gravity test is a straightforward test that looks at the concentration of a person's urine. The test is easy to do but may involve temporarily changing one's diet or stopping certain medications.
Urine concentration shows how well the kidneys are functioning or may point to an underlying condition that is altering the body's fluid status.
A person can usually treat mild dehydration by drinking more fluid. In severe cases, a person may require intravenous fluids.
In the case of overhydration, a doctor will recommend further testing to diagnose the cause and outline a treatment plan.