A urine sample is a painless way for your doctor to evaluate your health and test for abnormalities. When your doctor orders a urine sample test known as a urinalysis, one part of this testing may be for specific gravity.
A urine specific gravity test basically compares the density of urine to the density of water. This quick test can help determine how efficiently your kidneys are diluting your urine.
Urine that is too concentrated may indicate that your kidneys aren’t functioning properly, or that you may simply not be drinking enough water.
The main role of your kidneys is to determine how many substances, such as glucose and electrolytes (salts), should be in your blood. If you have too many, your kidneys will get rid of them through your urine. However, sometimes your kidneys don’t filter efficiently, you are dehydrated, or your body starts releasing nutrients it should not, like glucose. This is where specific gravity testing aids a physician in making a diagnosis.
Specific gravity testing can be used if your physician suspects any of the following conditions:
- heart failure
- diabetes insipidus (a condition in which the kidneys cannot hold onto, conserve, water)
- kidney failure
- kidney infection
- urinary tract infection
- hyponatremia/hypernatremia (low/elevated sodium level)
Sometimes your physician will order a specific gravity test be taken several times over the course of a day to determine how well your kidneys are filtering.
Obtaining a valid sample for specific gravity testing requires getting a sample of at least 1 to 2 oz. of urine. The best time to obtain the sample is first thing in the morning when your urine is the most concentrated.
You will likely be given a cup and an antibacterial wipe before going into a restroom. For the best sample, you should use the wipe to clean the area around the urethra to reduce the likelihood that bacteria will contaminate the sample.
Urinate a small amount and then place the cup under your urine stream (called mid-stream, clean-catch) to collect a sample large enough for testing. Once you’ve obtained enough urine for the sample, you can finish urinating into the toilet. .
Your medical provider will likely take the urine sample to a laboratory. The sample must be tested fairly quickly to ensure best results. The lab will use a special machine known as a refractometer that projects light into the sample to determine its density. This machine is considered more reliable than using a dipstick. In the dipstick method, a stick is placed in the urine and specific gravity is determined by how much it floats or sinks.
To understand urine concentrations, think about the color of your urine when you have not had anything to drink in some time. The urine is dark in color and usually the specific gravity is higher. On the other hand, when you are well-hydrated, your urine is lighter and the specific gravity is usually lower. The urine specific gravity measurement is meant to be more precise than determining your urine’s overall concentration by the color of your urine.
The actual number that your doctor will look at is the ratio of the density of your urine to the density of water. To put it another way, the specific density of water itself would be 1.000.
Ideally, urine specific gravity results fall between 1.002 and 1.030 if your kidneys are functioning at a normal level. However, if your specific gravity is above 1.010, this can indicate mild dehydration. The higher the number, the more dehydrated you may be.
If your urine specific gravity is high, this can indicate you may have extra substances in your urine. These include glucose, protein, bilirubin, red blood cells, white blood cells, crystals, and bacteria. If your urine specific gravity is higher than 1.035, the sample is either contaminated or very high in glucose.
Your doctor will use the results from your urine specific gravity test along with other urinalysis results to draw conclusions and possibly make a diagnosis. If your specific gravity results are abnormal, this could indicate some of the following conditions:
- excess substances in the blood
- kidney disease
- infection such as a bacterial urinary tract infection
Other aspects of a urinalysis include the measurements of the concentration of various cells — such as white blood cells, which can indicate an infection, or glucose, which can point to glucose intolerance or diabetes.
The urine specific gravity test involves urinating normally and is not associated with any harmful side effects. However, if you are experiencing painful urination due to a urinary tract infection, urinating may cause a burning or discomforting sensation. Always notify your doctor if you experience any discomfort on urination or any unexpected side effects.