A urine sample is a painless way for your doctor to evaluate your health and test for abnormalities. One thing your doctor may check for in your urine sample test (urinalysis) is specific gravity.
A urine specific gravity test 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’s too concentrated may indicate that your kidneys aren’t functioning properly, or simply that you aren’t 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’re dehydrated, or your body starts releasing nutrients it shouldn’t, such as glucose. This is where specific gravity testing can help your doctor make a diagnosis.
Specific gravity testing is useful if your doctor suspects any of the following conditions:
- heart failure
- diabetes insipidus (a condition marked by excessive thirst, and the excretion of large amounts of diluted urine)
- kidney failure
- kidney infection
- urinary tract infection
- hyponatremia/hypernatremia (low/elevated sodium levels)
You may have to complete a urine specific gravity test several times in one day. This allows your doctor to see how well your kidneys are filtering.
A valid sample for urine specific gravity testing contains at least 1 to 2 ounces of urine. The best time to obtain the sample is first thing in the morning, when your urine is the most concentrated.
Your doctor will give you a cup to collect a urine sample. For the best sample, you should use an antibacterial wipe to clean the area around your urethra. This will reduce the likelihood that bacteria will contaminate the sample.
Urinate a small amount and then place the cup under your urine stream. Urinate into the cup until you have a large enough sample. (You can finish urinating into the toilet.) This is known as the clean-catch or mid-stream method.
Your doctor likely will send the urine sample to a laboratory. Testing the sample while it’s fresh ensures best results. The lab will use a refractometer to project light into the sample and determine its density. This is more reliable than the dipstick method, in which a stick placed in the urine measures how much it sinks or floats.
To understand urine concentrations, think about the dark color of your urine when you haven’t had anything to drink in some time. Your urine is lighter and usually has lower specific gravity when you’re well hydrated. Urine specific gravity is a more precise measurement of your urine’s overall concentration than the color of your urine alone.
Your doctor will look at 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 will fall between 1.002 and 1.030 if your kidneys are functioning at a normal level. Specific gravity results above 1.010 can indicate mild dehydration. The higher the number, the more dehydrated you may be.
High urine specific gravity can indicate that you have extra substances in your urine, such as:
- red blood cells
- white blood cells
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 a diagnosis. Abnormal specific gravity results could indicate:
- excess substances in the blood
- kidney disease
- infection such as a bacterial urinary tract infection
A urinalysis can also measure 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 isn’t associated with any harmful side effects. However, if you have a urinary tract infection, urinating may cause a burning or painful sensation. Always notify your doctor if you experience discomfort urinating or any unexpected side effects.