What is a urine osmolality test?
Osmolality is the concentration of all particles in a fluid. For example, thick, sugary syrup has significantly higher osmolality than a cup of water with just a pinch of sugar. The urine osmolality test measures the amount of several compounds in your urine. Some of these compounds in your urine can include:
It’s the job of the kidneys to block or allow certain particles into the urine to maintain a balance within the body. Your doctor can use the urine osmolality test to check the levels of water and particles in your urine. Urine osmolality is a marker for how well the kidneys are working. It also helps your doctor diagnose certain problems.
In most cases, a doctor may also order a blood, or serum, osmolality test. Levels of the following commonly affect blood osmolality:
- salt, or sodium, which is one of the most important electrolytes in blood and urine
Your doctor may order a urine osmolality test if they want to check the level of water in your body. They may also use it to test your ability to produce urine. Your doctor may also order the test if you’re experiencing:
- an abnormally high or low blood sodium level
- excessive urination
- excessive thirst
- seizures or coma
- chronic diarrhea
Your doctor can also use it to:
- assess the function of your kidneys
- help determine if your electrolyte balance is normal and if your kidneys are working normally
- monitor drug treatment
- check how effective treatment is for any conditions that might affect your osmolality
You should eat a balanced diet in the days leading up to the test. In some cases, your doctor may tell you to restrict fluids for 12 to 14 hours before the test.
Some medications, such as dextran and sucrose, can interfere with the results of the urine osmolality test. For this reason, you must tell your doctor about all of the medications you’re taking.
Tell your doctor if you’ve had an X-ray involving dye or contrast medium in the days before the test. Either of these can interfere with your results.
The test requires a clean-catch urine sample. If you’re a woman, you’ll need to clean your labia and urethra. If you’re a man, you’ll to clean the head of your penis. You’ll then urinate briefly into the toilet. Stop the flow of urine momentarily and position the sterile cup. Begin urinating again, catching the flow in the cup until it’s about half full. Seal the cup as directed to avoid contaminating it.
Urine osmolality is measured in milliosmoles per kilogram of water (mOsm/kg). A normal result is typically 500 to 850 mOsm/kg but may be slightly higher or lower. The exact standards for normal results may vary depending on your doctor and lab. Talk to your doctor if you have any concerns.
Several conditions can cause high urine osmolality, including:
- congestive heart failure
- high glucose
- acute kidney injury
In rare cases, high urine osmolality can occur due to Addison’s disease.
Several conditions may cause low urine osmolality, including:
- excessive fluid intake, or over-hydration
- kidney failure
- renal tubular necrosis
Rarely, diabetes insipidus or aldosteronism can cause low osmolality. Osmolality will fluctuate as the body responds and corrects any temporary water imbalances. The urine test can indicate an imbalance, but it can’t pinpoint the exact cause. Depending on your results, you may need additional tests.
Both high and low urine osmolality have several causes. Some of these, such as dehydration, are relatively easy to treat. Others can be more serious or ongoing. Increased urine output and a high osmolality may occur due to your body flushing out a substance, such as excess glucose if you have diabetes. Your doctor will work with you to figure out what’s causing your abnormal results.