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
is the purpose of a urine osmolality test?
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
for the urine osmolality test
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
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.
do the results mean?
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,
- congestive heart failure
- high glucose
- acute kidney injury
In rare cases, high urine osmolality can occur due to
Several conditions may cause low urine osmolality,
- 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
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