Osmolality measures the concentration of all particles in a fluid. For example, a 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. The compounds typically present include:
In some cases, your doctor may choose to also order a blood (serum) osmolality test.
Your doctor may order a urine osmolality test if he or she wants to check your body’s water balance. Your doctor may also order the test if you are experiencing:
- abnormal blood sodium level (high or low)
- complicated urinary tract infection
- excessive urination
The test can also be useful in assessing the function of your kidneys.
Eat a balanced diet in the days leading up to the test. In some cases, your doctor may instruct 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 are taking.
Tell your doctor if you have had an X-ray involving dye/contrast medium in the days before the test. This can also interfere with your results.
The test requires a clean-catch urine culture. To do this, women will clean the labia and urethra and men will clean the head of the penis. You will then urinate briefly into the toilet. Stop the flow of urine momentarily and position the sterile cup you were given. Begin urinating again, catching the flow in the cup until it is about half full.
Urine osmolality is measured in milliosmoles per kilogram. A normal result is typically 500 to 850 milliosmoles per kilogram, but may go slightly higher or lower. If you have been placed on a fluid restriction for 12 to 14 hours before the test, your results should be over 850 milliosmoles per kilogram.
The exact standards for normal results may vary depending on your doctor and lab, so be sure to talk to your doctor if you have any concerns.
High urine osmolality may be caused by several conditions, including:
- congestive heart failure
- renal artery stenosis
In rare cases, high urine osmolality can be caused by Addison’s disease.
Low urine osmolality may also be caused by several conditions, including:
- excess fluid intake/overhydration
- kidney failure
- pyelonephritis (severe)
- renal tubular necrosis
Rarely, low osmolality can be caused by diabetes insipidus or aldosteronism.
There are several causes for both high and low urine osmolality. Some of these, such as dehydration, are relatively easy to deal with, while others can be more serious or ongoing. Your doctor will work with you to figure out what is causing your abnormal results.