Osmolality is the amount of osmoles (written as Osm) of solute per kilogram of solvent (expressed as Osm/kg), frequently confused with osmolarity, which differs in that it measures the concentration of a solution in litres (expressed as Osm/L). So what? In relation to urine testing, the two terms are interchangeable; what is more important is to know what a urine osmolality test is, why it is taken, what a normal urine osmolality reading is and how abnormal results are treated.
A urine osmolality test, also known as a urine osmolarity test, measures the concentration of certain particles in your urine, most usually including, but not limited to, chloride, glucose, potassium, sodium and urea. In preparation for this procedure, you are advised to eat a balanced diet for a few days before the test; it is also not uncommon to be told that you must inhibit your fluid intake for approximately 12 hours directly before the test itself. It is also crucial to notify your doctor of any medications that you are currently taking, for some substances can interfere with your urine osmolality (dextran and sucrose, for example).
Such a test is useful for the diagnosis of a variety of conditions. It may be taken if you are frequently urinating in order to examine your body’s production of its antidiuretic hormone (ADH) which serves to prevent excessive urination and regulate your fluid levels. This may be a sign of diabetes insipidus which can be verified by taking a urine osmolality test. It can also be used to examine the levels of sodium in your blood, as well as analyse the effectiveness of your kidneys.
Conveyed as a reading of milliosmoles per kilogram (mOsm/kg), a normal urine osmolality is around 500-800 mOsm/kg of water. If you have restricted your fluid intake 12-14 hours prior to the test, it is expected that your urine osmolality should be above 850 mOsm/kg.
Abnormal urine osmolalitycan be an indicator of a number of problematic health conditions. Below is a list of some of the potential causes of an abnormal urine osmolality that is either higher or lower than the 500-800 mOsm/kg average, as well as information on how to treat each cause.