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What is Urine Osmolality?

By H. Colledge
Updated May 17, 2024
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Urine osmolality is a measurement of the number of particles dissolved in a given weight of urine. It is effectively a urine concentration measure, and the result of a urine osmolality test is usually used, along with the results of other tests, to give a picture of the body's fluid balance and to investigate changes in the amount of urine produced. Urine osmolality can be used to help diagnose conditions such as heart failure, diabetes insipidus and diseases involving the kidneys.

A urine osmolality test involves taking what is known as a midstream urine sample. Before the test, the skin around the opening where urine leaves the body is carefully cleaned to avoid any contamination. The person begins urinating as normal, then places a clean container in the urine stream to collect a sample before urination ends. When investigating the body's fluid balance, a serum osmolality test, which measures the concentration of substances dissolved in the blood, is often required as well. This test involves a health professional drawing a sample of blood from a vein.

Where the results of a urine osmolality test show that osmolality is high, meaning that the urine is more concentrated, this can be associated with either an increased or a decreased production of urine. A greater output of urine, with high osmolality, indicates that something is being flushed out of the body. This can occur in the disease known as diabetes mellitus, where excessive amounts of a sugar called glucose are removed in the urine. If there is a high osmolality, but a low amount of urine, this could mean a person is suffering from dehydration, or that there is a reduced blood supply to the kidneys, perhaps as the result of congestive heart failure.

A lower than normal urine osmolality measurement may exist, together with a reduced output of urine, in diseases which affect the kidneys' ability to produce and concentrate urine. Low osmolality results may also be seen in cases of overhydration, caused by drinking too much water, where large amounts of dilute urine are produced. In a rare disease known as diabetes insipidus, urine osmolality is also typically low.

Diabetes insipidus is caused by problems with a hormone known as antidiuretic hormone, or ADH, which normally acts on the kidneys causing water to be conserved. Without the normal action of ADH, large amounts of urine are produced leading to dehydration, thirst, and high levels of sodium in the blood. Diabetes insipidus has more than one cause, and treatments vary accordingly, but the outlook is often positive .

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