Urine osmolality measures urine concentration and can help diagnose conditions such as heart failure, diabetes insipidus, and kidney diseases. A midstream urine sample is taken, and a serum osmolality test may also be required. High osmolality with increased urine output can indicate diabetes mellitus, while low osmolality can indicate kidney disease or overhydration. Diabetes insipidus is caused by problems with antidiuretic hormone and leads to dehydration and high blood sodium levels.
Urine osmolality is a measure of the number of particles dissolved in a given weight of urine. It is actually a measure of urine concentration, and the result of a urine osmolality test is usually used, along with the results of other tests, to provide a picture of the body’s fluid balance and to investigate changes in the amount of urine being 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 testing, the skin around the opening where urine leaves the body is thoroughly cleaned to avoid any contamination. The person begins to urinate normally, then inserts a clean container into the urine stream to collect a sample before urination is finished. When studying body fluid balance, a serum osmolality test, which measures the concentration of dissolved substances in the blood, is also often required. This test involves a health professional taking a blood sample from a vein.
Where the results of a urine osmolality test show that the osmolality is high, meaning that the urine is more concentrated, this may be associated with an increase or decrease in urine output. Increased urine output, with a high osmolality, indicates that something is being expelled from the body. This can occur in the disease known as diabetes mellitus, in which excessive amounts of a sugar called glucose are removed in the urine. If there is high osmolality, but a low amount of urine, this could mean that a person is suffering from dehydration or that there is reduced blood supply to the kidneys, perhaps due to congestive heart failure.
A lower-than-normal urinary osmolality measurement may exist, along with decreased urine output, in diseases that affect the kidneys’ ability to produce and concentrate urine. Results of low osmolality can also be observed in cases of overhydration, caused by drinking too much water, in which large quantities of diluted 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 retention. Without the normal action of ADH, large amounts of urine are produced leading to dehydration, thirst, and high blood sodium levels. Diabetes insipidus has more than one cause and treatments vary accordingly, but the outlook is often positive.
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