Urine osmolality: measuring how concentrated your urine is

Osmolality is the number of osmotically active particles dissolved in a kilogram of water. Lab Tests Online UK explains that it determines how water moves across membranes, and that urine osmolality is a measure of the kidney's ability to concentrate urine: the more concentrated the urine, the higher its osmolality, which is largely due to the presence of urea and creatinine.

Osmolarity, the number of particles per litre, is almost the same for clinical purposes and the terms are used interchangeably. Osmolality is measured directly in the laboratory with an osmometer, often by freezing point depression.

Blood osmolality is tightly regulated. When it rises, for example in dehydration, sensors in the hypothalamus promote secretion of antidiuretic hormone (ADH) from the pituitary gland, which signals the kidneys to reabsorb and conserve water, producing concentrated urine with high osmolality, while thirst increases water intake. When osmolality falls, for example after a large drink of water, ADH secretion is suppressed and the kidneys make large amounts of dilute urine with low osmolality.

If serum and urine osmolality are not in keeping with each other, this may indicate a problem with water balance that can show up as abnormal sodium results.

Serum and urine osmolality may be tested together in patients with a low serum sodium, a high serum sodium, an unusually high or low urine output, or excessive thirst. Lab Tests Online UK says serum osmolality is also measured when ingestion of toxic alcohols such as methanol and ethylene glycol is suspected, using the difference between measured and calculated osmolality. Very rarely stool osmolality is measured in diarrhoea.

A blood sample from a vein and a spot urine taken at the same time usually helps interpretation. Follow instructions provided, such as paired serum and urine samples before low sodium is treated, a first morning urine, or fluid restriction with timed sampling for a deprivation test. Tell your clinician about medications such as mannitol.

High plasma osmolality may be seen in dehydration with reduced urine output, in uncontrolled diabetes with high glucose, after ingesting toxins including alcohol, and in diabetes insipidus. Low plasma osmolality may be seen in water intoxication or disorders that cause the kidneys to retain water, such as the syndrome of inappropriate antidiuresis (SIAD), in which excess ADH causes a dilute serum with low sodium alongside inappropriately concentrated urine.

Urine osmolality gives information about whether the kidneys are responding properly. When blood sodium is low, the kidneys should excrete dilute urine with low osmolality. Large amounts of osmotically active substances such as glucose or mannitol in urine increase urine volume, termed osmotic diuresis, with water loss that can raise sodium and cause thirst.

Diabetes insipidus causes high urine output, above 3 litres a day, high sodium and thirst, due to too little ADH or failure of the kidneys to respond to it; high calcium or low potassium can also impair the kidney response. It is diagnosed with a water deprivation test in people with proven high urine volume and a spot urine osmolality that is inappropriately low for a high serum osmolality. Fluid is restricted and weight, serum and urine osmolality and sodium are measured frequently; failure to concentrate urine supports the diagnosis, and desmopressin may be given part-way to separate pituitary from kidney causes.

Lab Tests Online UK stresses that this test has risks and should only be performed carefully by specialist teams, so never attempt fluid restriction on your own.

Results guide further tests and treatment: fluid restriction or salt in SIAD, fluid replacement for dehydration, or desmopressin in diabetes insipidus. Seek urgent medical help for confusion, seizures, severe thirst with very large urine volumes, or collapse; call 999 or attend A&E.