Serum and Urine Osmolality

The term osmolality refers to the osmotic concentration of a fluid. The osmolality of serum, urine, or any other body fluid depends on the number of active ions or molecules in a solution. In laboratory reports, osmolality is expressed as "so many" milliosmoles per kilogram of water (mOsm/kg water). With a standard measurement of osmoles and of milliosmoles for clinical studies, the precise concentration of active solutes in the serum and urine can be calculated. Tests of both serum and urine osmolality can yield important information about a patient's ability to maintain a normal fluid balance status.

Kidneys that are healthy will excrete water in relation to the amount the person consumes. Patients with impaired renal function may not be able to concentrate urine. As a result, the urine osmolality will resemble that of plasma, approximately 290mOsm/Kg. A urine osmolality test may be done on an early morning urine sample as water depletion during the night should concentrate the urine. The test may also be done using multiple timed samples or on a cumulative sample collected over a 24 hour period.

Sodium, blood urea nitrogen, and blood glucose levels are major factors in determining serum osmolality. In severe dehydration serum osmolality will be increased, as there is less water in proportion to solutes in the serum or blood. Urine osmolality, like specific gravity, is a measurement of the concentration of urine. Urine osmolality reflects the total number of osmotically active particles in the urine, without regard to the size or weight of the particles. Substances such as glucose, proteins, or dyes increase the urinary specific gravity. Therefore, urine osmolality is a more accurate measurement of urine concentration than specific gravity, and urine osmolality can be compared with the serum osmolality to obtain an accurate picture of a patient's fluid balance.

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Expected values for osmolality:

• Serum osmolality: 282 - 295 mOsm/kg water; a serum osmolality of 285 mOsm correlates with a urine specific gravity of 1.010
• Urine osmolality: extreme range of 50 - 1400 mOsm/kg water, but average is about 500 - 800 mOsm. After an overnight fast, the urine osmolality should be at least 3 times the serum osmolality

With restricted fluid intake, urine osmolality should be greater than 800mOsm/Kg. A 24 hour urine osmolality should average between 500 and 800 mOsm/Kg. A random urine osmolality should average 300 and 900 mOsm/Kg.

Increased serum and urine osmolality (hyperosmolality) levels are seen in:

• Renal disease
• Congestive heart failure
• Addison's disease
• Dehydration
• Diabetes insipidus
• Hypercalcemia
• Diabetes mellitus/hyperglycemia
• Hypernatremia
• Alcohol ingestion
• Mannitol therapy
• Azotemia

Decreased serum and urine osmolality (hypoosmolality) levels are seen in:

• Sodium loss due to diuretic use and a low salt diet
• Hyponatremia
• Adrenocortical insufficiency
• SIADH
• Excessive water replacement/overhydration/water intoxication

Panic values for serum osmolality are values of less than 240 mOsm or greater than 321 mOsm. A serum of osmolality of 384 mOsm produces stupor. If the serum osmolality rises over 400 mOsm, the patient may have grand mal seizures. Values greater than 420 mOsm are fatal.

When the serum osmolality is normal or increased, the kidneys are conserving water. As the serum osmolality rises, the urine osmolality should also rise. The higher the number of millosmoles in the urine, the more concentrated the urine; this is the expected physiological response to dehydration

This table shows the relationship between serum and urine osmolality and the clinical significance of laboratory values.