Urine Osmolality Test

Alternate Names
urine osmolality

Definition of Urine Osmolality Test

A urine osmolality test is a measure of the concentration of dissolved minerals (like sodium or salt) and wastes (like BUN) in a person's urine relative to water content. Higher mineral content and lower amounts of water lead to a high osmolality. Osmolality also provides information about the body's ability to excrete certain materials (low excretion means kidney damage) and balance fluids like water (when someone drinks a lot of water, there is more in the urine and therefore a lower osmolality).

Who is a candidate for the test?

A urine osmolality test is done when either a water balance or kidney problem is suspected.

How is the test performed?

A urine sample—usually either a first morning sample or a sample at a time determined in advance by the healthcare provider-- is required for this test.

To collect a specimen, a person should:

• start urinating into the toilet
• move the container to catch a sample of urine (about 2 "shot glasses" full or about 60ccs)
• take away the container and finish urinating into the toilet

The container should be covered and brought to the healthcare provider or lab for testing. Sometimes the test itself will be supervised in the hospital.

What is involved in preparation for the test?

Since test preparations vary, a person should ask a healthcare provider for specific instructions. First morning samples often are collected after a fast (no food or water) of 8-12 hours or so.

What do the test results mean?

Urine osmolality is dependent on 3 things:

• kidney function
• water intake or lack thereof
• the level of hormones released from the pituitary gland that help reabsorb water in the kidney (called Antidiuretic Hormone or ADH).

If kidney function is normal, a person fasts for about 8 hours, and ADH is working, a maximum urine osmolality is 1200.

Older people gradually lose some kidney function with aging so they decrease to about 800-1000.

Osmolality is interpreted according to the body's water balance: when water is plentiful omolality should be low (minimum of about 60) and when it is not, osmolality is maximized (1200).

Inapproprite low osmolality is the abnormality most found and may be a sign of:

• diabetes insipidus, a disease which causes increased thirst and urination as a result excess water loss in the urine. It is the result of either less/no ADH or a kidney problem that limits response to ADH.
• excessive fluid intake, which occurs when a person drinks too much fluid, especially inappropriately not in response to thirst.