The osmolality urine test the concentration of particles in urine. Osmolality (particles/kg water) and osmolarity (particles/liter of solution) are sometimes confused, but for dilute fluids such as urine they are essentially the same.

A blood test may also be done to measure osmolality. See: Serum osmolality

This test helps evaluate your body's water balance and urine concentration.

Osmolality is a more exact measurement of urine concentration than the urine specific gravity test.

A "clean-catch" (midstream) urine sample is needed. For information on how to collect the sample, see clean catch urine culture.

Your health care provider will tell you if you need to stop taking any drugs that may interfere with the test. Drugs that can increase specific gravity measurements include dextran and sucrose.

Receiving intravenous dye (contrast medium) for an x-ray exam up to 3 days before the test can also interfere with results.

Eat a normal, balanced diet for several days before the test.

The test involves normal urination, and there is no discomfort.