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

Collect a "clean-catch" (midstream) urine sample.

To obtain a clean-catch urine sample, boys and men should wipe the head of the penis clean. Girls and women need to wipe between the vagina "lips" (labia) with soapy water and rinse well. Your doctor may give you a special clean-catch kit that contains a cleansing solution and sterile wipes.

Urinate a small amount into the toilet bowl to clear the urethra of any contaminants. Then, collect a sample of urine in a clean or sterile container. About 1 - 2 ounces of urine is needed for a test. Remove the container from the urine stream without stopping the flow. You may finish urinating into the toilet bowl. Take the sample to the lab.

For infants, the genital area is cleaned and dried, and then a collection device is attached to collect the urine. If you are asked to collect the urine, be sure the collection device is attached securely to prevent leakage. After your baby has urinated, the urine (at least 20 cc) is placed in a sterile container.

No special preparation is necessary for this test, but if the collection is being taken from an infant, a couple of extra collection bags may be necessary.

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