Osmolality - urine

Definition
The osmolality urine test measures the osmolality (concentration of particles) of the urine.

How the test is performed

Child or adult:
Collect a “clean-catch” (midstream) urine sample. To obtain a clean-catch sample, men or boys should clean the head of the penis. Women or girls need to wash the area between the lips of the vagina with soapy water and rinse well. As you start to urinate, allow a small amount to fall into the toilet bowl to clear the urethra of contaminants. Then, put a clean container under your urine stream and catch 1 to 2 ounces of urine. Remove the container from the urine stream. Cap and mark the container and give it to the health care provider or assistant.

Infant:
Thoroughly wash the area around the urethra. Open a Urine collection bag (a plastic bag with an adhesive paper on one end), and place it on the infant. For males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, the bag is placed over the labia. Diaper as usual over the secured bag.

This procedure may take a couple of attempts - lively infants can displace the bag, causing the specimen to be absorbed by the diaper. The infant should be checked frequently and the bag changed after the infant has urinated into the bag. The urine is drained into the container for transport to the laboratory.

How to prepare for the test
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.

How the test will feel
The test involves normal urination, and there is no discomfort.

Why the test is performed

Osmolality measures the concentration of particles in a solution, in this case, 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.

Osmolality is a more exact measurement of urine concentration than specific gravity because specific gravity depends on the precise nature of the molecules present in the urine. Specific gravity also requires correction for the presence of glucose or protein.

Normal Values


Normal Values
are as follows (normal value ranges may vary slightly among different laboratories):

     
  • Random specimen: 50 to 1400 mOsm/kg  
  • 12 to 14 hour fluid restriction: greater than 850 mOsm/kg

Note: mOsm/kg = milliosmoles per kilogram

What abnormal results mean


Abnormal results are indicated as follows:

Greater-than-normal measurements may indicate:

Lower-than-normal measurements may indicate:

     
  • Aldosteronism (very rare)  
  • Diabetes insipidus (rare)  
  • Excess fluid intake  
  • Renal tubular necrosis  
  • Severe pyelonephritis

Additional conditions under which the test may be performed:

     
  • Complicated UTI (pyelonephritis)  
  • Dilutional hyponatremia (SIADH)

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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