Osmolality

Definition

This test measures the osmolality (concentration of particles) of blood serum (the fluid portion of blood).

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to cause the veins to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For infants or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site, if there is any continued bleeding.

How to prepare for the test

Fast for 6 hours before the test.

For infants and children:

The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For general information on how you can prepare your child, see the following:

• Infant test or procedure preparation (birth to 1 year)
  
• Toddler test or procedure preparation (1 to 3 years)
  
• Preschooler test or procedure preparation (3 to 6 years)
  
• School age test or procedure preparation (6 to 12 years)
  
• Adolescent test or procedure preparation (12 to 18 years)
  

How the test will feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Osmolality measures the concentration of particles in solution. Osmolality increases with dehydration (loss of water without loss of solutes) and decreases with overhydration.