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Osmolality

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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.

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.

In healthy people, when osmolality in the blood becomes high, ADH (antidiuretic hormone) is secreted. ADH causes your kidney to reabsorb water. You then pass more concentrated urine. The reabsorbed water dilutes the blood, dropping blood osmolality back to normal.

Conversely, low blood osmolality suppresses ADH, reducing how much water the kidney reabsorbs. You pass dilute urine to get rid of the excess water, and blood osmolality increases.

Osmolality of the blood also changes if extraneous particles are added. Thus, if ethanol, methanol, ethylene glycol, or other compounds are ingested, the osmolality of the blood increases. Similarly, with diabetes, extra blood glucose increases blood osmolality.

• Review Date: 2/3/2006
• Reviewed By: Charles Silberberg, DO, Private Practice specializing in Nephrology, Affiliated with New York Medical College, Division of Nephrology, Valhalla, NY. Review provided by VeriMed Healthcare Network.

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