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.

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.

Normal Values

Normal values range from 280 to 303 mOsm/kg.

Note: mOsm/kg = milliosmoles per kilogram

What abnormal results mean

Higher-than-normal levels may indicate:

• Dehydration
• Diabetes insipidus
• Hyperglycemia
• Hypernatremia
• Consumption of methanol
• Consumption of ethylene glycol
• Renal tubular necrosis
• Stroke or head trauma resulting in deficient ADH secretion (cranial diabetes insipidus)
• Uremia

Lower-than-normal levels may indicate:

• Excess fluid intake
• Hyponatremia
• Overhydration
• Paraneoplastic syndromes associated with lung cancer
• Syndrome of inappropriate ADH secretion

What the risks are

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Special considerations

One interfering factor is mannitol (a diuretic).

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

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