Osmolality is a test that measures the concentration of all chemical particles found in the fluid part of blood.

A blood sample is needed. For information on how this is done, see: Venipuncture

Do not eat for 6 hours before the test. Your doctor may tell you to temporarily stop taking any drugs, such as Mannitol (a diuretic), that may interfere with test results.

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.

This test helps evaluate your body's water balance. Your doctor may order this test if you have signs of hyponatremia, water loss, or poisoning from harmful substances such as ethanol, methanol, or ethylene glycol. It may also be done if you have problems producing urine.

Osmolality increases with dehydration and decreases with overhydration.

In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH). This hormone causes your kidney to reabsorb water, which results in more concentrated urine. The reabsorbed water dilutes the blood, allowing the blood osmolality to fall back to normal.

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.

Normal values range from 275 to 295 milliosmoles per kilogram.

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

Higher than normal levels may be due to:

• Dehydration
• Diabetes insipidus
• Ethylene glycol poisoning
• Hyperglycemia
• Hypernatremia
• Methanol poisoning
• Stroke or head trauma resulting in deficient ADH secretion
• Uremia

Lower than normal levels may be due to:

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

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.

Other risks associated with having blood drawn are slight but may include:

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

Landry DW, Bazari H. Approach to the patient with renal disease. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 116.