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Osmolality - blood

Definition

Osmality is a test that measures the concentration of all chemical particles found in the fluid part 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 site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

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.

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

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

Normal values range from 280 to 303 milliosmoles per kilogram.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Higher than normal levels may indicate:

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

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)

References

Bazari H. Approach to the patient with renal disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 115.