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Osmolality - blood test

Definition

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

How the Test is Performed

A blood sample is needed.

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 medicines that may interfere with test results. Such medicines may include water pills (diuretics).

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing or a slight bruise. These soon go away.

Why the Test is Performed

This test helps check your body's water balance. Your doctor may order this test if you have signs of any of the following:

  • Low sodium (hyponatremia) or water loss
  • Poisoning from harmful substances such as ethanol, methanol, or ethylene glycol
  • Problems producing urine

In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH).

This hormone causes the kidneys to reabsorb water. This results in more concentrated urine. The reabsorbed water dilutes the blood. This allows blood osmolality to fall back to normal.

Low blood osmolality suppresses ADH. This reduces how much water the kidneys reabsorb. Dilute urine is passed to get rid of the excess water, which increases blood osmolality back toward normal.

Normal Results

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.

What Abnormal Results Mean

Higher than normal levels may be due to:

Lower than normal levels may be due to:

  • ADH oversecretion
  • Adrenal gland not working very well
  • Conditions linked to lung cancer
  • Drinking too much water or dilute fluid
  • Low sodium level (hyponatremia)
  • Underactive thyroid gland

Risks

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

Oh MS. Evaluation of renal function, water, electrolytes, and acid-base balance. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 14.

Pincus MR, Abraham NZ Jr. Toxicology and therapeutic drug monitoring. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 23.


Review Date: 8/4/2013
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.
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