Osmolality blood test

Definition

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

Osmolality in the urine can be measured with a urine test.

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

Follow any instructions from your health care provider about not eating before the test. Your provider 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. This soon goes away.

Why the Test is Performed

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

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 mOsm/kg (275 to 295 mmol/kg).

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

A higher than normal osmolality may be due to:

A lower than normal osmolality may be due to:

  • ADH oversecretion
  • Adrenal gland not working normally
  • Conditions linked to lung cancer (causing syndrome of inappropriate ADH production, or SIADH)
  • Other conditions or medicines that cause SIADH
  • Drinking too much water or fluid
  • Low sodium level (hyponatremia)
  • Underactive thyroid gland (hypothyroidism)

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Taking blood 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 lightheaded
  • Multiple punctures to locate veins
  • Blood accumulating under the skin (hematoma)
  • Infection (a slight risk any time the skin is broken)

References

Oh MS, Briefel G, Pincus MR. 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. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 15.

Verbalis JG. Disorders of water balance. In: Yu ASL, Chertow GM, Luyckx VA, Marsden PA, Skorecki K, Taal MW, eds. Brenner and Rector's The Kidney. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 15.

Related Links

Review Date: 8/20/2023
Reviewed By: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

adam.com