DefinitionOsmolality 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 PerformedHow to Prepare for the TestFollow 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 FeelWhen 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 PerformedThis 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 ResultsNormal 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 MeanA higher than normal osmolality may be due to:
A lower than normal osmolality may be due to:
RisksThere 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:
ReferencesOh 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. | |
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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. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. No warranty of any kind, either expressed or implied, is made as to the accuracy, reliability, timeliness, or correctness of any translations made by a third-party service of the information provided herein into any other language. © 1997- A.D.A.M., a business unit of Ebix, Inc. Any duplication or distribution of the information contained herein is strictly prohibited. | |
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Blood test
