Osmolality blood test
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.
Urine test
The osmolality urine test measures the concentration of chemicals in urine. Osmolality in the blood can be measured using a blood test.
Read Article Now Book Mark ArticleHow the Test is Performed
Blood sample is needed
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
Read Article Now Book Mark ArticleHow 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:
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Low sodium (hyponatremia) or water loss
Low sodium
Low blood sodium is a condition in which the sodium level in the blood is lower than normal. The medical name of this condition is hyponatremia....
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Ethanol
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Ethylene glycol is a colorless, odorless, sweet-tasting chemical. It is poisonous if swallowed. Ethylene glycol may be swallowed accidentally, or it...
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- Excessive thirst
- Excessive urination
In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH).
Antidiuretic hormone
Antidiuretic blood test measures the level of antidiuretic hormone (ADH) in blood.
Read Article Now Book Mark ArticleThis 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:
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Diabetes insipidus
Diabetes insipidus
Diabetes insipidus (DI) is an uncommon condition in which the kidneys are unable to prevent the excretion of water. DI is not the same as diabetes me...
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High blood sugar level (hyperglycemia)
High blood sugar level
A blood sugar test measures the amount of a sugar called glucose in a sample of your blood. Glucose is a major source of energy for most cells of the...
Read Article Now Book Mark Article - High level of nitrogen waste products in the blood (uremia)
Uremia
Prerenal azotemia is an abnormally high level of nitrogen waste products in the blood.
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Stroke or head trauma resulting in decreased ADH secretion
Stroke
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Water loss (dehydration)
Water loss
Dehydration occurs when your body does not have as much water and fluids as it needs. Dehydration can be mild, moderate, or severe, based on how much...
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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)
Lung cancer
Small cell lung cancer (SCLC) is a fast-growing type of lung cancer. It spreads much more quickly than non-small cell lung cancer. There are two typ...
Read Article Now Book Mark ArticleSIADH
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a condition in which the body makes too much antidiuretic hormone (ADH). ADH is ...
Read Article Now Book Mark Article - Other conditions or medicines that cause SIADH
- Drinking too much water or fluid
- Low sodium level (hyponatremia)
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Underactive thyroid gland (hypothyroidism)
Hypothyroidism
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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.
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Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
-
Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
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.