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Osmolality - blood test
Osmolality is a test that measures the concentration of all chemical particles found in the fluid part of blood.
Osmolality can also be measured with a urine test.
How 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.
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 doctor may order this test if you have signs of any of the following:
-
Low sodium (
hyponatremia
) or water loss
Hyponatremia
Low sodium level is a condition in which the amount of sodium (salt) in the blood is lower than normal. The medical name of this condition is hypona...
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Poisoning from harmful substances such as
ethanol
,
methanol
, or
ethylene glycol
Ethanol
Ethanol poisoning is caused by drinking too much alcohol. This article is for information only. Do NOT use it to treat or manage an actual poison ex...
Read Article Now Book Mark ArticleMethanol
Methanol is a nondrinking type of alcohol used for industrial and automotive purposes. This article discusses poisoning from an overdose of methanol...
Read Article Now Book Mark ArticleEthylene glycol
Ethylene glycol is a colorless, odorless, sweet-tasting chemical. It is poisonous if swallowed. Ethylene glycol may be swallowed accidentally, or it...
Read Article Now Book Mark Article - Problems producing urine
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.
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.
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 level may be due to:
-
Diabetes insipidus
Diabetes insipidus
Diabetes insipidus is an uncommon condition in which the kidneys are unable to prevent the excretion of water.
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High blood sugar level (
hyperglycemia
)
Hyperglycemia
A blood glucose 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 t...
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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.
Read Article Now Book Mark Article - High sodium level (hypernatremia)
-
Stroke
or head trauma resulting in decreased ADH secretion
Stroke
A stroke occurs when blood flow to a part of the brain stops. A stroke is sometimes called a "brain attack. " If blood flow is cut off for longer th...
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Water loss (
dehydration
)
Dehydration
Dehydration occurs when your body does not have as much water and fluids as it should. Dehydration can be mild, moderate, or severe, based on how muc...
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Lower than normal levels may be due to:
- ADH oversecretion
- Adrenal gland not working normally
-
Conditions linked to
lung cancer
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 Article - Drinking too much water or fluid
- Low sodium level (hyponatremia)
-
Underactive thyroid gland (
hypothyroidism
)
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. This condition is often called underactive thyroid....
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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 lightheaded
- 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: 8/29/2015
Reviewed By: Laura J. Martin, MD, MPH, ABIM Board Certified in Internal Medicine and Hospice and Palliative Medicine, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Oglvie, PhD, and the A.D.A.M. Editorial team.
