Blood Osmolality Test (Serum Osmolality Test)

Written by Gretchen Holm | Published on June 1, 2012
Medically Reviewed by Jason Baker, MD on May 12, 2014

What Is a Blood Osmolality Test?

Osmolality is a measure of how much of one substance has been dissolved in another substance. The greater the concentration of substance dissolved, the higher the osmolality. Very salty water, for example, has higher osmolality than water with just a hint of salt.

The blood osmolality test, also known as a serum osmolality test, measures the amounts of several chemicals in your serum. Serum is the liquid part of your blood. The blood osmolality test identifies how much dissolved blood urea nitrogen, glucose, and sodium are in your serum.

When your body is functioning properly, it makes specific adjustments to maintain the correct osmolality. For example, you may need to urinate frequently if your blood osmolality is too low. Your body does this as a way of getting rid of excess water. This raises the osmolality of your blood.

All this test involves on your part is allowing a healthcare provider to take a blood sample.

Why Doctors Perform a Blood Osmolality Test

Your doctor may order a blood osmolality test to check your body’s salt/water balance. This can give more insight into possible medical conditions. For example, your doctor might order this test if he or she thinks that you might have:

  • dehydration
  • loss of water
  • low sodium (hyponatremia)
  • diabetes insipidus
  • poisoning from certain substances, such as ethanol, ethylene glycol, and methanol

Preparing for a Blood Osmolality Test

Your doctor will usually ask you to fast for six hours before your test.

Some drugs, such as mannitol, can interfere with the results of the blood osmolality test. For this reason, you must tell your doctor about all of the medications you are taking.

Normal Blood Osmolality Test Results

Blood osmolality is measured in milliosmoles per kilogram. A normal result is typically 275 to 303 milliosmoles per kilogram.

The exact standards for normal results may vary depending on your doctor and lab, so be sure to ask your doctor if you have any concerns.

What Do Abnormal Blood Osmolality Test Results Mean?

High blood osmolality may be caused by poisoning from:

  • ethanol
  • ethylene glycol
  • methanol

High blood osmolality can also be caused by several other conditions, including:

  • dehydration
  • diabetes insipidus
  • head trauma
  • hyperglycemia (high blood sugar)
  • hypernatremia (high sodium)
  • stroke
  • uremia (accumulation of toxins in the blood)

Low blood osmolality may also be caused by various conditions, including:

  • excess fluid intake/overhydration
  • hyponatremia (low sodium)
  • paraneoplastic syndromes (lung cancer)
  • syndrome of inappropriate ADH secretion (SIADH)

There are several causes for both high and low blood osmolality. Some of these causes are less serious than others. Your doctor will use the information from this test to identify the cause and the best course of treatment.

Risks of a Blood Osmolality Test

Slight risks associated with this test are present any time you have blood drawn. These include some fairly minor risks, such as:

  • lightheadedness
  • pain at the puncture site
  • slight bleeding
  • slight bruising

In rare cases, you might experience more serious symptoms, such as:

  • excessive bleeding
  • fainting
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • inflammation of the vein

Be sure to tell your doctor if you have any of these symptoms.

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