What Is a Serum Osmolality Test?

Your blood is a little like a liquid chemistry set. Along with oxygen, it contains proteins, minerals, hormones, and a long list of chemicals. Your body usually does a good job balancing all these things.

But sometimes you can have too much of a mineral or chemical -- or too little. This can trigger reactions in your body, some of which can cause serious health problems.

If your doctor thinks you have such a chemical imbalance in your blood, she may recommend that you get a serum osmolality test.

“Osmolality” refers to the concentration of dissolved particles of chemicals and minerals -- such as sodium and other electrolytes -- in your serum. Higher osmolality means more particles in your serum. Lower osmolality means they’re more diluted.

A serum osmolality test is a way to check the fluid-to-particle balance in your body. It can help your doctor diagnose several possible conditions. You may also hear it called an “osmolality serum” test. Serum is the fluid in your veins and arteries minus the blood cells. So you will have blood taken anytime you get a “serum test.”

Why Would I Get One?

The main reason to get this test is if you’re showing signs of dehydrationor other problems related to your fluid levels. The main one is hyponatremia. This condition happens when your sodium levels are too low and your body starts retaining fluid.

Sodium is one of the major electrolytes in your bloodstream. (Others include magnesium and potassium.) Electrolytes are chemicals that help cells absorb nutrients and get rid of waste products, among other important functions. One of sodium’s other main jobs is to balance water levels inside cells and throughout your body.

You may also have a serum osmolality test if you have a problem with antidiuretic hormone (ADH). ADH helps your body retain water rather than losing it when you pee.When your body makes more ADH, your kidneys make less urine. Your urine then becomes very concentrated. A rise in osmolality causes your body to make more ADH. If your osmolality is lower, you’ll have less ADH.

Too much or too little ADH can cause health problems. One reaction to low sodium levels is a seizure. If you have seizures or noticeable changes in your urine or how often you pee, your doctor may recommend that you get a serum osmolality test.


How Is It Done?

You don’t need to do anything special before the test. But there are a couple of things your doctor will want to know:

  • All of the medicines and supplements you take
  • Whether you’ve been drinking a lot of water

Your doctor or another health care professional will take a small sample of blood from a vein in your arm. It takes only a few minutes and it shouldn’t hurt, apart from the needle prick. You’ll get a bandage on your skin afterward.

There is a slight chance of bleeding, bruising, or infection at the site where the blood is drawn. Some people get a little lightheaded during a blood test, but this is temporary.

What the Results Mean

You may get your serum osmolality test results back within an hour or possibly 24 hours.

Your results will be measured in milliosmoles per kilogram (mOsm/kg). What’s important for you to know are the numbers:

  • For adults, the normal result range is between 285 and 295.
  • For children, it’s between 275 and 290.

A result higher than the normal range could point to one of these conditions:

  • Hypernatremia (too much sodium)
  • Diabetes insipidus (the kidneys make too much urine)
  • Dehydration (low fluid levels throughout the body)
  • Uremia (too much urea and other waste products in the blood)
  • Hyperglycemia (high blood sugar)
  • Ingestion of a toxic substance (ethanol and other alcohols)
  • Diabetic ketoacidosis

A result lower than the normal range means you could have one of these conditions:

  • Hyponatremia (too little sodium)
  • Overhydration (too much fluid retained in the body)

Other Tests

Your doctor may order a urine osmolality test to check for the concentration of dissolved particles in your pee. The results of the urine and blood tests should help your doctor find the cause of the chemical imbalance.

You may also get an ADH blood test.

Once your doctor makes a diagnosis, she will recommend a treatment plan to help you restore balance to the chemicals, electrolytes, minerals, and other substances circulating in your bloodstream.

WebMD Medical Reference Reviewed by Sabrina Felson, MD on February 16, 2019



Palomar College: “Blood Components.”

University of New Mexico Comprehensive Cancer Center: “Electrolyte Imbalance.”

University of Rochester Medical Center: “Osmolality.”

Mayo Clinic: “Hyponatremia: Definition.”

National Heart, Lung, and Blood Institute: “What to Expect with Blood Tests.”

Beaumont Laboratory: “Osmolality, Blood.”

American Diabetes Association, “Hyperglycemia (High Blood Glucose).”

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