Serum Osmolality Test

Medically Reviewed by Carmelita Swiner, MD on September 02, 2022
Written by WebMD Editorial Contributors
3 min read

A serum osmolality test looks for a chemical imbalance in your blood. You may also hear it called an osmolality serum test.

Serum is the fluid in your veins and arteries minus the blood cells. You will have blood taken anytime you get a serum 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 you have more particles in your serum. Lower osmolality means the particles are more diluted.

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.

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. If your doctor thinks you have such a chemical imbalance in your blood, they may recommend that you get this test.

Here are some reasons your health care provider may order a serum osmolality test:

  • You show signs of dehydration or other problems related to your fluid levels, like hyponatremia, which happens when your sodium levels are too low and your body starts retaining fluid.
  • 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.
  • You’ve had a seizure, because it is one thing your body may do when it has too little sodium.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.

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

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

It’s a blood test. 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 shouldn’t hurt, apart from the needle prick. You’ll get a bandage on your skin afterward.

  1. 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.

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)

Your doctor may suggest further testing including:

  • Urine osmolality test. This test checks 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 any chemical imbalance.
  • ADH blood test. This test checks the level of ADH hormone in your blood.

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