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Osmolality (Blood)

Does this test have other names?

Serum osmolality, osmolality serum

What is this test?

This test measures the concentration of dissolved particles (osmolality) in your blood.

This test can help diagnose a fluid or electrolyte imbalance, including dehydration. Electrolytes are mineral salts that help move nutrients into your cells and waste products out of your cells.

Electrolytes also control your acidity and pH levels. The more dilute your blood and urine are, the lower the concentration of particles. The less water in your blood, the greater the concentration of particles. Osmolality increases when you are dehydrated and decreases when you have a fluid buildup.

Your body has a unique way to control osmolality. When osmolality increases, it triggers your body to make antidiuretic hormone (ADH). Your kidneys then keep more water inside your body and your urine becomes more concentrated. When osmolality decreases, your body doesn't make as much ADH. Your blood and urine become more dilute.

Why do I need this test?

You may need this test if you have seizures or problems with ADH. You may also have this test if you are dehydrated or if your healthcare provider thinks you might have diabetes insipidus. Diabetes insipidus happens when your body makes less ADH. It's also called vasopressin. This condition can also happen if your kidneys are not responding to ADH, even though you are making enough of it. Signs and symptoms of diabetes insipidus include:

  • Frequent need to urinate

  • Very dilute urine

  • Dizziness when standing

  • Extreme thirst

  • Low blood pressure, which can lead to shock and organ failure

You might also have this test if you have symptoms of hyponatremia. Hyponatremia is a condition in which your body retains fluid because it doesn't have enough sodium, an electrolyte. Symptoms of hyponatremia include:

  • Nausea and vomiting

  • Headache

  • Mental confusion

  • Fatigue

  • Restlessness

  • Irritability

  • Muscle weakness

  • Spasms or cramps

  • Seizures or passing out

You may also have this test if you are in a coma. When osmolality increases, it can cause fatal grand mal seizures.

What other tests might I have along with this test?

Your healthcare provider may also order a urine osmolality test. The results of both urine osmolality and blood osmolality tests help figure out the cause of osmolality problems.

Your provider may also order:

  • Antidiuretic hormone test to measure the amount of ADH in your blood

  • Antidiuretic hormone suppression test to look at problems with ADH

What do my test results mean?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

Results are given in milliosmoles per kilogram (mOsm/kg). Normal results are:

  • 275 to 295 mOsm/kg for adults and older adults

  • 275 to 290 mOsm/kg for children

If your levels are higher or lower, it may mean you have one of these conditions:

  • Hyponatremia

  • Hypernatremia

  • Hyperglycemia

  • Diabetes insipidus

  • Kidney problems

A level that's beyond normal range may also be caused by blood loss, as from trauma, or prolonged vomiting or diarrhea.

How is this test done?

The test is done with a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Eating a poor diet or drinking too much water can affect your results. Intense exercise and being under stress can also affect your results. Certain medicines and the illicit drug ecstasy can also affect your results.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use. Also tell your provider if you have been drinking a lot of water. 

Medical Reviewers:

  • Fraser, Marianne, MSN, RN
  • Haldeman-Englert, Chad, MD