Blood Osmolality Test

Blood Osmolality Test- Why am I having this test?

Blood osmolality test is used to gain information about the balance of fluid and certain chemicals in your blood.

Your health care provider may ask you to have this test to determine if you have an electrolyte balance or abnormal fluid status. This test may also be used to help evaluate the following:

  • Not having enough water in your body (dehydration).
  • Medicines used in the treatment of seizures.
  • Swelling in the abdomen caused by fluid buildup in the abdomen (ascites).
  • Acid–base imbalance in your blood.
  • The balance of sodium levels in your body.
  • Problems with antidiuretic hormone (ADH) function in your body.
  • Kidney disease.
  • Possible poisoning.

What is being tested?

This test measures the balance between water and dissolved particles in the blood. These particles include minerals (electrolytes) such as salt (sodium) and sugar (glucose). This balance is needed because:

  • As the amount of water in the blood increases, the number of particles decreases. When this happens, your blood becomes less concentrated (diluted).
  • As the amount of water in the blood decreases, the number of dissolved particles increases. When this happens, your blood becomes more concentrated.

This fluid–particle balance is controlled by certain hormones in your blood, such as the antidiuretic hormone (ADH). ADH acts on your kidneys to control how much fluid is kept in your bloodstream and how much is lost through urination.

What kind of sample is taken?

A blood sample is required for this test. It is usually collected by inserting a needle into a blood vessel or by sticking a finger with a small needle.

How are the results reported?

Your test results will be reported as a value. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

  • Adult or elderly: 285–295 mOsm/kg water or 285–295 mmol/kg (SI units).
  • Child: 275–290 mOsm/kg water.

What do the results mean?

Results that are higher than the reference range may indicate a number of health conditions. These may include:

  • Higher-than-normal level of sodium in your blood (hypernatremia).
  • Higher-than-normal level of glucose in your blood (hyperglycemia).
  • Higher-than-normal level of calcium in your blood (hypercalcemia).
  • Very high levels of glucose in your blood without ketones in your urine (hyperosmolar hyperglycemic nonketotic syndrome, HHNS).
  • Excess ketones in your body resulting from the breakdown of fat (ketosis).
  • Higher-than-normal level of urea, creatinine, and other nitrogen compounds in your blood (azotemia).
  • Low levels of water in the body (dehydration).
  • Mannitol therapy for seizure disorders.
  • Buildup of urea in your blood due to decreased kidney function (uremia).
  • Diabetes insipidus.
  • Kidney disease.
  • Kidney infection.
  • Recent ingestion of or poisoning with:
    • Ethanol.
    • Methanol.
    • Antifreeze (ethylene glycol).

Results that are lower than the reference range may also indicate a number of health conditions. These may include:

  • Having too much water in your blood (overhydration).
  • Lower-than-normal level of sodium in your blood (hyponatremia).
  • Syndrome of inappropriate ADH (SIADH) secretion.
  • Complication of some types of cancers.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

  • When will my results be ready?
  • How will I get my results?
  • What are my treatment options?
  • What other tests do I need?
  • What are my next steps?

Summary

  • The blood osmolality test is performed to gain information about the balance of fluid and certain chemicals in your blood.
  • This test measures the balance between water and dissolved particles in the blood, which is controlled by certain hormones in your body.
  • Results outside the normal range for this test may indicate a number of conditions and diseases, including dehydration, overhydration, diabetes, kidney disease, and ingestion of certain chemicals, such as methanol.
  • Talk with your health care provider about what your results mean.
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