A blood osmolality test measures the blood for the concentration of certain dissolved particles. This test can show whether a person has an irregular balance of fluids in their body.

Osmolality tests measure the amount of certain substances in body fluids. As such, a blood osmolality test measures the blood. A health expert may also refer to a blood osmolality test as a serum or plasma osmolality test or a blood osmolarity test.

A doctor may recommend this test if a person is displaying symptoms of a fluid imbalance or certain types of poisoning.

In this article, we discuss what a blood osmolality test involves and why a person may require one.

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A blood osmolality test is a test that measures the concentration of particles in the blood. The term osmolality refers to a measure of how much one substance has dissolved in another substance. For example, very salty water will have a higher osmolality than water with limited salt.

The body is able to make adjustments to blood osmolality to help it function better. For example, a person may urinate frequently if their blood osmolality is too low. This is because urination will help the body remove excess water, which will raise the osmolality of the blood.

A blood osmolality test can help evaluate the body’s sodium and water balance and its ability to produce and concentrate urine. It can also help identify the presence of certain poisons. A doctor may request this test for people with:

  • a low serum sodium concentration
  • a high serum sodium concentration
  • unusually high urine output
  • unusually low urine output
  • excessive thirst

A doctor may request a blood osmolality test if a person displays symptoms that indicate a fluid imbalance. Typically, a medical professional may suggest this test to:

  • check the balance between water and certain chemicals in the blood
  • help diagnose dehydration
  • help diagnose overhydration
  • help diagnose diabetes insipidus
  • determine if a person has ingested a poison

A healthcare professional can provide any directions a person will need to follow before taking this test. Typically, they will ask a person questions about any medications or supplements they are taking.

Additionally, they will ask about a person’s diet, how much water they are drinking, stress levels, and about physical activity. This is because all these factors could affect test results.

To perform this test, a medical professional will need to collect a sample of blood. Depending on a person’s age, they will typically require 1–5 milliliters of blood. To do so, they will likely use a needle to collect the blood sample from a vein in the arm. The process of drawing blood from a vein is also known as venipuncture.

To begin, the medical professional will clean the area, then wrap an elastic band, or tourniquet, around the arm. This will cause the vein to swell, which will make it easier to collect blood. They will then insert a needle into the vein and draw a blood sample into a vial.

After collecting the sample, they will remove the needle and elastic band. They will also clean the injection site and bandage it if necessary. The technician will then label the sample and send it to a laboratory for testing.

Other tests

A doctor may also suggest a person undergoes a urine osmolality test. This is similar to a blood osmolality test, but instead, it measures the concentration of particles in the urine. Additionally, they may recommend tests to measure antidiuretic hormone (ADH). This hormone is responsible for regulating the balance of fluid and salt in the body.

Test results can vary depending on a number of factors, such as a person’s age, gender, health history, and the lab a doctor sends the sample to. A doctor will be able to explain what a person’s test results mean.

Typically, serum osmolality should range from 275–295 milliosmoles per kilogram (mOsm/kg). If a person has levels higher than this range, it could indicate:

Test results lower than this range could suggest:

While a venipuncture procedure is generally safe, it can involve some risks. These may include discomfort, minor bleeding, and bruising.

In rare cases, a person may experience more severe complications, such as:

  • fainting
  • infection at the site of the needle puncture
  • excessive bleeding
  • phlebitis, which is inflammation of a vein
  • hematoma, which is a blood clot under the skin

If a person feels dizzy during a venipuncture, they should tell the medical professional to help avoid injury. For most people, the benefit of a blood osmolality test outweighs these risks.

A blood osmolality test is a test that measures the concentration of certain substances in the blood. A doctor may order this test to help establish whether a person has a fluid imbalance.

To perform the test, a healthcare professional will draw a blood sample, which is then sent to a lab for testing. A doctor can then interpret the results to help determine if a person has a fluid imbalance.