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Blood Osmolality Test

What is a blood osmolality test?

Osmolality is a measure of how much one substance has dissolved in another substance. The greater the concentration of the substance dissolved, the higher the osmolality. Very salty water has higher osmolality than water with just a hint of salt.

When your body is functioning properly, it makes specific adjustments to maintain an appropriate osmolality. For example, you may need to urinate frequently if your blood osmolality is too low. This helps your body get rid of excess water, raising the osmolality of your blood.

The blood osmolality test is also known as a serum osmolality test. Serum is the liquid part of your blood.

The serum test is used mainly to evaluate hyponatremia, a below normal level of sodium in the bloodstream.

Physicians may also use this test in consideration with the measured amounts of blood urea nitrogen, glucose, and sodium in your serum. Urea is a byproduct of protein breaking down in the body. 

Certain toxins and therapies that affect an individual’s fluid balance can also be evaluated with serum osmolality testing.

Both serum and urine osmolality tests may be evaluated together in order to compare and diagnose any diseases that influence osmolality in these areas.

All you need to do for this test is provide a sample of your blood.

Why do doctors perform a blood osmolality test?

Your doctor may order a blood osmolality test to check your body’s salt/water balance. This can help them determine if you have certain medical conditions. For example, your doctor might order this test if they suspect you have any of the following:

  • dehydration
  • hyponatremia, a deficiency of sodium in the bloodstream
  • an excess of sodium in the bloodstream
  • kidney damage
  • poisoning from certain substances, such as ethanol, ethylene glycol, or methanol
  • They can also use it to check for signs of several other conditions.

How should you prepare for a blood osmolality test?

To conduct a blood osmolality test, your doctor will collect a sample of your blood to send to a laboratory for testing.

They may ask you to fast for six hours before your blood is drawn. You may also need to avoid drinking certain liquids.

Your doctor may also ask you to avoid taking certain drugs before your blood is drawn. Some drugs, such as mannitol, can interfere with the test results.

It’s important to tell your doctor about any medications you’re taking, including prescription and over-the-counter drugs.

How will your blood be collected?

A trained medical professional will collect a sample of your blood at your doctor’s office or another site. They’ll use a needle to collect the blood, likely from a vein in your arm.

To start, they’ll clean the area with an antiseptic. Then they’ll wrap an elastic band around your arm, causing your vein to swell. A needle will be inserted into the vein and a sample of your blood will draw into a vial.

Once the blood is collected the needle and elastic band will be removed from your arm. The technician will then clean the injection site and, if needed, bandage it. Your blood sample will be labeled and sent to a laboratory for testing.

What do the test results mean?

The lab wills end the test results to your doctor. The results may be “normal” or “abnormal,” which your doctor will interpret for you.

Normal results

Blood osmolality is measured in milliosmoles per kilogram. A normal result is typically 275 to 295 milliosmoles per kilogram. The exact standards for normal results may vary, depending on your doctor and lab.

Abnormal results

Abnormal results typically fall outside the range of 275 to 295 milliosmoles per kilogram.

Abnormally high blood osmolality can result from a variety of conditions, including:

  • dehydration
  • diabetes insipidus
  • head trauma
  • stroke
  • hyperglycemia, or high blood sugar
  • hypernatremia, or high blood sodium
  • uremia, or an accumulation of toxins in your blood
  • poisoning from ethanol, ethylene glycol, or methanol

Abnormally low blood osmolality can be caused by several conditions, including:

  • excess fluid intake or over hydration
  • hyponatremia, or low blood sodium
  • paraneoplastic syndromes, a type of disorder that affects some people with cancer
  • syndrome of inappropriate ADH secretion (SIADH)

Some of these causes are less serious than others. Your doctor will use the results of your test to help develop a diagnosis. They may also order additional tests or exams.

What are the risks involved in a blood osmolality test?

Any blood draw involves some risks. These include such as lightheadedness or pain at the puncture site. You may also experience slight bleeding or bruising.

In rare cases, you might experience more serious complications, such as:

  • fainting
  • excessive bleeding
  • hematoma, an accumulation of blood under your skin
  • phlebitis, an inflammation of your vein
  • infection at the puncture site

If you suspect you’ve developed any serious side effects, contact your doctor. For most people, the benefits of this test outweigh the risks.

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