osmolality test

Osmolality is a test that measures the concentration of all chemical particles found in the fluid part of blood.

Osmolality can also be measured with a urine test.

What are osmolality tests?

Osmolality tests measure the amount of certain substances in blood, urine, or stool. These include glucose (sugar), urea (a waste product made in the liver), and several electrolytes, such as sodium, potassium, and chloride. Electrolytes are electrically charged minerals. They help control the amount of fluids in your body. The test can show whether you have an unhealthy balance of fluids in your body. An unhealthy fluid balance can be caused by many different conditions. These include excess salt intake, kidney disease, heart disease, and some types of poisoning.

Other names: serum osmolality, plasma osmolality urine osmolality, stool osmolality, osmotic gap

What are they used for?

Osmolality tests may be used for a variety of reasons. A blood osmolality test, also known as a serum osmolality test, is most often used to:

  • Check the balance between water and certain chemicals in the blood.
  • Find out if you have swallowed a poison such as antifreeze or rubbing alcohol
  • Help diagnose dehydration, a condition in which your body loses too much fluid
  • Help diagnose overhydration, a condition in which your body retains too much fluid
  • Help diagnose diabetes insipidus, a condition that affects the kidneys and can lead to dehydration

Sometimes blood plasma is also tested for osmolality. Serum and plasma are both parts of the blood. Plasma contains substances including blood cells and certain proteins. Serum is a clear fluid that does not contain these substances.

A urine osmolality test is often used along with a serum osmolality test to check the body’s fluid balance. The urine test may also be used to find out the reason for increased or decreased urination.

A stool osmolality test is most often used to find out the reason for chronic diarrhea that is not caused by a bacterial or parasitic infection.

Why do I need an osmolality test?

You may need a serum osmolality or urine osmolality test if you have symptoms of a fluid imbalance, diabetes insipidus, or certain types of poisoning.

Symptoms of a fluid imbalance and diabetes insipidus are similar and may include:

  • Excessive thirst (if dehydrated)
  • Nausea and vomiting
  • Headache
  • Confusion
  • Fatigue
  • Seizures

Symptoms of poisoning will be different depending on the type of the substance that was swallowed, but may include:

  • Nausea and vomiting
  • Convulsions, a condition that causes uncontrollable shaking of your muscles
  • Difficulty breathing
  • Slurred speech

You may also need a urine osmolality if you have trouble urinating or are urinating too much.

You may need a stool osmolality test if you have chronic diarrhea that can’t be explained by a bacterial or parasitic infection or another cause such as intestinal damage.