What are osmolality tests?

Osmolality tests measure the number 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 number 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. The 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.

What happens during an osmolality test?

During a blood test (serum osmolality or plasma osmolality):
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

During a urine osmolality test:
Your health care provider will need to collect a sample of your urine. You will receive a container to collect the urine and special instructions to make sure that the sample is sterile. These instructions are often called the "clean catch method." The clean catch method includes the following steps:

• Wash your hands.
• Clean your genital area with a cleansing pad given to you by your provider. Men should wipe the tip of their penis. Women should open their labia and clean from front to back.
• Start to urinate into the toilet.
• Move the collection container under your urine stream.
• Collect at least an ounce or two of urine into the container, which should have markings to indicate the amounts.
• Finish urinating into the toilet.
• Return the sample container to your health care provider.

During a stool osmolality test:
You will need to provide a stool sample. Your provider will give you specific instructions on how to collect and send in your sample. Your instructions may include the following:

• Put on a pair of rubber or latex gloves.
• Collect and store the stool in a special container given to you by your health care provider or a lab. You may get a device or applicator to help you collect the sample.
• Make sure no urine, toilet water, or toilet paper mixes in with the sample.
• Seal and label the container.
• Remove the gloves and wash your hands.

Return the container to your health care provider or the lab as soon as possible. If you think you may have trouble delivering your sample in time, talk to your health care provider.

Will I need to do anything to prepare for the test?

You may need to fast (not eat or drink) for 6 hours before the test or limit fluids 12 to 14 hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Are there any risks to osmolality tests?

There is very little risk to having a blood test. You may have slight pain or bruise at the spot where the needle was put in, but most symptoms go away quickly.
There is no risk to having a urine or stool test.