What are osmolality tests?

Osmolality tests measure the amount of certain substances in your blood, urine, or stool (poop). The tests can show how your kidneys are working and whether you have an unhealthy fluid balance.

The substances that the tests measure include:

• Glucose (sugar).
• Urea (a waste product made in the liver).
• Several electrolytes, such as sodium, potassium, and chloride. Electrolytes are minerals that have an electrical charge when they are dissolved in water or body fluids. They help control the balance of fluids in the body. They also help balance the acids and bases (pH balance) in your body.

Osmolality is the concentration of particles dissolved in your body fluid. The concentration is based on the amount of particles that are in a specific amount of that fluid. Different factors can make it go up or down:

• Your blood osmolality increases when the amount of water in your blood decreases or the number of substances such as sodium, chloride, and glucose increases. When your blood osmolality goes up it causes your kidney to hold onto water. This makes your urine more concentrated.
• Your blood osmolality decreases when the amount of water in your blood increases. This makes your urine more diluted.

This process helps your body maintain a healthy fluid balance. But sometimes this balance can be thrown off. Many different conditions and factors can cause an unhealthy fluid balance, such as. too much 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 substances in your blood.
• Find out if you have swallowed a poison such as antifreeze or rubbing alcohol.
• Check for an overdose of drugs such as salicylates, commonly found in aspirin.
• Help diagnose dehydration, a condition in which your body loses too much fluid.
• Help diagnose overhydration, a condition in which your body retains (holds in) too much fluid.
• Help diagnose diabetes insipidus, a condition that affects the kidneys. It causes frequent urination, which can lead to dehydration.

Sometimes, your blood plasma is also tested for osmolality by measuring the amount of sodium dissolved in it.

A urine osmolality test is often used along with a serum osmolality test to check your 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 substance that was swallowed but may include:

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

You may also need a urine osmolality test if you have trouble urinating or are urinating too much. This test measures how well your kidneys are working.

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:

You will need to give a urine sample for the test. A health care professional may give you a cleansing wipe, a small container, and instructions for how to use the "clean catch" method to collect your urine sample. It's important to follow these instructions so that germs from your skin don't get into the sample:

1. Wash your hands with soap and water and dry them.
2. Open the container without touching the inside.
3. Clean your genital area with the cleansing wipe.
   • For a penis, wipe the entire head (end) of the penis. If you have a foreskin, pull it back first.
   • For a vagina, separate the labia (the folds of skin around the vagina) and wipe the inner sides from front to back.
4. Urinate into the toilet for a few seconds and then stop the flow. Start urinating again, this time into the container. Don't let the container touch your body.
5. Collect at least an ounce or two of urine into the container. The container should have markings to show how much urine is needed.
6. Finish urinating into the toilet.
7. Put the cap on the container and return it as instructed.

If you have hemorrhoids that bleed or are having your menstrual period, tell your provider before your test.

During a stool osmolality test:

You will need to provide a fresh sample of loose or liquid stool for your test. Your provider will probably give you a container or kit with instructions on how to collect the sample. There are different ways to collect stool samples so follow the instructions carefully. In general, you'll need to:

• Label the container with your name, the collection date, and time.
• Collect a stool sample as instructed. This usually involves using a clean, dry container, or special paper or plastic wrap placed over the toilet to catch the stool.
• Make sure the stool doesn't mix with any urine, toilet paper, or water from the toilet.
• Close the container tightly.
• Wash your hands well with soap and water.
• Return the container according to the instructions. This is important to get an accurate test result.

If you're collecting a sample from diapers, you'll get special instructions for using plastic wrap inside of a clean diaper. In certain cases, a provider may use a swab to take a stool sample from the rectum.

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 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 bruising 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.

What do the results mean?

If your serum osmolality results were not normal, it may be a sign of:

• Dehydration or overhydration
• Too much or too little salt in the blood
• Antifreeze or other types of poisoning
• Diabetes insipidus
• Stroke

If your urine osmolality results were not normal, it may be a sign of:

• Dehydration or overhydration
• Heart failure
• Liver disease
• Kidney disease

If your stool osmolality results were not normal, it may be a sign of:

• Factitious diarrhea, a condition caused by the overuse of laxatives
• Malabsorption, a condition that affects your ability to digest and take in nutrients from food

If you have questions about your results, talk to your provider. To understand the results of an osmolality test, your provider may consider your symptoms, medical history, and the results of other blood tests.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about osmolality tests?

Your provider may order more tests along with or after your osmolality test. These may include:

• Complete blood count (CBC)
• Blood urea nitrogen (BUN) test
• Blood glucose test
• Electrolyte panel
• Albumin blood test
• Fecal occult blood test (FOBT)