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.

Why this test is done

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.

How to prepare

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.

Sample type

Blood Urine Stool

Other names for this test

serum osmolality, plasma osmolality, urine osmolality, stool osmolality, osmotic gap