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Formal name: Osmolality (plasma, urine, stool)

The Test Sample

What is being tested?

Osmolality is a measure of the number of dissolved particles in a fluid. The osmolality test reflects the concentration of substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood, urine, or sometimes stool. It is used to evaluate the balance between water and dissolved particles in the blood and urine, to detect the presence of substances that may affect this balance, and to assess the kidneys' ability to concentrate urine.

Water balance in the body is a dynamic process that is regulated by controlling the amount of water excreted in the urine and by increasing or decreasing water drinking by regulating "thirst." Osmotic sensors in the body perceive and react to increases and decreases in the amount of water and particles in the bloodstream. When blood osmolality increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, the hypothalamus secretes antidiuretic hormone (ADH), which tells the kidneys to conserve water. This results in more concentrated urine with higher urine osmolality and more dilute blood with lower osmolality. As blood osmolality decreases, ADH secretion is suppressed, the kidneys excrete increased amounts of dilute urine, the amount of water in the body decreases, and blood osmolality returns to normal.

Blood osmolality is primarily a measure of sodium dissolved in the serum. Sodium is the major electrolyte in the blood, urine, and stool. It works with potassium, chloride, and CO2 (in the form of bicarbonate) to maintain electrical neutrality in the body and acid-base balance. Sodium comes into the body in the diet and is normally conserved or excreted in the urine by the kidneys to maintain its concentration in the blood within a healthy range.

Urine osmolality primarily measures the waste products urea and creatinine. Urea and creatinine are produced and removed by the body at a relatively constant rate.

Glucose and urea are not electrolytes but as particles (molecules), they do contribute to osmolality. Normally their contributions are small, but when someone has high blood glucose (hyperglycemia, as found in diabetes) or high blood urea (seen in diseases such as kidney failure), their influence can be significant.

Glucose is osmotically active. This means it can draw water out of the body's cells, increasing the amount of fluid in circulation, which in turn increases the amount of dilute urine produced. Mannitol, a drug used to treat cerebral edema, also has this property. Toxins such as methanol, isopropyl alcohol, ethylene glycol, propylene glycol, and acetone, and drugs such as acetylsalicylic acid (aspirin) can also affect osmolality when ingested in sufficiently large amounts.

Osmolality can be measured or it can be estimated from the major solutes expected to be in the blood. The difference between measured and calculated (estimated) results is called the "osmotic gap" or "osmolal gap." An increase in the osmotic gap (greater than 10) indicates the presence of other substances such as toxins, aspirin, or mannitol. The osmotic gap is sometimes used for evaluation when a person is being treated with mannitol or when someone is suspected of having ingested a toxin such as methanol or too much of a drug such as aspirin. Other common causes of an elevated osmolal gap are alcoholic ketoacidosis, kidney failure, diabetic ketoacidosis, and shock.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. A random urine sample is collected using a clean catch method (see description under "Urinalysis: How is the sample collected for testing?"). A fresh (refrigerated or frozen within about 30 minutes of collection), liquid stool that is not contaminated by urine is collected in a clean container. Bacteria in the stool can change the results of the test within a short period of time.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation may be needed or you may be instructed to fast (nothing to eat or drink except water) for 6 hours before the test. Follow any instructions you are given. Because some medications can interfere with this testing, inform your health care provider of all of the medications you are taking, especially if you are taking mannitol.