Osmolality

The osmolality test is an indirect measurement of the number of particles (solutes) in a fluid (solvent). It is used to evaluate the water balance in the body and is based on the principle that changes in the number of solutes result in changes in the physical properties of the fluid they are in. Water balance in the body is a dynamic process that is regulated by increasing or decreasing the amount of water coming into the body in response to “thirst” and by controlling the amount of water excreted in the urine.

Osmotic sensors in the body register and react to increases and decreases in the amount of water and particles in the bloodstream. When osmolality increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, the hypothalamus (a tiny gland in the brain) secretes antidiuretic hormone (ADH), which tells the kidneys to conserve water. This results in a more concentrated urine (higher urine osmolality) and a more dilute plasma. When osmolality decreases in the blood, indicating either an increase in the amount of water in the blood or a decrease in the number of particles, ADH secretion is suppressed and the kidneys excrete increased amounts of dilute urine (lower urine osmolality). As the amount of water in the body decreases, plasma osmolality returns to normal.

Serum osmolality primarily measures sodium and urine osmolality primarily measures the waste products urea and creatinine. Sodium is the major electrolyte in the blood, urine, and stool. It works with potassium (which is found primarily inside cells), 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 by the kidneys in the urine to maintain its concentration in the blood within a narrow range. 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 a patient has a high blood sugar (hyperglycemia, as found in diabetes) or a high blood urea (seen in diseases such as renal failure), their influence can be significant. Glucose is osmotically active. 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 (excess fluid in the brain), also has this property. Urea, however, can move into cells; it does not draw water out of them.

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. Osmolality can be measured or calculated from the major solutes expected to be in the blood. The difference between measured and calculated results is called the “osmotic gap.” Toxins, acetylsalicylic acid, and mannitol can be detected as an increase in the osmotic gap.

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 cup. Bacteria in the stool can change the results of the test within a short period of time.

No test preparation is needed.