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.

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.