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.