Osmolality is the number of milliosmoles/ kg (mOsm/kg ) of solvent. It is the concentration of the particles that is dissolved in a fluid. This is the clinical laboratory measurement using an osmometer.
Example: Urine osmolality: extreme range of 50 – 1400 mOsm/kg water, but average is about 500 – 800 mOsm. After an overnight fast, the urine osmolality should be at least 3 times the serum osmolality.
Example: Serum osmolality: 282 – 295 mOsm/kg water