A Serum Osmolality Test measures dissolved particles (like sodium, glucose, urea) in your blood to check your body’s water balance, helping diagnose dehydration, overhydration, kidney issues, or electrolyte imbalances, and detect certain toxins like antifreeze or alcohol by seeing if particles in the blood match what’s expected (Osmolal Gap). It involves a blood draw, often with fasting, and reveals if your body has too much or too little water relative to salts