Test Details: Osmolality measures the amount of dissolved substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood and sometimes in urine. A blood (serum) osmolality test is primarily a measure of sodium dissolved in the serum (the liquid portion of blood). Sodium is the major electrolyte in the blood and urine. It works with potassium, chloride, and CO2 (in the form of bicarbonate) to maintain electrical neutrality in the body and acid-base balance.