The concentration of different solute particles (osmoles) in the serum, expressed in mOsm/kg. Sometimes used synonymously with serum osmolarity. Serum osmolality can be measured using an osmometer (normal range is 275-299 mOsm/kg) or estimated by calculating the serum osmolarity (in mOsm/L): 2 × (serum sodium concentration in mEq/L) + (serum glucose concentration in mg/dL)/18 + (serum urea concentration in mg/dL)/2.8.