Serum osmolality: Components
Basic, Organ-Based and Clinical Sciences
Serum osmolality is an estimate of the dissolved solutes in serum. Serum osmolality can be estimated as follows:
Serum osmolality = 2 x [Na+] + [glucose/18] + [BUN/2.8]
Sodium ions account for almost 90% of the solute dissolved in serum. In the equation, sodium is multiplied by 2 to account for the osmotically active anion associated with each sodium cation. In pathological states, glucose and urea also contribute to extracellular osmolality, but when these values are normal their effect on osmolality is small.
Osmolality is tightly regulated to between 280-290 mOsm/kg. These narrow limits are maintained by regulating both water intake and excretion. An osmolal gap is any deviation between the calculated and measured osmolality. A gap indicates a high concentration of osmotically active solute in the plasma like ethanol, mannitol, methanol, ethylene glycol, or isopropyl alcohol. Osmolal gaps can be seen in patients with CKD, ketoacidosis (high concentration of ketone bodies), or those receiving large volumes of glycine (like during a TURP).