The difference between the measured serum osmolarity and the calculated serum osmolarity. Normally, the calculated serum osmolarity is primarily determined by circulating levels of sodium salts (chloride and bicarbonate), glucose, and urea. An increased serum osmolar gap occurs when other solutes (e.g., ethanol, methanol, propylene glycol) are present in high enough concentrations to increase the measured osmolarity by more than 10 mOsmol/L.