The normal measured serum osmolality is 280-300 mOsm/kg H2O. The serum osmolality is calculated with the equation (sodium X 2) + BUN/3 + glucose/20 + ethanol/4. The BUN, glucose and ethanol are in mg/dl. The limit for the difference between measured and calculated is 10-20 in most textbooks. The list of substances causing an osmolar gap is limited because certain serum levels must be obtained before any substance can contribute to the serum osmolality. These substances include sorbitol, mannitol, glycerin, isoniazid, ethanol, isopropanol, methanol, acetone and ethylene glycol. The presence of ethanol is the most common confounder for calculating the gap to check for the presence of another agent. Please see the table below for the information necessary to make these adjustments.

  Serum Osm increase
per each 1 mg/dl
Concentration (mg/dl) increase
per each 1 mOsm/kg H2O
Methanol 0.34 2.6
Ethanol 0.22 4.3
Ethylene glycol 0.20 5.0
Acetone 0.18 5.5
Isopropanol 0.17 5.9
From Kulig, K. et al. Toxic effects of methanol, ethylene glycol and isopropyl alcohol. Topics in Emergency Medicine. 6 (2): 16.