Calculation of Serum Osmolality

The serum osmolality may be estimated by the following equation:

Osmolality = 1.86 Na + (Glucose/18) + (BUN/2.8) + 9 (or) 1.91 Na + (Glucose/15) + (BUN/2.25). The difference between this calculated value and the measured osmolality usually ranges from +13 to -13 mosm/Kg. Larger discrepancies are seen when the formula is simplified (e.g., using 2 x Na).  Large disparities between measured and calculated osmolalities are most commonly encountered in alcohol intoxications.  Serum osmolality is increased by 0.26 mosm/kg for each 1.0 mg/dl of ethanol in serum.