Calculated vs measured plasma osmolalities revisited.

The osmolalities of 100 plasma samples were measured and compared with the osmolalities calculated from the plasma concentrations (mmol/L) of sodium, potassium, glucose, and urea by several different formulae. The formula recommended by Dorwart and Chalmers (Clin Chem 21: 190, 1975) gave inferior results to those obtained with our "most accurate" formula: osmolality = 1.89 Na + 1.38 K + 1.03 urea + 1.08 glucose + 7.45. We recommend using this formula for calculation of osmolality on equipment linked to a computer. However, for simplicity, and to reduce the possibility of calculation errors, the following formula can be used for manual calculations: osmolality = 1.86 (Na + K) + glucose + urea + 10.