Comparison of methods for calculating serum osmolality form chemical concentrations, and the prognostic value of such calculations.

Sodium, potassium, glucose, blood urea nitrogen, and osmolality were determined for 715 hospital-patient sera. The chemical concentrations were used to calculate osmolalities according to 13 different methods taken from the literature. The goodness of the comparison between calculated and measured osmolality was quite similar for several of the better methods. The aggreement was unimproved when molal chemical concentrations were used instead of molar values. The difference between measured and calculated osmolality was unrelated to whether a patient was discharged from the hospital after a short or long period or ultimately died. The equation we found to yield the most accurately calculated osmolalities is Osmolality = 1.86 Na + (Glu/18) + - (BUN/2.8) + 9.