For calculating osmolality, the simplest formula is the best.

Plasma osmolalities were measured in 100 normal, 100 general hospital and 100 intensive care patients, and compared with the osmolalities calculated from the plasma concentrations of sodium, potassium, glucose and urea, using five different published formulae. The mean osmolar gaps in the 100 consecutive intensive care patients and the 100 general hospital patients were not significantly different from the mean osmolar gap in the 100 normal individuals. The formula which gave the least difference between the measured and calculated osmolality was 2 X Na + urea + glucose, where the concentrations of sodium, urea and glucose were measured in mmol/l.