Derivation and validation of a new formula for plasma osmolality estimation

Plasma osmolality is a physic and chemical property of interest in emergency medicine. This magnitude can be measured at the laboratory, but it is usually estimated with equations. A huge variety of formulas for calculating osmolality have been published, most of them relying on sodium, urea and glucose. The purpose of this study is to develop a new equation for plasma osmolality calculation. In addition we assess the new equation in a sample of healthy individuals.

We used results of sodium, potassium, glucose, urea and osmolality recovered from our patient's database. Multivariate lineal regression was carried-out, considering sodium and potassium as separated variables and as unique variable. In a second phase the obtained equations were tested in a sample of healthy blood-donors. Osmolality was measured by freezing point depression.

In the first phase, 1362 plasma determinations for sodium, potassium, glucose, urea and osmolality were analyzed. All of included variables had a significant correlation with measured osmolality, being the highest correlation with sodium plus potassium and the lowest one was with potassium alone. The formulas obtained for the osmolality estimation were 1.86*Na + 1.6*(Glucose/18) + 1.12*(Urea/6) + 21 (A) and 1.88*(Na + K) + 1.59*(Glucose/18) + 1.08*(Urea/6) + 10.6 (B). Assess of the new equations in a sample of healthy individuals showed better results than equations previously published. The lowest difference versus measured osmolality was produced by formula B.

The equations produced in this study perform better in the estimation of plasma osmolality than previously published formulas. We recommend introducing formula B in the clinical chemistry routine.