To determine the osmole gap utilizing 18 previously published formulae for the estimation of serum osmolality in cats.


Serum samples were frozen at −80°C after routine biochemical analysis. An Advanced Micro Osmometer 3300 was used to measure serum osmolality. Eighteen previously reported formulae were used to calculate osmolality from biochemical analysis results. The calculated osmolality was subtracted from the measured osmolality to determine the osmole gap. Osmole gaps for azotaemic and hyperglycaemic cats were compared to those from cats without azotaemia or hyperglycaemia using each formula.


The osmole gaps varied dependent on the formula used and the presence or absence of hyperglycaemia or azotaemia. Eleven formulae led to calculated osmolality and osmole gaps that were not statistically different when hyperglycaemia, azotaemia or both were present. Four of these 11 formulae resulted in osmole gaps near zero. For each formula used, the osmole gap increased with increasing osmolality.

Clinical Significance

Multiple formulae to calculate serum osmolality can be used, but they result in significantly different osmole gaps. Clinicians should be aware of the specific reference interval for the formula being used. The formula [2(Na+) + glucose + BUN] is recommended as it is easy to use and reliable even in the presence of hyperglycaemia and/or azotaemia.