Evaluation of the effects of glucose on osmolal gap using freezing point depression and vapor pressure methods

The measurement of serum osmolality, and the calculation of osmolal gap (OG) from a calculated osmolality are widely used in clinical and emergency medicine. In this study, the possible effects of blood glucose on OG were investigated by freezing point depression and vapor pressure methods. The concentrations of sodium, glucose, blood urea nitrogen and osmolalities of 2640 samples were measured. There were two methods for calculating serum osmolality: freezing point depression method (n = 2399) and vapor pressure method (n = 241). The OG was positively associated with glucose in glucose 110-450 mg/dL (r = 0.191, p < 0.001) and glucose > 450 mg/dL (r = 0.372, p < 0.001), but not in glucose < 110 mg/dL (r = 0.017, p = 0.711) in freezing point depression method. However, OG had no correlation with glucose regardless of glucose level in vapor pressure method. In freezing point depression method, compared with the groups of glucose <110 and 110-450 mg/dL, the group with glucose >450 mg/dL had higher OG (p < 0.001) and higher prevalence of OG > 10 mOsm/Kg H₂O (p < 0.001). Our study demonstrated that OG is impacted by increasing blood glucose concentration using freezing point depression method, special attention should be made to blood glucose concentrations when using freezing point depression method to determine OG.