Osmolal gap in hemodialyzed uremic patients.

Osmolality is an expression of the number of particles in a given weight of solvent (mOsm). Measured osmolality is determined by the osmometer, and calculated osmolality is estimated by 2xNa + UN/2.8 + glucose/18. The difference between measured and calculated osmolality is the osmolal gap. The purpose of the present study is to determine the measured and the calculated osmolality and the osmolal gap in hemodialyzed uremic patients, pre- and post-hemodialysis (HD). In 24 uremic patients under regular HD, blood samples pre- and post-HD were collected, and serum osmolality measured (osmometer) and calculated (2xNa + UN/2.8 + glucose/18) and the osmolal gap (measured-calculated osmolality) were determined. Also, the same parameters were determined in 22 healthy subjects (control). According to our findings, the measured osmolality in patients is significantly higher pre- and post-HD in comparison to that of controls, but post-HD is significantly lower than pre-HD. Also, calculated osmolality is significantly higher pre- and post-HD in comparison to that of controls, but the value post-HD is significantly lower than the pre-HD. The osmolal gap of patients pre-HD (11 ± 2.08) and post-HD (7.29 ± 1.94) is significantly higher (P < 0.001) in comparison to that of controls (3.18 ± 1.46); also, the value post-HD is significantly decreased in comparison to the value pre-HD (P < 0.001). Uremic hemodialyzed patients present high measured and calculated osmolality pre-HD that remains high post-HD in comparison to that of controls in spite of the significant decrease post-HD in comparison to that of pre-HD. Also, the osmolal gap is high pre-HD and, in spite of the decrease, remains high post-HD. In comparison to that of controls, the high osmolal gap indirectly indicates the presence of unidentified endogenous osmoles in the serum of uremic patients which partly are removed during HD.