Effect of alcohols and selected solvents on serum osmolality measurements

The method by which serum osmolality is measured can significantly affect the result if certain volatiles or solvents are present in the specimen. Commonly available solvents and alcohols were added to aliquots of pooled human serum to produce toxicologically relevant concentrations. Increasing concentrations of carbon tetrachloride, chloroform, mono-n-butyl ether (butyl cellosolve), 1, 1,1 trichloroethylene, toluene, and xylene did not change vapor pressure (VP) or freezing point depression (FPD) osmolality. Acetone, ethanol, isopropanol, and methanol in increasing concentrations produced a linear increase in FPD osmolality, but no change in VP osmolality. Only ethylene glycol produced a linear increase in VP and FPD osmolality across the range of concentrations studied. Despite the excellent correlation between osmolality and ethanol concentration in prepared serum samples, this relationship could not accurately predict patient ethanol concentrations from FPD osmolality. The osmolal gap, "delta" osmolality, (measured FPD minus calculated osmolality) did not correlate with the difference between measured FPD and VP osmolalities. Patient ethanol levels could not be predicted with accuracy using an equation based on the osmolal gap or "delta" osmolality.