the divisor should be 4.6 but empiric data shows that ethanol does not behave as an ideal osmole. Osmolar gap (OG) The osmolar gap is the difference between the measured osmolality and the calculated osmolarity. The difference in units is attributed to the difference in the way that blood solutes are measured in the laboratory versus the way they are calculated. The laboratory value measures the freezing point depression, properly called osmolality while the calculated value is given in units of osmolarity. Even though these values are presented in different units, when there is a small amount of solute compared to total volume of solution, the absolute values of osmolality vs. osmolarity are very close. Often, this results in confusion as to which units are meant. For practical purposes, the units are considered interchangeable. The resulting "osmolar gap" can be thought of as either osmolar or osmolal, since both units have been used in its derivation. Measured osmolality is abbreviated "MO", calculated osmolarity is abbreviated "CO", and the osmolality gap is abbreviated "OG". Clinically the osmolar gap is used to detect the presence of an osmotically active particle that is not normally found in plasma, usually a toxic alcohol such as ethanol, methanol or isopropyl alcohol. See also Osmotic concentration Urine osmolality Serum Osmolarity vs. Osmolality References Blood tests