Osmolal or Osmolar Gap

Definitions

Osmolarity = the total number of particles in solution per liter

Osmolality = the number of particles per kilogram of solvent

Background

Serum osmolality should be measured by the freezing-point depression method rather than by the vapor pressure method, since the latter technique will underestimate the concentrations of volatile alcohols (methanol, ethylene glycol, and isopropanol).

Calculations

Calculated Osm

2 x Na + (glu/18) + (BUN/2.8) + (ethanol/4.6) + (methanol/2.6) + (ethylene glycol/5) + (acetone/5.5) + (isopropanol/5.9)

Osmolal Gap

Measured osm - Calculated osm

(Normal = (-)14-10 mOsm/L)

Estimating Toxic Alcohol Concentration

Toxic alcohol (mg/dL) = Osmolal gap x Conversion Factor

Parent compounds are responsible for the osmolar gap while their metabolites are responsible for anion gap changes

DDX (Increased)

Alcohols

    -ethanol

    -methanol

    -ethylene glycol

    -isopropanol

Sugar

    -glycerol

    -mannitol

Ketones

    -acetone

Other

    -propylene glycol

    -osmotic contrast dyes

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