Table 1:

Stepwise approach to calculating the Osmol Gap

1 Determining the osmol gap begins with obtaining a measured serum osmolality from the laboratory. The normal measured serum osmolality is typically between 285 – 290 mOsm/L

2. The calculated serum osmolality is then determined by the following equation: $${\mathbf{OSM}}_{\mathbf{calc}}\hspace{0.17em}\mathbf{(}\mathbf{mOsm}\mathbf{/}\mathbf{L}\mathbf{)}\mathbf{=}\mathbf{(}\mathbf{2}\mathbf{\times }\mathbf{N}{\mathbf{a}}^{\mathbf{+}}\mathbf{)}\mathbf{+}\mathbf{(}\mathbf{BUN}\hspace{0.17em}\mathbf{(}\mathbf{mg}\mathbf{/}\mathbf{dL}\mathbf{)}\mathbf{/}\mathbf{2.8}\mathbf{)}\mathbf{+}\mathbf{(}\mathbf{glucose}\hspace{0.17em}\mathbf{(}\mathbf{mg}\mathbf{/}\mathbf{dL}\mathbf{)}\mathbf{/}\mathbf{18}\mathbf{)}$$

3. When attempting to determine the osmolar gap, it is crucial to also check an ethanol level. Ethanol, like the other alcohols, is a low molecular weight substance that will also increase the osmolar gap, altering the results.

4. Calculation of ethanol’s contribution to the serum osmolality is achieved with the following equation: $${\mathbf{OSM}}_{\mathbf{calc}}\hspace{0.17em}\mathbf{(}\mathbf{mOsm}\mathbf{/}\mathbf{L}\mathbf{)}\mathbf{=}\mathbf{(}\mathbf{2}\mathbf{\times }\mathbf{N}{\mathbf{a}}^{\mathbf{+}}\mathbf{)}\mathbf{+}\mathbf{(}\mathbf{BUN}\hspace{0.17em}\mathbf{(}\mathbf{mg}\mathbf{/}\mathbf{dL}\mathbf{)}\mathbf{/}\mathbf{2.8}\mathbf{)}\mathbf{+}\mathbf{(}\mathbf{glucose}\mathbf{(}\mathbf{mg}\mathbf{/}\mathbf{dL}\mathbf{)}\mathbf{/}\mathbf{18}\mathbf{)}\mathbf{+}\mathbf{(}\mathbf{ethanol}\hspace{0.17em}\mathbf{(}\mathbf{mg}\mathbf{/}\mathbf{dL}\mathbf{)}\mathbf{/}\mathbf{4.25}\mathbf{)}$$

5. The osmol gap is measured as the difference between the calculated serum osmolality (OSM calc) and the measured osmolality (OSM meas). The difference between the two should be less than or equal to 20 $${\mathbf{OSM}}_{\mathbf{calc}}\hspace{0.17em}\mathbf{-}{\mathbf{OSM}}_{\mathbf{meas}}\mathbf{=}\le \mathbf{20}\hspace{0.17em}\mathbf{mOsm}\mathbf{/}\mathbf{L}$$

6. Intoxication with methanol or ethylene glycol produce a high anion gap metabolic acidosis. The anion gap can be calculated with the following formula: $$\mathbf{Sodium}\mathbf{-}\mathbf{(}\mathbf{chloride}\mathbf{+}\mathbf{bicarbonate}\mathbf{)}\mathbf{=}\mathbf{12}\mathbf{\pm }\mathbf{4}$$

7. It is important to remember, isopropanol poisoning will cause an elevated osmol gap withouta metabolic acidosis, making this calculation important in differentiating between possible intoxicants.