Osmolal Gap

Calculates the osmolal gap to screen for unmeasured osmotically active substances, particularly toxic alcohols (methanol, ethylene glycol), mannitol, or other exogenous toxins.

mOsm/kg
Measured by freezing point depression or vapor pressure.
mEq/L (mmol/L)
Serum sodium.
mg/dL
Glucose in mg/dL. If mmol/L, multiply by 18.
mg/dL
Blood urea nitrogen in mg/dL.
mg/dL
If ethanol present, add to calculated osmolality. Conversion factor: divide mg/dL by 4.6 for mOsm/L, or use 1.2-1.25 mmol/L factor.

References

  1. Serum osmolal gap — UpToDate (2024)
  2. Harmonisation of Osmolal Gap - Can We Use a Common Formula? — Clinical Biochemist Reviews/PMC (2016)

Note: The Smithline-Gardner formula [2*Na + Glucose + Urea] in SI units is recommended for harmonization. The Dorwart-Chalmers formula [1.86*Na + Glucose/18 + BUN/2.8 + 9] was historically used but overestimates. When ethanol is present, consider it separately.

Warnings & Limitations

  • Normal gap varies by laboratory and formula used. Most use <10 mOsm/kg as upper normal limit.
  • Ethanol contribution to gap uses factor of 1.2-1.25 mmol/L rather than expected 1.0 based on molecular weight.
  • Elevated OG is an EARLY finding in toxic alcohol poisoning; anion gap elevation appears LATER after metabolism to acids.
  • CKD and DKA can elevate OG due to accumulated endogenous solutes.
  • Always correlate with anion gap, clinical picture, and history.