Acid-Base & ElectrolytesEvidence-Based Golden Standard
Serum Osmolality & Osmolar Gap
Calculates serum osmolality and the osmolar gap to screen for occult toxic alcohol ingestions (methanol, ethylene glycol, isopropanol, propylene glycol).
In Conventional (mg/dL): Calculated Osm = 2 × Na + (Glucose / 18) + (BUN / 2.8) + (Ethanol / 4.6)
In SI (mmol/L): Calculated Osm = 2 × Na + Glucose + Urea + Ethanol
Osmolar Gap = Measured Osmolality - Calculated Osmolality
Published Clinical Benchmark Cases
Clinical Parameters & Inputs
Unit: mEq/L
mEq/L
Unit: mg/dL
mg/dL
Unit: mg/dL (BUN)
mg/dL (BUN)
Unit: mOsm/kg
mOsm/kg
Unit: mg/dL
mg/dL
Enter 0 if not measured or not suspected.
Computed OutputNormal Osmolar Gap (≤ 10 mOsm/kg)
0mOsm/kg
Diagnostic Interpretation
Osmolar Gap is NORMAL (0 mOsm/kg). Unmeasured toxic alcohol osmoles are unlikely. If unexplained metabolic acidosis is present, prioritize non-toxic-alcohol HAGMA etiologies (DKA, lactic acidosis, uremia, aspirin).
Calculation Breakdown
Calculated Serum Osmolality
2 × 140 + 5 + 5 =290 mOsm/kg
Osmolar Gap
290 (Measured) - 290 (Calculated) =0 mOsm/kg
Clinical Decision Pearls
- An osmolar gap > 10 mOsm/kg in a patient with unexplained high anion gap metabolic acidosis is an emergency requiring immediate toxicology consultation, fomepizole administration, or hemodialysis.
- Early in methanol or ethylene glycol ingestion, the parent alcohol causes a high osmolar gap with a normal anion gap. Later, as the alcohol is metabolized into toxic acids (formate, oxalate), the osmolar gap falls while the anion gap rises dramatically.
Limitations & Confounders
- Must be measured using a Freezing-Point Depression osmometer; vapor-pressure osmometers miss volatile alcohols (ethanol, methanol, isopropanol).
Evidence & Citations
- Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmol gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011;58(3):480-484.
- Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659.