Anion gap and osmolar gap (high)
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Calculations
Anion Gap
AG = Na - (Cl + HCO3)
Calculated Osm
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)
- Normal = 275-295
Osmolal Gap
Osmolal Gap = measured osm - calculated osm
- Normal = (-)14-10 mOsm/L
Differential Diagnosis
Toxicological
- Methanol
- Ethanol glycol
- Aspirin (Salicylate) Toxicity
- Iron
- INH
- Paraldehyde
- Alcoholic Ketoacidosis
- Inhalants - CN, CO, Hydrogen Sulfide
- Toluene
Non-Toxicological
CAT MUDPILERS
- C-Cyanide
- A-ASA, Alcohol
- T-Toluene
- M-Methanol, Metformin
- U-Uremia
- D-DKA
- P-Paraldehyde, Post-ictal lactic acidosis (transient, 60-90 min), Phenformin (withdrawn in 1970s)
- I-Iron, INH, Inhalants, Inborn Errors
- L-Lactic Acidosis
- E-Ethylene glycol, Ethanol
- R-Rhabdomyolysis
- S-Salicylates, Solvents, Starvation
Evaluation
Toxic Alcohols Anion/Osmolar Gaps
| Substance | Osmolar gap | Metabolic acidosis | Anion gap | Ketones | Ca Oxalate crystals | Reduced vision | Management |
|---|---|---|---|---|---|---|---|
| Ethanol | + | +/- (if ketoacidosis) | +/- (if ketoacidosis) | +/- | - | - | Mainly supportive |
| Ethylene glycol | + (early)* | + | + | - | + | - | Fomepizole, Thiamine, Pyridoxine, +/- Dialysis |
| Methanol | + (early)* | + | + | - | - | + | Fomepizole or ethanol, Folinic acid/Folic acid, +/- Dialysis |
| Isopropyl alcohol | + | - | - | + (acetonemia without acidosis) | - | - | Mainly supportive, +/- Dialysis if severe |
| Propylene glycol | + | + | + (lactic acidosis) | - | - | - | D/C offending agent (e.g. IV lorazepam/diazepam), supportive, +/- Dialysis |
- Osmolar gap → Anion gap transition: For all toxic alcohols, the osmolar gap is elevated early (parent compound present) and decreases over time as the alcohol is metabolized into organic acid metabolites, which then produce an anion gap metabolic acidosis. A normal osmolar gap does NOT exclude toxic alcohol ingestion if presentation is delayed.
- Key distinguishing features
- Isopropyl alcohol: The only toxic alcohol that causes ketosis without metabolic acidosis (metabolized to acetone, not an organic acid)
- Ethylene glycol: Ca oxalate crystals in urine + anion gap metabolic acidosis + renal failure
- Methanol: Visual disturbances (blurred vision, "snowfield" vision, blindness) + anion gap metabolic acidosis + optic disc hyperemia on fundoscopy