Anion Gap and Osmolar Gap (High)

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

• Lactic Acidosis
• DKA
• Uremia

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• P-Paraldehyde, Post-ictal lactic acidosis (transient, 60-90 min), Phenformin (withdrawn in 1970s)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, Inhalents, Inborn Errors
• L-Lactic Acidosis
• E-Ethylene glycol, Ethanol
• R-Rhabdomyolysis
• S-Salicylates, Solvents, Starvation

Evaluation

Toxic Alcohols Anion/Osmolar Gaps

	Osmolar gap	Anion gap
Ethanol	+	+ if ketoacidosis
Ethylene glycol	+	+
Methanol	+	+
Isopropyl alcohol	+	-

See Also

• Osmolal or Osmolar Gap
• Anion Gap (High)
• Toxidromes

External Links

• MDCalc - Serum Osmolality/Osmolarity
• MDCalc - Anion Gap

References

Authors

Ross Donaldson, Neil Young, Kevin Lu, Joel Clinton Miller, Richard Whiddon, Jordan Swartz, Daniel Ostermayer