Anion gap

Background

AG = Na - (Cl + HCO3)

Normal Anion Gap = 12+/-4 (8-16)

Cutoffs for "normal" Anion Gap are laboratory and equipment specific. Newer technology and equipment have been shown to measure "low" AG in otherwise normal, healthy people.

Elevated Anion Gap

DDx

MUDPILES

• M - Methanol
• U - Uremia
• D - DKA
• P - Paraldehyde
• I - Iron, INH
• L - Lactic acidosis
• E - Ethylene glycol
• S - Salicylates

Also:

• Starvation/ETOH ketoacidosis
• Carbon Monoxide (CO), CN poisoning (incr. lactate)
• toluene
• chronic Tylenol use

Low Anion Gap

DDX

• Lab error
• Decreased "unmeasured" anions
  • hypoalbuminemia
• Increased "unmeasured" cations
  • Hyperkalemia
  • Hypercalcemia
  • Hypermagnesemia
  • Lithium Toxicity
• Increased cationic paraprotein
  • Multiple myeloma
  • Polyclonal IgG gammopathy
• Pseudo
  • Severe Hypernatremia (>170 meq/L)
    • True conc of Na is underestimated
  • Marked hyperlipidemia
    • Leads to overestimation of plasma Cl conc)
  • Bromide intoxication (e.g. for myasthenia gravis and some herbal medications)
    • Machine mistakenly reads Br as Cl

• AG falls by 2.5 meq/L for every 1 g/dL reduction in albumin concentration

See Also

Anion Gap and Osmolar Gap (High)