Anion gap

Background

A normal gap is 12 +/- 4 (i.e. 8 to 16). The anion gap is the difference between measured cations and measured anions in serum. This difference does not reflect a true disparity between positive and negative charges, given that serum actually is electrically neutral when all serum cations and anions are measured. Rather, the anion gap is a measurement artifact resulting from the fact that only certain cations and anions are routinely measured. Anion gap metabolic acidosis is secondary to the addition of endogenous or exogenous acid

• AG = Na - (Cl + HCO3)
• Normal Anion Gap = 12+/-4 (8-16)
• 12-20 mEq/L when including K+

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

Differential Diagnosis

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 (increased lactate)
• Toluene
• Chronic acetaminophen use

KILR

• K – Ketoacidosis - (diabetic, Alcohol, Starvation)
• I – Ingestion - (Salicylates, Acetaminophen, Methanol, Ethylene glycol, CO, CN, Iron, INH)
• L – Lactic acidosis - (infection, hemorrhage, hypoperfusion, Alcohol, Metformin)
• R – Renal - (Uremia)

Low Anion Gap

Differential Diagnosis

• Lab error
• Decreased "unmeasured" anions
  • Hypoalbuminemia
  • Bromide toxcity
• 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)
  • Elevated serum bromide (e.g. for myasthenia gravis and some herbal medications)
    • Assay mistakenly reads Br as Cl
  • Elevated serum iodide

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

See Also

Anion Gap and Osmolar Gap (High)