Anion gap acidosis

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 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.

Clinical Features

• Signs/symptoms of underlying disease
• Compensatory tachypnea

Differential Diagnosis

Anion gap metabolic acidosis

• Lactic acidosis
  • Sepsis, shock, liver disease, CO, CN, metformin, methemoglobin
  • Short bowel syndrome
  • Propylene glycol infusions for lorazepam and phenobarbital
• Renal failure
  • Uremia
• Ketoacidosis
  • DKA
  • Alcoholic Ketoacidosis
  • Starvation ketoacidosis
• Ingestions
  • Acetaminophen toxicity
  • Aspirin toxicity
  • Increased osm gap
    • Methanol
    • Ethylene glycol
  • Normal osm gap
    • Salicylate toxicity
    • Iron
    • INH

Evaluation

• AG = Na - (Cl + HCO3)
• Normal Anion Gap = 12+/-4 (8-16)
• 12-20 mEq/L when including K+
• Primary acidosis if pH <7.38
• HCO3 <24 = metabolic acidosis
• Always determine if there is another acid/base process occurring
  • Primary respiratory acidosis if pCO2 > pCO2expected
  • Primary respiratory alkalosis if pCO2 < pCO2expected
    • use Winter's formula: PCO2 (expected) = (1.5 x [HCO3–] + 8) ± 2
    • In acute setting PCO2 should fall by 1 mmHg for every 1 mEq fall in HCO3
  • Concurrent metabolic alkalosis if delta-delta > 28
  • Delta-Delta = (AG - 12) + HCO3

Management

• Treat source
• Correct any respiratory acidosis
• Bicarbonate
  • HCO3 dose in mEq = 0.5(wt in kg) x (24 - measured HCO3)
  • Each bicarb 0.5mEq/kg causes 1 meq/L rise in HCO3
  • Consider for:
    • Bicarb <4
    • pH <7.20 AND shock/myocardial irritability
    • Severe hyperchloremic acidemia
      • Lost bicarbonate would take days to replenish

See Also

• Non anion gap acidosis