Anion Gap Study Notes

Formula:

$$AG = Na^+ - (Cl^- + HCO₃^-)$$

Normal Range: ~8–12 mEq/L (lab dependent; some use 3–11 or 6–12).

High Anion Gap (>12 mEq/L)

Meaning: → High Anion Gap Metabolic Acidosis (HAGMA) = buildup of unmeasured acids.

Mechanism:

• Excess acids → ↑unmeasured anions

• HCO₃⁻ consumed buffering them → ↓HCO₃⁻ → ↑gap

Causes (MUDPILES CAT mnemonic):

• Methanol

• Uremia (renal failure)

• DKA / Alcoholic / Starvation ketoacidosis

• Paraldehyde

• INH / Iron overdose

• Lactic acidosis (sepsis, hypoxia, shock, metformin, liver failure)

• Ethylene glycol

• Salicylates (aspirin overdose)

• CO / Cyanide poisoning

• Alcoholic ketoacidosis

• Toluene

Clinical Note: Urgent problem – think shock, sepsis, toxins, renal failure.

Low Anion Gap (<3–6 mEq/L)

Meaning: Rare. Often artifact or due to fewer unmeasured anions.

Mechanism & Causes:

1. ↓Albumin (most common; liver disease, malnutrition, nephrotic syndrome, burns)

2. ↑Unmeasured Cations

   • Paraproteins (Multiple Myeloma, Waldenström’s)

   • Hyperkalemia, hypercalcemia, hypermagnesemia

   • Lithium toxicity

3. Lab artifact / assay interference

   • Bromide/Iodide intoxication → falsely ↑Cl⁻

   • Severe hyperlipidemia/hyperviscosity → pseudohyponatremia

   • Measurement error (Na⁺, Cl⁻, HCO₃⁻)

Clinical Note: Usually less dangerous by itself, but important clue to underlying disease.

Key Points

• Always interpret with albumin → Corrected AG = AG + 2.5 × (4 – albumin in g/dL).

• AG ≠ diagnosis → must combine with: ABG, electrolytes, lactate, renal/liver tests, history.

• High AG → urgent metabolic acidosis.

• Low AG → uncommon, often hypoalbuminemia or lab artifact.