Anion Gap Calculator - Acid-Base Balance Assessment

The anion gap is a key diagnostic tool for metabolic acidosis: (1) Differential Diagnosis: Distinguishes high AG from normal AG metabolic acidosis, narrowing the diagnostic scope. (2) Detect Mixed Disorders: Identifies complex acid-base disturbances through delta gap analysis. (3) Toxicology Screening: Elevated AG is an important clue for certain drug toxicities. (4) Assess Severity: Higher AG typically indicates more severe conditions. (5) Monitor Treatment: Dynamic AG monitoring helps evaluate treatment efficacy.

Albumin-corrected AG should be used in: (1) Albumin < 4.0 g/dL (hypoalbuminemia). (2) Albumin > 4.5 g/dL (hyperalbuminemia, rare). (3) Liver cirrhosis, nephrotic syndrome, malnutrition. (4) Critically ill patients (often have hypoalbuminemia). (5) Multiple trauma or post-major surgery patients. Uncorrected AG may miss high AG metabolic acidosis, while corrected AG more accurately reflects acid-base status.

Traditionally, the normal anion gap range is 8-16 mEq/L (average ~12 mEq/L). Important considerations: (1) Different laboratories may vary slightly (typically 6-14 or 8-16 mEq/L). (2) Modern automated analyzers measure chloride more precisely, slightly lowering normal AG values. (3) Albumin levels affect AG and correction should be considered. (4) AG > 20 mEq/L almost always indicates metabolic acidosis. (5) AG > 30 mEq/L usually indicates severe metabolic acidosis.

The most common cause of elevated AG is metabolic acidosis, but not the only one: (1) Metabolic Acidosis (most common): DKA, lactic acidosis, uremia. (2) Metabolic Alkalosis: AG may be mildly elevated in severe metabolic alkalosis. (3) Laboratory Error: Pseudohyponatremia or hyperchloremia, hyperlipidemia. (4) Dehydration: Hemoconcentration causing elevated ion concentrations. (5) Massive Antibiotic Infusion: Penicillins, carbenicillin. Therefore, elevated AG requires clinical context and other blood gas analysis results for accurate interpretation.

MUDPILES is a mnemonic for causes of high anion gap metabolic acidosis: M - Methanol toxicity, U - Uremia (renal failure), D - Diabetic Ketoacidosis (DKA), P - Paraldehyde (now rare), I - INH (isoniazid), Iron poisoning, L - Lactic acidosis, E - Ethanol, Ethylene glycol, S - Salicylates (aspirin). This mnemonic helps clinicians quickly recall and evaluate potential causes of elevated AG in emergency settings, guiding targeted diagnostic tests and treatments.

The delta gap evaluates mixed acid-base disorders: Delta Gap = Measured AG - 12. Expected HCO₃⁻ = 24 - Delta Gap. Interpretation: (1) Measured HCO₃⁻ ≈ Expected HCO₃⁻: Pure high AG metabolic acidosis. (2) Measured HCO₃⁻ > Expected HCO₃⁻: High AG metabolic acidosis + metabolic alkalosis. (3) Measured HCO₃⁻ < Expected HCO₃⁻: High AG metabolic acidosis + normal AG metabolic acidosis. This approach helps identify complex mixed disorders that require comprehensive management.