Notes · Physiology · 23 October 2026

Acid-Base Disorders: Anion Gap

Definition

• Anion Gap (AG) is the calculated difference between the measured cations and anions in plasma, reflecting the concentration of unmeasured ions.
• Formula (in mmol/L): AG = [Na⁺] + [K⁺] – ([Cl⁻] + [HCO₃⁻]).
• Because potassium contributes little, many clinicians use the simplified AG: AG = [Na⁺] – ([Cl⁻] + [HCO₃⁻]).
• Normal AG values vary with laboratory method but are generally 8‑12 mmol/L (without K⁺) or 12‑16 mmol/L (with K⁺).

Epidemiology

• High‑anion‑gap metabolic acidosis (HAGMA) accounts for ≈30‑40 % of all metabolic acidoses encountered in emergency departments and intensive‑care units.
• The most common causes in hospitalized adults are lactic acidosis (sepsis, shock) and renal failure.
• In developing countries, diabetic ketoacidosis (DKA) remains a leading cause of HAGMA in young adults.

Aetiology (MUDPILES mnemonic)

Non‑MUDPILES causes: hyperchloremic metabolic acidosis (normal AG) from large‑volume normal saline, renal tubular acidosis, GI bicarbonate loss.

Pathophysiology

1. Charge balance – Plasma must remain electrically neutral; the sum of cations equals the sum of anions.
2. Measured ions – Sodium (Na⁺) and potassium (K⁺) are the principal extracellular cations; chloride (Cl⁻) and bicarbonate (HCO₃⁻) are the principal measured anions.
3. Unmeasured ions – Include albumin, phosphate, sulfate, organic acids (lactate, keto‑bodies), and calcium/magnesium.
4. Increase in AG – Occurs when acidic unmeasured anions accumulate faster than they can be buffered or excreted, displacing bicarbonate and lowering serum HCO₃⁻.
5. Albumin effect – Albumin carries a negative charge; for each 1 g/dL decrease in serum albumin, the AG falls by ≈2.5 mmol/L. Adjusted AG = measured AG + 2.5 × (4.0 – albumin [g/dL]).
6. Compensatory mechanisms – Respiratory system attempts to blow off CO₂ (↑ pCO₂ → ↓ HCO₃⁻) and kidneys increase ammonium excretion; these are limited in acute settings.

Clinical Features

• General: Nausea, vomiting, abdominal pain, lethargy, confusion; severity correlates with pH.
• Respiratory compensation: Kussmaul respirations (deep, rapid breathing) in severe metabolic acidosis.
• Specific toxin clues:
  • Methanol – visual blurring, “snowfield” vision.
  • Ethylene glycol – flank pain, hematuria, calcium oxalate crystals.
  • Salicylates – tinnitus, hyperthermia.
• Signs of underlying disease:
  • Sepsis – fever, hypotension, warm extremities.
  • DKA – polyuria, polydipsia, fruity breath.

Investigations

Quick bedside calculation

1. Measured AG = Na⁺ – (Cl⁻ + HCO₃⁻)
2. Corrected AG = Measured AG + 2.5 × (4.0 – albumin [g/dL])
3. Delta AG = Corrected AG – 12 (normal)
4. Delta HCO₃⁻ = 24 – measured HCO₃⁻
5. Delta ratio = Delta AG ÷ Delta HCO₃⁻
   • Ratio ≈ 1–2 → pure HAGMA
   • Ratio > 2 → concurrent normal‑AG metabolic acidosis (e.g., diarrhea)
   • Ratio < 1 → concurrent metabolic alkalosis (e.g., vomiting)

Management (Guidelines up to 2024)

1. Identify and treat the underlying cause – this is the cornerstone; AG will normalize once the source is removed.
2. Supportive care
   • Airway & breathing – Ensure adequate ventilation; consider non‑invasive ventilation for severe Kussmaul respirations.
   • Circulation – Restore perfusion with isotonic crystalloids (e.g., 0.9 % saline) or balanced solutions (Ringer’s lactate) as per the 2023 Surviving Sepsis Campaign.
3. Specific antidotes (per 2024 toxicology guidelines)
   • Methanol / Ethylene glycol – Intravenous fomepizole 15 mg/kg loading, then 10 mg/kg every 12 h; if unavailable, ethanol 0.5 g/kg loading then 0.1 g/kg/h. Add hemodialysis when plasma level > 50 mg/dL (methanol) or > 30 mg/dL (ethylene glycol) or severe acidosis (pH < 7.1).
   • Salicylate toxicity – Sodium bicarbonate infusion (1–2 mEq/kg bolus, then 150 mEq/L infusion) to alkalinize urine and promote excretion; consider dialysis if salicylate > 100 mg/dL.
   • DKA – Fluid resuscitation 15–20 mL/kg isotonic saline, then insulin infusion 0.1 U/kg/h after potassium > 3.3 mmol/L; monitor glucose and replace potassium as needed.
4. Bicarbonate therapy – Indicated only when pH < 7.1 or severe hemodynamic compromise (per 2022 American College of Physicians guideline). Give 1 mmol/kg of NaHCO₃ in 5 % dextrose, repeat titrated to pH > 7.2. Avoid over‑correction.
5. Renal replacement therapy – Initiate early (within 6 h) for refractory acidosis, oliguria, or toxin accumulation per 2023 KDIGO AKI guideline.

Complications

• Cardiovascular – Decreased myocardial contractility, arrhythmias, hypotension.
• Neurological – Cerebral edema (especially in DKA children), seizures, coma.
• Respiratory – Fatigue from sustained hyperventilation leading to respiratory failure.
• Renal – Acute tubular necrosis from severe hypoperfusion or toxin exposure.
• Electrolyte disturbances – Hyperkalemia (cellular shift) in acidosis; hypokalemia after correction due to insulin and bicarbonate therapy.

High‑Yield Exam Points

• Normal AG range: 8‑12 mmol/L (without K⁺). Adjust for albumin: each 1 g/dL ↓ albumin → AG ↓ 2.5 mmol/L.
• Delta ratio: 1–2 = pure HAGMA; >2 = mixed HAGMA + normal‑AG metabolic acidosis; <1 = mixed HAGMA + metabolic alkalosis.
• MUDPILES mnemonic for HAGMA causes – remember the “L” stands for lactic acidosis (most common in ICU).
• Fomepizole is the first‑line antidote for both methanol and ethylene glycol poisoning; dose does not need adjustment for renal failure.
• DKA management: “Fluid‑insulin‑electrolyte” sequence; never start insulin before potassium is ≥ 3.3 mmol/L.
• Bicarbonate therapy: Only when pH < 7.1 or severe hemodynamic instability; otherwise, treat underlying cause.
• Adjusted AG formula: AG_adj = AG_measured + 2.5 × (4.0 – albumin [g/dL]).

Practice questions

1. A 45‑year‑old man presents with sepsis. Labs: Na⁺ 138 mmol/L, K⁺ 4.2 mmol/L, Cl⁻ 100 mmol/L, HCO₃⁻ 14 mmol/L, albumin 3.0 g/dL, lactate 6 mmol/L. What is the corrected anion gap?
Answer: 20 mmol/L — Measured AG = 138 + 4.2 – (100 + 14) = 28.2 mmol/L; albumin correction = 2.5 × (4.0 – 3.0) = 2.5; corrected AG = 28.2 – 2.5 ≈ 20 mmol/L.

2. Which of the following conditions is most likely to cause a normal anion gap metabolic acidosis?
A) Lactic acidosis
B) Diabetic ketoacidosis
C) Chronic diarrhea
D) Methanol poisoning
Answer: C — Chronic diarrhea leads to loss of bicarbonate without accumulation of unmeasured anions, keeping the AG normal.

3. A patient with suspected ethylene glycol ingestion has a plasma osmolar gap of 18 mOsm/kg and a measured AG of 22 mmol/L. Which of the following is the best next step?
A) Start intravenous sodium bicarbonate only
B) Administer fomepizole and arrange urgent hemodialysis
C) Give intravenous insulin infusion
D) Begin high‑dose thiamine therapy
Answer: B — Fomepizole blocks glycolate formation; severe AG and osmolar gap indicate need for dialysis.

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