How to determine the type of metabolic acidosis from an arterial blood gas (ABG) result?

Determining the Type of Metabolic Acidosis from ABG

Calculate the anion gap using the formula [Na⁺] - ([HCO₃⁻] + [Cl⁻]) to immediately classify metabolic acidosis into high anion gap (>12 mEq/L) versus normal anion gap (hyperchloremic) categories, which directs your diagnostic workup and treatment approach. 1, 2

Step-by-Step Algorithmic Approach

Step 1: Confirm Metabolic Acidosis

• Verify pH <7.35 with bicarbonate <22 mmol/L on ABG 1, 3
• Check that PaCO₂ is appropriately reduced (approximately 1 mmHg decrease for every 1 mmol/L fall in bicarbonate) to confirm this is a primary metabolic process, not respiratory 2
• If PaCO₂ is elevated despite low pH, suspect mixed disorder with concurrent respiratory acidosis 2

Step 2: Calculate the Anion Gap

• Use the formula: Anion Gap = [Na⁺] - ([HCO₃⁻] + [Cl⁻]) 2, 4
• Normal anion gap is typically 8-12 mEq/L 2
• An elevated anion gap (>12 mEq/L) indicates accumulation of unmeasured anions, most commonly organic acids 2, 4

Step 3: Classify by Anion Gap

High Anion Gap Metabolic Acidosis (>12 mEq/L)

• Indicates organic acid accumulation - check lactate level immediately 4
• Elevated lactate (>2 mmol/L) suggests lactic acidosis from tissue hypoperfusion, sepsis, or shock 5, 4
• If lactate normal, measure serum ketones for diabetic ketoacidosis or alcoholic ketoacidosis 1
• Consider toxic ingestions (methanol, ethylene glycol, salicylates) if history suggests and osmolar gap is elevated 1
• Check creatinine and urea for uremic acidosis in renal failure 4
• In fire victims with severe acidosis (pH <7.20) and lactate ≥10 mmol/L, suspect cyanide poisoning and consider empiric hydroxocobalamin 5

Normal Anion Gap Metabolic Acidosis (8-12 mEq/L)

• Indicates bicarbonate loss or impaired renal acid excretion 2
• Check serum chloride - hyperchloremia (Cl⁻ >108 mEq/L) confirms hyperchloremic metabolic acidosis 4
• Measure urine anion gap ([Na⁺] + [K⁺] - [Cl⁻]) to distinguish renal from GI causes 2
• Positive urine anion gap suggests renal tubular acidosis 2
• Negative urine anion gap suggests GI bicarbonate loss (diarrhea) 2
• Review medication history for acetazolamide or other carbonic anhydrase inhibitors 1

Step 4: Correct for Albumin (Critical Pitfall)

• Calculate albumin-corrected anion gap, as hypoalbuminemia masks elevated anion gap 4
• For every 1 g/dL decrease in albumin below 4 g/dL, add 2.5 mEq/L to the calculated anion gap 4
• In critically ill patients, 98% have unmeasured anions when properly corrected for albumin 4

Step 5: Check for Mixed Disorders

• Calculate the delta-delta ratio: (Change in anion gap) / (Change in bicarbonate from normal) 2
• Ratio of 1-2 suggests pure high anion gap acidosis 2
• Ratio <1 suggests concurrent normal anion gap acidosis 2
• Ratio >2 suggests concurrent metabolic alkalosis 2
• In ICU patients, 80% have hyperchloremia and 62% have elevated lactate, indicating multiple simultaneous mechanisms 4

Key Clinical Pearls

Severity Assessment

• pH <7.1 with base deficit >10 indicates severe acidosis requiring urgent intervention 1
• In CO poisoning, pH <7.20 correlates with 30-50% short-term mortality regardless of carboxyhemoglobin levels 5
• Plasma lactate ≥10 mmol/L in fire victims warrants empiric cyanide antidote treatment 5

Common Diagnostic Pitfalls to Avoid

• Never rely on standard anion gap calculation alone in critically ill patients - unmeasured anions are present in 98% when albumin is corrected 4
• The anion gap is not useful for detecting hyperlactatemia without measuring lactate directly 4
• Multiple mechanisms coexist in most ICU patients - look for overlapping causes rather than assuming a single etiology 4
• End-tidal CO₂ correlates with bicarbonate (r=0.553) but ABG remains the gold standard for diagnosis and treatment guidance 6

Treatment Implications Based on Type

• High anion gap acidosis: Treat the underlying cause - bicarbonate does not improve outcomes in lactic acidosis or DKA unless pH <7.0 1, 7
• Normal anion gap acidosis: Consider bicarbonate therapy when pH <7.1 after ensuring adequate ventilation 1
• Sodium bicarbonate is specifically indicated for tricyclic antidepressant toxicity with QRS >120 ms 1
• Target pH 7.2-7.3, not complete normalization, to avoid complications 1