What constitutes a diagnosis of metabolic acidemia?

Diagnosis of Metabolic Acidosis

Metabolic acidosis is diagnosed by a serum bicarbonate level <22 mmol/L, arterial pH <7.35, and appropriate respiratory compensation with a decreased PaCO2, along with clinical assessment of the underlying cause through anion gap calculation. 1

Diagnostic Criteria

Metabolic acidosis is characterized by:

• Primary reduction in serum bicarbonate (HCO3-) concentration (<22 mmol/L)
• Secondary decrease in arterial PCO2 (approximately 1 mmHg for every 1 mmol/L fall in serum HCO3-)
• Reduced blood pH (<7.35)
• Appropriate respiratory compensation (decreased PCO2)

Laboratory Parameters

• Arterial blood gases showing:
  • pH <7.35
  • Decreased HCO3- (<22 mmol/L)
  • Decreased PCO2 (compensatory)
• Serum electrolytes with calculated anion gap
• Venous blood samples are usually sufficient unless oxygenation status is needed 2

Classification Using Anion Gap

The anion gap calculation is essential for categorizing metabolic acidosis:

Anion Gap Formula

Anion Gap = [Na+] - ([Cl-] + [HCO3-]) 1

• Normal anion gap: 8-12 mmol/L
• High anion gap metabolic acidosis (HAGMA): >12 mmol/L
• Normal anion gap (hyperchloremic) metabolic acidosis: 8-12 mmol/L

Important Correction Factor

• Albumin correction: For every 1 g/dL decrease in albumin below normal, add 2.5 to the calculated anion gap 1, 3

Severity Classification

Metabolic acidosis severity can be classified based on total CO2 level:

• Mild: Total CO2 ≥19 mmol/L
• Moderate: Total CO2 between 10-18 mmol/L
• Severe: Total CO2 <10 mmol/L 1

Diagnostic Algorithm

1. Measure arterial or venous blood gases and serum electrolytes
2. Confirm metabolic acidosis: pH <7.35, HCO3- <22 mmol/L
3. Calculate anion gap: [Na+] - ([Cl-] + [HCO3-])
4. Correct for albumin if hypoalbuminemia is present
5. Categorize based on anion gap:
   • High anion gap: Consider ketoacidosis, lactic acidosis, toxic ingestions, renal failure
   • Normal anion gap: Consider GI bicarbonate loss, renal tubular acidosis, hyperchloremic states

Common Causes by Anion Gap Category

High Anion Gap Metabolic Acidosis

• Diabetic ketoacidosis
• Lactic acidosis
• Renal failure
• Toxic ingestions (methanol, ethylene glycol, salicylates)

Normal Anion Gap Metabolic Acidosis

• Diarrhea (bicarbonate loss)
• Renal tubular acidosis
• Ureterosigmoidostomy
• Early renal failure
• Hyperchloremic states

Clinical Correlation

The diagnosis must always be interpreted in the context of the patient's clinical presentation 1:

• Acute metabolic acidosis (minutes to days): Often from overproduction of organic acids (ketoacids, lactic acid)
• Chronic metabolic acidosis (weeks to years): Often reflects bicarbonate wasting or impaired renal acidification 4

Important Considerations

• Overlapping disorders can occur, with mixed acid-base disturbances present in many critically ill patients 5
• Hypoalbuminemia (common in critically ill patients) can mask an elevated anion gap if not corrected for 3
• Base excess calculation can help determine the metabolic component of acid-base disorders 2
• Treatment should always target the underlying cause rather than just correcting the pH 6, 7

The diagnosis of metabolic acidosis requires a systematic approach that includes assessment of pH, bicarbonate levels, anion gap calculation with appropriate corrections, and correlation with the clinical picture to determine the underlying etiology.