What is the cutoff for diagnosing metabolic alkalosis in terms of bicarbonate level?

Metabolic Alkalosis Diagnostic Cutoff

Metabolic alkalosis is diagnosed when serum bicarbonate is >28 mEq/L (or >30 mEq/L depending on the reference range) with an arterial pH >7.45. 1, 2

Specific Diagnostic Thresholds

Standard Criteria

• Serum bicarbonate (HCO3-): >28 mEq/L - This represents the primary elevation that defines metabolic alkalosis 1, 3
• Arterial pH: >7.45 - The elevated pH confirms the alkalemic state 1, 2
• PaCO2: Elevated (compensatory) - Respiratory compensation occurs with hypoventilation to retain CO2 and normalize pH 1, 3

Severity Classification

The evidence provides context for severity grading based on bicarbonate levels and pH:

• Severe metabolic alkalosis:

  • Serum bicarbonate >55 mEq/L 4
  • Arterial pH >7.60 4
  • Associated with stupor, seizures, and significantly increased mortality 4, 2

• Moderate metabolic alkalosis:

  • Serum bicarbonate 40-55 mEq/L (extrapolated from clinical context)
  • pH 7.50-7.60

• Mild metabolic alkalosis:

  • Serum bicarbonate 28-40 mEq/L
  • pH 7.45-7.50

Clinical Context from Guidelines

Important caveat: The diabetic ketoacidosis guidelines provide useful reference points for normal bicarbonate ranges. In these guidelines, resolution of acidosis is defined as bicarbonate ≥18 mEq/L and pH ≥7.3, while normal bicarbonate is listed as >15 mEq/L. 5 This helps establish that bicarbonate levels above approximately 26-28 mEq/L represent the threshold for alkalosis.

Diagnostic Workup Beyond the Cutoff

Once metabolic alkalosis is identified by the bicarbonate and pH cutoffs, further evaluation requires:

• Urine chloride concentration to differentiate chloride-responsive (<20 mEq/L) from chloride-resistant (>20 mEq/L) causes 6, 1
• Serum potassium and chloride levels - hypokalemia and hypochloremia commonly accompany metabolic alkalosis 1, 3
• Assessment of volume status - extracellular volume depletion versus expansion guides treatment 1, 3

Common Pitfalls

• Do not rely on pH alone - compensatory respiratory acidosis can normalize pH despite significantly elevated bicarbonate 1, 3
• Recognize that mortality increases as pH increases - severe alkalosis (pH >7.60) requires urgent intervention 4, 2
• In renal failure patients, the kidney's ability to excrete excess bicarbonate is impaired, making even moderate elevations more clinically significant 4, 3