ABG Compensation Rules

Compensation rules help determine if the body’s response to acid-base disturbances is appropriate, and aid in detecting mixed disorders. Use these formulas and guidelines to interpret arterial blood gases (ABGs) with confidence.

Normal ABG Values

  • pH: 7.35 – 7.45
  • PaCO₂: 35 – 45 mmHg
  • HCO₃⁻: 22 – 26 mEq/L
  • PaO₂: 80 – 100 mmHg

Metabolic Disorders

Metabolic Acidosis — Winter’s Formula

Expected PaCO₂ = (1.5 × [HCO₃⁻]) + 8 ± 2

If measured PaCO₂ is > expected, there is a concurrent respiratory acidosis.
If measured PaCO₂ is < expected, there is a concurrent respiratory alkalosis.

Metabolic Alkalosis

Expected PaCO₂ = 40 + 0.7 × ([HCO₃⁻] − 24) ± 5

PaCO₂ should rise about 0.7 mmHg for every 1 mEq/L increase in HCO₃⁻ above 24.

Respiratory Disorders

Respiratory Acidosis

  • Acute: HCO₃⁻ increases by 1 mEq/L for every 10 mmHg ↑ PaCO₂ above 40
  • Chronic: HCO₃⁻ increases by 4 mEq/L for every 10 mmHg ↑ PaCO₂ above 40

Respiratory Alkalosis

  • Acute: HCO₃⁻ decreases by 2 mEq/L for every 10 mmHg ↓ PaCO₂ below 40
  • Chronic: HCO₃⁻ decreases by 5 mEq/L for every 10 mmHg ↓ PaCO₂ below 40

How to Apply Compensation Rules

  1. Identify the primary acid-base disorder (look at pH, PaCO₂, HCO₃⁻).
  2. Calculate expected compensation using the relevant formula above.
  3. Compare the expected and actual PaCO₂ or HCO₃⁻ to check for additional (mixed) disorders.

Related Tools & Guides

Last updated: October 2025