ABG Compensation Rules

Compensation rules help determine whether the body’s response to an acid-base disturbance is appropriate and help uncover mixed disorders. Use these formulas and patterns to interpret arterial blood gases with more confidence.

Want to apply these rules in real cases? Use the ABG Calculator for live interpretation or practice with ABGenius.

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 greater than expected, there is concurrent respiratory acidosis.
If measured PaCO₂ is less than expected, there is 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 increase in PaCO₂ above 40
  • Chronic: HCO₃⁻ increases by 4 mEq/L for every 10 mmHg increase in PaCO₂ above 40

Respiratory Alkalosis

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

How to Apply Compensation Rules

  1. Identify the primary acid-base disorder by looking at pH, PaCO₂, and HCO₃⁻.
  2. Calculate expected compensation using the appropriate formula.
  3. Compare the expected and actual PaCO₂ or HCO₃⁻ to look for mixed disorders.

Clinical Tip

Compensation helps move pH toward normal, but it does not usually fully normalize the disturbance unless the process is chronic or fully compensated. If the measured value falls outside the expected compensation range, think about a second acid-base process.

Related Tools & Guides

Last updated: 2026