Step-by-Step ABG Interpretation

Learn a fast, repeatable approach to arterial blood gas interpretation. Then practice with ABG Practice Quizzes or test real values using the ABG Calculator.

Interpreting ABGs becomes simple when you follow a structured process. Use this 6-step framework to identify acid-base disorders, evaluate compensation, and assess oxygenation quickly and accurately.

Step 1 — Assess pH

Determine if the blood is acidic or alkaline. A pH below 7.35 indicates acidemia, while above 7.45 indicates alkalemia.

Step 2 — Check PaCO₂

PaCO₂ represents the respiratory component. High PaCO₂ = respiratory acidosis. Low PaCO₂ = respiratory alkalosis.

Step 3 — Check HCO₃⁻

HCO₃⁻ reflects the metabolic component. Low = metabolic acidosis. High = metabolic alkalosis.

Step 4 — Identify the Primary Disorder

Determine which component (PaCO₂ or HCO₃⁻) explains the pH change. That defines the primary disorder.

Step 5 — Evaluate Compensation

Compare measured values to expected compensation. Use ABG compensation rules or Winter’s formula to detect mixed disorders.

Step 6 — Assess Oxygenation

  • Normal: PaO₂ ≥ 80 mmHg
  • Mild hypoxemia: 60–79
  • Moderate hypoxemia: 40–59
  • Severe hypoxemia: < 40

Clinical Tip

Always interpret ABGs in clinical context. A normal pH does not rule out a compensated disorder—always evaluate PaCO₂ and HCO₃⁻ together.

Related Tools

Common ABG Questions

How do you interpret an ABG step-by-step?

Start by assessing pH, then evaluate PaCO₂ and HCO₃⁻ to determine if the disorder is respiratory or metabolic. Next, check compensation and finally assess oxygenation.

What are normal ABG values?

Normal ABG values include pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–29 mEq/L, and PaO₂ 80–100 mmHg.

How do you tell if an ABG is respiratory or metabolic?

Compare pH with PaCO₂ and HCO₃⁻. If PaCO₂ explains the pH change, it’s respiratory. If HCO₃⁻ explains it, it’s metabolic.

What is compensation in ABG interpretation?

Compensation is the body’s attempt to normalize pH. The lungs adjust PaCO₂ and the kidneys adjust HCO₃⁻ to counteract acid-base disturbances.

Last updated: 2026