ABG Interpretation Guide

Step-by-Step ABG Interpretation

Learn a fast, repeatable approach to arterial blood gas interpretation using a simple 6-step framework used by RTs, ICU nurses, and critical care students. Then practice with ABG Practice Quizzes or test real values using the ABG Calculator.

What this guide teaches

This page teaches a fast bedside framework for ABG interpretation: identify acidemia vs alkalemia, determine whether the primary process is respiratory or metabolic, evaluate compensation, and classify oxygenation in under 60 seconds.

  • • Built for RT, RN, PA, NP, and ICU learners
  • • Focused on real bedside ABG interpretation
  • • Designed for exams, rounds, and clinical practice
  • • Pairs with ABG drills and compensation tools

Interpreting ABGs becomes much easier when you follow the same structured process every time. Instead of trying to memorize isolated values, use this stepwise ABG interpretation method to quickly identify acid-base disorders, evaluate compensation, and assess oxygenation with confidence.

Step 1 — Assess pH

Start with pH to determine whether the blood is acidic or alkaline. A pH below 7.35 indicates acidemia. A pH above 7.45 indicates alkalemia. This tells you whether the primary process is pushing the patient toward acidosis or alkalosis.

Step 2 — Check PaCO₂

PaCO₂ reflects the respiratory component of acid-base balance. High PaCO₂ suggests respiratory acidosis from hypoventilation. Low PaCO₂ suggests respiratory alkalosis from hyperventilation. Ask whether the CO₂ change explains the pH.

Step 3 — Check HCO₃⁻

HCO₃⁻ reflects the metabolic component. Low bicarbonate suggests metabolic acidosis. High bicarbonate suggests metabolic alkalosis. Compare the bicarbonate trend to the pH and determine whether it explains the acid-base disturbance better than PaCO₂.

Step 4 — Identify the Primary Disorder

Determine which variable best explains the pH shift. If PaCO₂ explains the pH change, the primary disorder is respiratory. If HCO₃⁻ explains it, the primary disorder is metabolic. This is the core step in ABG interpretation and defines the primary acid-base disorder.

Step 5 — Evaluate Compensation

Next, determine whether the opposite system is compensating appropriately. Respiratory disorders should trigger renal compensation through bicarbonate change. Metabolic disorders should trigger respiratory compensation through ventilation. Use ABG compensation rules or Winter’s formula to detect mixed disorders.

Step 6 — Assess Oxygenation

Finally, assess oxygenation using PaO₂. This step does not define acid-base status, but it tells you how severe the patient’s oxygenation failure is and whether immediate escalation may be needed.

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

Clinical Tip

Always interpret ABGs in clinical context. A normal pH does not rule out pathology. Patients with compensated respiratory failure, chronic CO₂ retention, or mixed acid-base disorders may have a near-normal pH with clinically important abnormalities in both PaCO₂ and HCO₃⁻.

Common ABG mistakes to avoid

  • Do not diagnose from pH alone — always compare PaCO₂ and HCO₃⁻.
  • Do not ignore compensation — mixed disorders are common in ICU patients.
  • Do not confuse oxygenation failure with acid-base failure.
  • Do not assume a normal pH means the ABG is normal.

Related ABG Tools

Common ABG Questions

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

Start with pH, then evaluate PaCO₂ and HCO₃⁻ to determine whether the primary disorder is respiratory or metabolic. Next, assess compensation and finish by checking oxygenation.

What are normal ABG values?

Normal ABG values are 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 shift, it is respiratory. If HCO₃⁻ explains the pH shift, it is metabolic.

What is compensation in ABG interpretation?

Compensation is the body’s attempt to normalize pH. The lungs compensate for metabolic disorders by changing ventilation, while the kidneys compensate for respiratory disorders by adjusting bicarbonate.

Last updated: 2026