ABG Interpretation Fundamentals

An ABG result sits in front of you with four numbers. If you don't know the systematic order for reading them, you'll misclassify the disorder every time.

Arterial blood gas interpretation starts with three core values: pH (normal 7.35–7.45), PaCO2 (normal 35–45 mmHg), and HCO3⁻ (normal 22–26 mEq/L). PaO2 (normal 80–100 mmHg) tells you about oxygenation but not acid-base status — keep it separate in your mind. Use a consistent three-step method. Step 1: Look at pH. Below 7.35 is acidosis; above 7.45 is alkalosis. Step 2: Check PaCO2 — this is the respiratory marker. CO2 is an acid, so a high PaCO2 means respiratory acidosis and a low PaCO2 means respiratory alkalosis. Step 3: Check HCO3⁻ — this is the metabolic marker. HCO3⁻ is a base, so a low HCO3⁻ means metabolic acidosis and a high HCO3⁻ means metabolic alkalosis. Match whichever value moves in the same direction as the pH shift to identify the primary disorder. If both PaCO2 and HCO3⁻ are abnormal, the one that explains the pH direction is the primary cause; the other reflects compensation. Master this framework first — each specific disorder and compensation pattern builds directly on these steps.

Key Distinctions

Don't confuse PaO2 with PaCO2 — PaO2 reflects oxygenation, PaCO2 reflects ventilation and acid-base. Students reverse the CO2 relationship: high CO2 means acidosis (not alkalosis) because CO2 converts to carbonic acid. A pH of 7.35–7.45 doesn't always mean normal — both values can be abnormal with full compensation, so always evaluate all three numbers.

Clinical Pearl

ROME: Respiratory Opposite (pH up, CO2 down), Metabolic Equal (pH up, HCO3⁻ up). Match the direction to find the primary culprit.