Interpreting arterial blood gases (ABGs) can seem complicated, but by following a structured approach, you can quickly identify acid-base imbalances and oxygenation status. Here’s a simple six-step process to follow every time.
Determine if the blood is acidic or alkaline. A pH below 7.35 indicates acidemia, while a pH above 7.45 indicates alkalemia. This tells you whether you’re dealing with an acidosis or alkalosis.
PaCO₂ reflects the respiratory component. If pH is low and PaCO₂ is high, the primary problem is respiratory acidosis. If pH is high and PaCO₂ is low, it’s respiratory alkalosis.
HCO₃⁻ reflects the metabolic component. A low bicarbonate indicates metabolic acidosis, while a high bicarbonate indicates metabolic alkalosis.
The direction of pH and the abnormal component (PaCO₂ or HCO₃⁻) will tell you if it’s respiratory or metabolic. The other component’s response indicates compensation.
Compare the measured PaCO₂ or HCO₃⁻ against expected compensation ranges. Use the compensation rules or Winter’s formula to determine if the response is appropriate.
Finally, assess oxygenation using PaO₂ and SaO₂. Classify hypoxemia:
Always interpret ABGs in the context of the patient’s clinical picture. A normal ABG doesn’t always mean the patient is stable — trends over time are often more meaningful than single results.
Last updated: October 2025