ABG Interpretation Made Easy | 6-Step Approach

Full transcript

Interpreting an arterial blood gas can feel overwhelming, but with a fixed six-step sequence, it becomes completely manageable. Today we will walk through every step using a real clinical example so you can apply this approach confidently at the bedside. Our six steps are: check the pH, check the PaCO₂, check the bicarbonate, assess compensation, calculate the anion gap, and finally look for a mixed disorder. Following these steps in order every single time prevents mistakes and keeps your thinking clear and systematic. Step one: check the pH. A pH below 7.35 means acidemia, and a pH above 7.45 means alkalemia. In our example the pH is 7.25, which is clearly below the normal range, so we are dealing with acidemia, and the primary process must be an acidosis. Steps two and three identify the primary disorder. PaCO₂ is 28, which is low, pointing toward respiratory alkalosis, but bicarbonate is also low at 13, pointing toward metabolic acidosis. Because the pH is low and the bicarbonate is low, the primary disorder is metabolic acidosis, and the low PaCO₂ is compensation. Step four checks compensation using Winter's formula. We multiply bicarbonate by 1.5 and add 8, giving an expected PaCO₂ of 27.5 plus or minus 2. The actual PaCO₂ is 28, which falls right inside that range, confirming that respiratory compensation is appropriate and no second primary disorder is hiding here. Step five calculates the anion gap using sodium minus the sum of chloride and bicarbonate, giving us 27, which is well above the normal upper limit of 12. Step six uses the delta ratio to screen for a mixed metabolic disorder. Our final interpretation is high-anion-gap metabolic acidosis with appropriate respiratory compensation — a complete and confident ABG read in six steps.