What is an easy way to interpret an arterial blood gas (ABG) result?

How to Interpret Arterial Blood Gas (ABG) Results: A Systematic Approach

Use a three-step systematic method to interpret ABG results: first evaluate pH to determine acidemia or alkalemia, then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1, 2

Step 1: Evaluate the pH

• pH < 7.35 indicates acidemia 2
• pH > 7.45 indicates alkalemia 2
• pH 7.35-7.45 is normal (but may still have compensated disorders) 2

This first step tells you the overall acid-base status and guides your interpretation of the remaining values.

Step 2: Examine PaCO2 (Respiratory Component)

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 2
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 2
• Normal PaCO2 is 35-45 mmHg 2

The key principle here is that CO2 moves in the opposite direction of pH - when CO2 is high, pH drops (acidosis), and when CO2 is low, pH rises (alkalosis). 3

Step 3: Evaluate HCO3/Base Excess (Metabolic Component)

• Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 2
• Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 2
• Normal HCO3 is 22-26 mmol/L 2

The metabolic component moves in the same direction as pH - when bicarbonate is low, pH drops (acidosis), and when bicarbonate is high, pH rises (alkalosis). 3

The "RoMe" Memory Aid

"Respiratory opposite, Metabolic equal" - This simple mnemonic helps you remember that respiratory changes move opposite to pH, while metabolic changes move in the same direction as pH. 3

Step 4: Calculate Delta Ratio (When Anion Gap Elevated)

When you identify metabolic acidosis with an elevated anion gap, calculate the delta ratio as (Anion Gap - 12) / (24 - HCO₃⁻) to identify mixed acid-base disorders. 1

• Delta ratio < 1 suggests concurrent normal anion gap metabolic acidosis 1
• Delta ratio > 2 suggests concurrent metabolic alkalosis 1

This step is particularly important in critically ill patients where multiple pathophysiologic processes may coexist. 1

Assess Oxygenation

• PaO2 < 60 mmHg indicates significant hypoxemia requiring intervention 2
• Target SpO2 88-92% for COPD and hypercapnic respiratory failure 2, 4
• Target SpO2 94-98% for most other patients 4

A critical pitfall: normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia, especially in patients on supplemental oxygen. 1, 4

Determine Compensation Status

• Uncompensated: pH abnormal, only one component (respiratory or metabolic) abnormal 3
• Partially compensated: pH still abnormal, but both respiratory and metabolic components are abnormal in compensatory directions 3
• Fully compensated: pH normalized, both components abnormal 3

Common Clinical Scenarios

Respiratory Acidosis (High CO2, Low pH)

• Initiate non-invasive ventilation for pH < 7.35 and PaCO2 > 49 mmHg despite optimal medical therapy 1, 2
• Common causes: COPD exacerbation, neuromuscular weakness, oversedation 4

Metabolic Acidosis (Low HCO3, Low pH)

• Check anion gap to differentiate causes 1
• Consider diabetic ketoacidosis, renal failure, lactic acidosis, or toxins 4

Mixed Disorders

• When pH is severely abnormal despite apparent compensation, suspect a mixed disorder 1
• Calculate delta ratio to confirm 1

Critical Management Points

• Repeat ABG within 60 minutes after starting oxygen therapy or changing FiO2 in COPD patients 4
• After each oxygen titration in patients with baseline hypercapnia, perform ABG to monitor for worsening CO2 retention 1, 4
• A rise in PaCO2 > 7.5 mmHg indicates clinically unstable disease requiring further optimization 1

Technical Considerations

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand 1, 4
• Either arterial or venous blood can measure COHb levels (they are similar) 5
• Use local anesthesia for all non-emergency ABG sampling 4