How do you interpret an Arterial Blood Gas (ABG) result?

How to Interpret Arterial Blood Gas (ABG) Results

Use a systematic three-step approach: first evaluate pH to determine acidemia versus alkalemia, then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1

Step 1: Evaluate pH (Acidemia vs Alkalemia)

• pH < 7.35 indicates acidemia 1
• pH > 7.45 indicates alkalemia 1
• This is your starting point—the pH tells you the overall acid-base status and guides the rest of your interpretation 2, 1

Step 2: Assess the Respiratory Component (PaCO2)

• PaCO2 > 45 mmHg indicates respiratory acidosis 2
• PaCO2 < 35 mmHg indicates respiratory alkalosis 1
• When PaCO2 > 45 mmHg with low pH, this confirms respiratory acidosis 1
• When PaCO2 < 35 mmHg with high pH, this confirms respiratory alkalosis 1

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

• Base excess < -2 or HCO3- < 22 indicates metabolic acidosis 1
• Base excess > +2 or HCO3- > 26 indicates metabolic alkalosis 1
• Base excess is particularly useful in trauma, shock, sepsis, and diabetic ketoacidosis to quantify metabolic acidosis and guide resuscitation 3

Step 4: Determine Compensation Status

• In primary respiratory disorders, base excess should remain normal initially 3
• In chronic respiratory disorders, base excess will change to compensate 3
• The degree of compensation helps determine if the disorder is acute, chronic, or mixed 3
• Changes in base excess over time provide valuable information about resuscitation effectiveness in critically ill patients 3

Step 5: Assess Oxygenation

• Check PaO2 and oxygen saturation (SaO2), with normal range >94% in most patients 2
• Use the PaO2/FiO2 ratio to assess severity of hypoxemia in critically ill patients 2
• Standard pulse oximetry (SpO2) cannot differentiate carboxyhemoglobin and may give falsely normal readings in carbon monoxide poisoning 2

Clinical Management Based on ABG Results

For Respiratory Acidosis (pH < 7.35, PaCO2 > 6.5 kPa/49 mmHg):

• Initiate non-invasive ventilation (NIV) when pH < 7.35 and PaCO2 > 6.5 kPa persist despite optimal medical therapy 2, 1
• Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 2, 1
• Repeat ABG after each oxygen titration to monitor for worsening hypercapnia 1

For Severe Metabolic Acidosis:

• Sodium bicarbonate therapy should be limited to patients with severe acidosis (arterial pH < 7.1 and base deficit < 10) 2

Key Clinical Indications for ABG Testing

• All critically ill patients to assess oxygenation, ventilation, and acid-base status 1
• Patients with shock or hypotension 1
• Oxygen saturation fall below 94% on room air or supplemental oxygen 1
• Suspected diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, shock, and sepsis 1
• When starting oxygen in COPD patients, especially with known CO2 retention 1

Important Technical Considerations

• In critically ill patients, arterial samples are preferred over capillary samples 2
• Use local anesthesia for all ABG specimens except in emergencies 2
• Patients undergoing radial ABG should have an Allen's test first to ensure dual blood supply to the hand 2
• In arterial samples, arterial samples are preferred, but venous blood may be used for measuring carboxyhemoglobin levels when CO body stores are in equilibrium 2

Common Pitfalls to Avoid

• Don't rely solely on pulse oximetry in suspected carbon monoxide poisoning—it will be falsely reassuring 2
• Don't forget to adjust the P(A-a)O2 cutoff to ≥20 mmHg for patients aged ≥65 years when diagnosing hepatopulmonary syndrome 1
• Always consider mixed disorders—a normal pH doesn't rule out significant acid-base disturbances if both respiratory and metabolic components are abnormal 3
• Monitor base excess trends over time rather than single values, especially in resuscitation scenarios 3