What is an Arterial Blood Gas (ABG) test?

What is an Arterial Blood Gas (ABG) Test?

An arterial blood gas (ABG) test is a diagnostic procedure that measures the levels of oxygen (PaO2), carbon dioxide (PaCO2), and pH (acid-base balance) in arterial blood to assess a patient's oxygenation, ventilation, and metabolic status. 1, 2

Core Components Measured

ABG analysis directly measures three primary values and calculates additional parameters:

• pH: Determines acidemia (pH < 7.35) or alkalemia (pH > 7.45), reflecting the body's acid-base balance 1
• PaCO2 (Partial Pressure of Carbon Dioxide): Indicates ventilation status, with values > 45 mmHg suggesting respiratory acidosis and < 35 mmHg suggesting respiratory alkalosis 1
• PaO2 (Partial Pressure of Oxygen): Assesses oxygenation adequacy, with values affected by hyperventilation and hypoventilation 2
• Bicarbonate (HCO3) and Base Excess: Calculated values that identify metabolic components, where HCO3 < 22 or base excess < -2 indicates metabolic acidosis, while HCO3 > 26 or base excess > +2 indicates metabolic alkalosis 1

Primary Clinical Indications

The British Thoracic Society and American College of Physicians recommend ABG testing in specific clinical scenarios:

• All critically ill patients to assess oxygenation, ventilation, and acid-base status 1, 3, 4
• Shock or hypotension, where initial blood gas measurement should be obtained from an arterial sample 3, 4
• Oxygen saturation falling below 94% on room air or supplemental oxygen 3
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 3
• Suspected metabolic conditions including diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, shock, and sepsis 1, 3

Systematic Interpretation Approach

The American Thoracic Society recommends a three-step systematic approach:

1. Evaluate pH first to determine if acidemia or alkalemia is present 1
2. Examine PaCO2 to identify the respiratory component of the disorder 1
3. Evaluate base excess/bicarbonate to identify the metabolic component 1

The degree of compensation helps determine if the acid-base disorder is acute, chronic, or mixed 4

Special Populations and Monitoring Requirements

COPD and hypercapnic patients require particular attention:

• ABG should be checked when starting oxygen therapy, especially in patients with known CO2 retention 3, 4
• After oxygen titration, ABG must be performed within 60 minutes to ensure adequate oxygenation without precipitating respiratory acidosis 3
• Patients with baseline hypercapnia require ABG monitoring after each titration of oxygen flow rate 1, 3

Oxygen therapy titration protocol per British Thoracic Society guidelines:

• Start at 1 L/min and titrate up in 1 L/min increments until SpO2 > 90% 5
• Perform ABG to confirm target PaO2 ≥ 8 kPa (60 mmHg) at rest has been achieved 5

Technical Considerations

Pre-procedure requirements:

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 5, 3
• Obtain informed consent with discussion of possible risks 5
• Use local anesthesia for all ABG specimens except in emergencies 3

Alternative sampling methods:

• Capillary blood gases (CBG) can substitute for ABG when re-measuring PaCO2 and pH at different oxygen flow rates during oxygen titration 5
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 5

Critical Pitfalls to Avoid

• Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia, as pulse oximetry will be normal in patients with abnormal acid-base status or ventilation 3
• Pulse oximetry cannot detect low blood oxygen content due to anemia or assess ventilation status 3
• Failing to repeat ABG after changes in oxygen therapy in patients at risk for CO2 retention can miss dangerous hypercapnia 3
• Relying solely on pulse oximetry when acid-base status and ventilation need assessment is inadequate 3

Management of Abnormal Results

For acute hypercapnic respiratory failure:

• The British Thoracic Society recommends initiating non-invasive ventilation (NIV) for pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 1
• Controlled oxygen therapy targeting SpO2 88-92% is recommended for COPD and all causes of acute hypercapnic respiratory failure 1

For patients developing respiratory acidosis during assessment:

• A rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization and reassessment after 4 weeks 5
• If this occurs on two repeated occasions while apparently clinically stable, domiciliary oxygen should only be ordered in conjunction with nocturnal ventilatory support 5