What is an Arterial Blood Gas (ABG)?

What is an Arterial Blood Gas (ABG)?

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

Core Components Measured

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

• pH: Indicates the acid-base status of blood, with normal range 7.35-7.45; values below 7.35 indicate acidemia and above 7.45 indicate alkalemia 3, 4
• PaCO2 (partial pressure of carbon dioxide): Reflects ventilation status, with normal range 35-45 mmHg; elevated levels indicate respiratory acidosis while decreased levels indicate respiratory alkalosis 3, 5
• PaO2 (partial pressure of oxygen): Indicates oxygenation status and helps identify respiratory failure 2
• Bicarbonate (HCO3) and base excess: Calculated values that reflect the metabolic component of acid-base balance, with normal bicarbonate 22-26 mmol/L 3, 4

Clinical Purpose and Applications

ABG analysis serves multiple critical diagnostic functions:

• Assessment of critically ill patients: The American College of Chest Physicians recommends ABG testing for all critically ill patients to evaluate oxygenation, ventilation, and acid-base status 4, 5
• Diagnosis of acid-base disorders: ABG enables identification of respiratory acidosis/alkalosis, metabolic acidosis/alkalosis, and mixed disorders including diabetic ketoacidosis, renal failure, sepsis, and shock 3, 2
• Monitoring oxygen therapy: After initiating or adjusting oxygen therapy, ABG should be performed within 60 minutes to ensure adequate oxygenation without precipitating respiratory acidosis or worsening hypercapnia 5
• Guiding ventilation management: ABG results directly inform decisions about initiating non-invasive ventilation, adjusting mechanical ventilation parameters, or proceeding to intubation 3, 4

Systematic Interpretation Approach

The American Thoracic Society recommends a three-step method for interpreting ABG results:

• Step 1: Evaluate pH to determine if acidemia (pH < 7.35) or alkalemia (pH > 7.45) is present 3, 4
• Step 2: Examine PaCO2 to identify the respiratory component; PaCO2 > 45 mmHg with low pH indicates respiratory acidosis, while PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 3
• Step 3: Evaluate base excess and bicarbonate to identify the metabolic component; base excess < -2 or HCO3 < 22 indicates metabolic acidosis, while base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 3

Key Clinical Indications

When to order an ABG:

• All patients with shock or hypotension should have initial blood gas sampling from an arterial source 4, 5
• Oxygen saturation falling below 94% on room air or supplemental oxygen 4, 5
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 5
• Suspected metabolic conditions such as diabetic ketoacidosis or metabolic acidosis from renal failure, trauma, or sepsis 3, 5
• Patients with COPD or known CO2 retention when starting oxygen therapy 4, 5
• After each titration of oxygen flow rate in patients with baseline hypercapnia 4, 5

Technical Considerations

Sampling technique:

• The radial artery is the most common site; perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 4, 5
• Either arterial or venous blood may be used for measuring carboxyhemoglobin levels, as COHb levels are similar in both 6
• Local anesthesia should be used for all ABG specimens except in emergencies 5
• For non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation 4, 5

Critical Pitfalls to Avoid

Common errors in ABG interpretation and management:

• Normal oxygen saturation does not rule out acid-base disturbances: Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status, ventilation problems, or low blood oxygen content due to anemia 5
• Standard pulse oximeters cannot differentiate carboxyhemoglobin: Two-wavelength pulse oximeters measure COHb similarly to oxyhemoglobin, potentially masking carbon monoxide poisoning even when COHb levels exceed 25% 6
• Older blood gas machines may report falsely normal oxygen saturation: Machines without CO oximeters calculate oxygen saturation from PaO2 and pH rather than directly measuring it, which can report 97-98% saturation even when 40% of hemoglobin is bound to carbon monoxide 6
• Failing to repeat ABG after oxygen therapy changes: This is particularly dangerous in patients at risk for CO2 retention, where a rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further optimization 4, 5

Alternative Monitoring Methods

When repeated ABG sampling is needed:

• Capillary blood gases (CBG) can replace ABG for re-measuring PaCO2 and pH during oxygen titration 4
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 4
• Fingertip pulse CO oximetry can measure carboxyhemoglobin at the scene, though laboratory-based spectrophotometry should confirm results for patients being considered for hyperbaric oxygen therapy 6