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), pH (acid-base balance), and bicarbonate in blood drawn from an artery to assess respiratory function, oxygenation status, ventilation adequacy, and metabolic acid-base disturbances. 1, 2

Core Components Measured

The ABG test directly measures three primary parameters and calculates additional values 2, 3:

• pH: Indicates acid-base status, with normal range 7.35-7.45. Values <7.35 indicate acidemia; >7.45 indicate alkalemia 4
• PaCO2 (partial pressure of carbon dioxide): Reflects ventilation status, with normal values 35-45 mmHg. Elevated levels indicate hypoventilation or respiratory acidosis 5, 4
• PaO2 (partial pressure of oxygen): Indicates oxygenation status, with normal values >90 mmHg on room air at sea level 6
• Bicarbonate (HCO3-): Calculated value reflecting metabolic component, with normal range 22-26 mmol/L 4
• Base excess/deficit: Indicates metabolic acid-base disturbances, with normal range -2 to +2 4

Clinical Indications for ABG Testing

Critical Care Settings

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status. 5, 7, 4 This is a Grade D recommendation from the British Thoracic Society.

Specific critical situations requiring immediate ABG analysis include 5, 7:

• Shock or hypotension (systolic blood pressure <90 mmHg) - initial blood gas must be from arterial sample 5
• Unexpected fall in oxygen saturation below 94% in patients breathing air or oxygen 5, 7
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 5, 7

Respiratory Compromise

ABG analysis is essential when patients require increased oxygen to maintain constant saturation, particularly in 5, 7:

• Patients with risk factors for hypercapnic respiratory failure who develop acute breathlessness, drowsiness, or features of CO2 retention 5
• COPD exacerbations or other fixed airflow obstruction conditions 5, 4

Metabolic Disturbances

ABG testing is indicated for patients at risk of metabolic conditions including diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, shock, and sepsis. 5, 4

Technical Considerations

Sampling Technique

• Local anesthesia should be used for all ABG specimens except in emergencies (Grade A recommendation) 5, 7
• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand from both radial and ulnar arteries 7, 6
• For critically ill patients, arterial sampling is mandatory; for most non-critical patients, arterialized earlobe blood gases may be acceptable for pH and PCO2 measurement 5, 7

Optimal Sampling Site

In patients on extracorporeal support (ECMO), arterial blood gases should come from a right radial arterial line as this best represents cerebral perfusion, though it doesn't perfectly reflect coronary blood oxygenation 5

Monitoring and Follow-up

Oxygen Therapy Titration

ABG analysis must be performed within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration, particularly in COPD patients. 7

After each titration of oxygen flow rate in patients with baseline hypercapnia, repeat ABG analysis is mandatory 7, 4

Management Thresholds

The British Thoracic Society recommends specific ABG-guided interventions 5, 4:

• Initiate non-invasive ventilation when pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) persist despite optimal medical therapy (Grade A) 5, 4
• Target oxygen saturation of 88-92% for all causes of acute hypercapnic respiratory failure 5, 4

Critical Pitfalls to Avoid

A normal oxygen saturation does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy. 5, 7 Pulse oximetry will be normal in patients with normal PaO2 but abnormal pH, PCO2, or low blood oxygen content due to anemia 5

Common errors include 7, 4:

• Failing to recognize that normal SpO2 doesn't rule out significant acid-base disturbances or hypercapnia 7, 6
• Not repeating ABG measurements after changes in oxygen therapy in patients at risk for CO2 retention 7, 4
• Relying solely on pulse oximetry when acid-base status and ventilation assessment are needed 7
• Overlooking metabolic conditions that may cause acid-base disturbances 7