When should an arterial blood gas (ABG) be ordered?

When to Order Arterial Blood Gas (ABG)

Arterial blood gas (ABG) should be ordered in all critically ill patients, patients with unexpected oxygen saturation drops below 94%, deteriorating respiratory status, risk factors for hypercapnic respiratory failure, or when blood gas results would significantly impact patient management. 1

Critical Indications for ABG Testing

• All critically ill patients require ABG measurement, with samples obtained from arterial rather than venous sources 1
• Patients in peri-arrest situations or those requiring immediate medical intervention 1
• After return of spontaneous circulation following cardiopulmonary resuscitation to guide ongoing oxygen therapy 1
• In critical illness including major trauma, sepsis, shock, and anaphylaxis 1
• Cases of carbon monoxide poisoning, where pulse oximetry readings may be falsely normal 1

Respiratory Indications

• Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen 1
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) 1
• Previously stable patients who deteriorate clinically and require increased FiO2 to maintain constant oxygen saturation 1
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating oxygen saturation, drowsiness or other features of carbon dioxide retention 1
• Major head injury cases, prior to securing the airway 1
• Acute asthma, pneumonia, and acute respiratory exacerbations requiring oxygen therapy 1

Metabolic Indications

• Patients with breathlessness who may have metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure 1
• Suspected acid-base disturbances requiring precise diagnosis and management 2

Monitoring and Follow-up Indications

• After oxygen titration is complete to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis or worsening hypercapnia 1
• Within 60 minutes of starting oxygen therapy in patients with COPD or other risk factors for hypercapnic respiratory failure 1
• Within 60 minutes of any change in inspired oxygen concentration in at-risk patients 1
• When there is an unexpected change in "track and trigger" systems such as a sudden rise in NEWS or an unexpected fall in oxygen saturation of 3% or more 1

Special Considerations

• For patients with baseline hypercapnia, ABG should be monitored after each titration of oxygen flow rate to detect respiratory acidosis 1
• In patients with COPD aged 50 years or more, ABG should be checked before giving an FiO2 of more than 28% via Venturi mask or 2 L/min via nasal cannulae 1
• For home oxygen assessment, patients should undergo two ABG measurements at least 3 weeks apart during a period of clinical stability 1

Technical Considerations

• Local anesthesia should be used for all ABG specimens except in emergencies 1
• Patients undergoing radial ABG should have an Allen's test performed first to ensure dual blood supply to the hand 1
• Informed consent should be obtained for the procedure with discussion of possible risks 1
• For most non-critical patients requiring blood gas sampling, arterialised earlobe blood gases may be used as an alternative to obtain accurate pH and PCO2 measurements, though PO2 is less accurate in these samples 1

Pitfalls and Caveats

• A normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy 1
• Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2, or with low oxygen content due to anemia 1
• In carbon monoxide poisoning, pulse oximetry readings may be falsely normal, necessitating ABG regardless of oximeter readings 1
• When using earlobe blood gas samples, oximetry should be monitored carefully and a repeat arterial specimen taken if there are concerns about accuracy 1

Remember that the requirement for increased concentration of oxygen is itself an indication for urgent clinical reassessment and usually repeat blood gas measurements 1.