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

When to Order Arterial Blood Gas (ABG)

Arterial blood gas analysis should be ordered in all critically ill patients, patients with unexpected oxygen desaturation below 94%, deteriorating respiratory status, suspected acid-base disturbances, or when monitoring response to respiratory interventions. 1

Primary Indications for ABG Testing

Critical Illness

• All critically ill patients require ABG analysis to assess oxygenation, ventilation, and acid-base status 1
• Initial blood gas measurement should be obtained from an arterial sample in patients with shock or hypotension (systolic blood pressure <90 mm Hg) 1
• Use the highest feasible inspired oxygen during cardiopulmonary resuscitation, then obtain ABG once spontaneous circulation returns 1

Respiratory Compromise

• 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
• Most previously stable patients who deteriorate clinically and require increased fraction of inspired oxygen (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

Metabolic Disturbances

• Patients with breathlessness who are thought to be at risk of metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure 1
• Suspected acid-base diseases including severe sepsis, septic shock, hypovolemic shock, and diverse metabolic diseases 2

Monitoring and Titration

• After oxygen titration to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis and/or worsening hypercapnia 1
• Within 60 minutes of starting oxygen therapy and within 60 minutes of a change in inspired oxygen concentration in COPD patients 1
• After each titration of oxygen flow rate in patients with baseline hypercapnia 1

Special Considerations

COPD and Risk of Hypercapnia

• For patients with COPD or other conditions causing fixed airflow obstruction, ABG should be checked when:
  • Starting oxygen therapy, especially if the patient has known CO2 retention 1
  • Increasing oxygen requirements despite stable clinical status 1
  • Developing drowsiness, headache, or flushing (signs of CO2 retention) 1

Normal SpO2 Limitations

• 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 be normal in patients with normal PaO2 but abnormal pH or PaCO2, or with low blood oxygen content due to anemia 1
• Oximetry has limitations and cannot detect hypercapnia or acidosis 1, 2

Technical Considerations

• Local anesthesia should be used for all ABG specimens except in emergencies 1
• For most non-critical patients, either arterial blood gases or arterialized earlobe blood gases may be used to measure pH and PCO2 1
• Patients undergoing radial ABG should be assessed with an Allen's test first to ensure dual blood supply to the hand 1

Avoiding Unnecessary ABGs

• Implementation of evidence-based protocols for ABG ordering can reduce unnecessary testing without affecting patient outcomes 3
• Routine daily ABGs without clinical indication should be avoided 3
• Consider using capillary blood gases for re-measuring PaCO2 and pH during oxygen titration when appropriate 1

Common Pitfalls

• Failing to recognize that a normal SpO2 does not rule out significant acid-base disturbances or hypercapnia 1, 2
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 1
• Not repeating ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 1
• Relying solely on pulse oximetry in situations where acid-base status and ventilation need to be assessed 1, 2

By following these evidence-based guidelines for ABG ordering, clinicians can appropriately assess patients' respiratory and metabolic status while avoiding unnecessary testing.