How should I interpret an arterial blood gas analysis?

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Last updated: October 23, 2026 • View editorial policy

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How to Analyze an Arterial Blood Gas (ABG)

Use a systematic stepwise approach to ABG interpretation, starting with assessment of oxygenation (PaO₂ and SpO₂), followed by pH to identify acidemia or alkalemia, then PaCO₂ to determine respiratory contribution, and finally bicarbonate (HCO₃⁻) to assess metabolic components, while examining the PaCO₂-to-HCO₃⁻ ratio to identify mixed disorders 1, 2.

When to Obtain an ABG

Blood gases are indicated in 3:

  • All critically ill patients
  • Patients with shock or hypotension (systolic BP <90 mmHg) - use arterial sample
  • Unexpected SpO₂ fall below 94% in patients on air or oxygen
  • Deteriorating oxygen saturation (≥3% fall) or increasing breathlessness in chronic hypoxemia
  • Patients requiring increased FiO₂ to maintain constant saturation
  • Risk factors for hypercapnic respiratory failure with acute breathlessness, drowsiness, or CO₂ retention features

Use local anesthesia for all ABG specimens except emergencies 3. For most patients, either arterial or arterialized earlobe samples work for pH and PaCO₂, though earlobe samples underestimate PaO₂ by 0.5-1 kPa 3.

Systematic Interpretation Algorithm

Step 1: Assess Oxygenation

  • PaO₂ and SpO₂: Determine if hypoxemia is present
  • Critical caveat: Normal SpO₂ does not exclude acid-base disturbances, especially if patient is on supplemental oxygen 3
  • Pulse oximetry will be normal with adequate PaO₂ but can miss abnormal pH, PaCO₂, or anemia

Step 2: Evaluate pH

  • pH <7.35: Acidemia present
  • pH >7.45: Alkalemia present
  • pH 7.35-7.45: Normal, but check for compensated disorders

Step 3: Determine Respiratory Component (PaCO₂)

  • PaCO₂ >6.0-6.5 kPa (45-49 mmHg): Respiratory acidosis or compensation for metabolic alkalosis
  • PaCO₂ <4.7 kPa (35 mmHg): Respiratory alkalosis or compensation for metabolic acidosis
  • For acute hypercapnic respiratory failure: PaCO₂ ≥6.5 kPa with pH <7.35 after optimal medical therapy indicates need for NIV 4

Step 4: Assess Metabolic Component (HCO₃⁻)

  • HCO₃⁻ <22 mEq/L: Metabolic acidosis or compensation for respiratory alkalosis
  • HCO₃⁻ >26 mEq/L: Metabolic alkalosis or compensation for respiratory acidosis

Step 5: Examine the PaCO₂-to-HCO₃⁻ Ratio

This is critical for identifying mixed disorders 1:

  • Plot paired values on a coordinate system to visually define the acid-base disorder
  • Evaluate whether compensation is appropriate or if multiple disorders coexist
  • The ratiometric relationship between lungs (volatile acid as CO₂) and kidneys (plasma bicarbonate maintenance) allows precise identification of dominant disturbances

Clinical Application for Hypercapnic Respiratory Failure

For patients with type 2 respiratory failure 4:

  • PaCO₂ between 6.0-6.5 kPa is unlikely to contribute significantly to acidosis
  • Provide optimal medical care and controlled flow oxygen (targeting SpO₂ 88-92% in at-risk patients) 3
  • Recheck ABG after 60 minutes of optimal therapy
  • Initiate NIV if pH <7.35, PaCO₂ ≥6.5 kPa, and RR >23 breaths/min persist 4

Key Pitfalls to Avoid

  1. Don't rely solely on pulse oximetry: Normal SpO₂ can mask significant pH or PaCO₂ abnormalities, particularly in patients on supplemental oxygen 3

  2. Distinguish respiratory acidosis from mixed metabolic-respiratory acidosis: Limit NIV use when large metabolic contribution exists; confirm persisting respiratory acidosis with repeat ABG 4

  3. Account for oxygen therapy: Use appropriate target ranges (88-92% for hypercapnic risk, 94-98% if PaCO₂ normal) 3

  4. Consider timing: Allow 30-60 minutes after oxygen adjustment before rechecking blood gases to assess true response 3

  5. Recognize incomplete compensation: True compensation rarely returns pH completely to normal; if pH is normal with abnormal PaCO₂ and HCO₃⁻, suspect mixed disorder 1

References

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.