How do you interpret an arterial blood gas (ABG) result?

How to Interpret an Arterial Blood Gas (ABG)

Use a systematic, step-by-step approach: first assess oxygenation (PaO2 and SaO2), then determine acid-base status by evaluating pH, PaCO2, and HCO3- in that specific order. 1, 2

Step 1: Assess Oxygenation Status

• Check PaO2: Normal is >80 mmHg. Values below this indicate hypoxemia. 1
• Evaluate oxygen saturation (SaO2): Normal is >94% in most patients. 1, 2
• Calculate PaO2/FiO2 ratio to assess severity of hypoxemia, particularly in critically ill patients. 1, 2

Critical pitfall: Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia, especially if the patient is on supplemental oxygen. 3 Pulse oximetry cannot differentiate carboxyhemoglobin and may give falsely normal readings in carbon monoxide poisoning. 1, 2

Step 2: Determine Acid-Base Status

A. Evaluate pH (Normal: 7.35-7.45)

• pH <7.35 = Acidemia 1
• pH >7.45 = Alkalemia 1

B. Assess PaCO2 (Normal: 35-45 mmHg)

• PaCO2 >45 mmHg indicates respiratory acidosis 2, 3
• PaCO2 <35 mmHg indicates respiratory alkalosis 1

C. Assess HCO3- (Normal: 22-26 mEq/L)

• HCO3- <22 mEq/L indicates metabolic acidosis 1
• HCO3- >26 mEq/L indicates metabolic alkalosis 1

Step 3: Identify the Primary Disturbance

Use the "RoMe" technique (Respiratory opposite, Metabolic equal): 4

• Respiratory disorders: pH and PaCO2 move in opposite directions

  • If pH is low and PaCO2 is high = respiratory acidosis 3
  • If pH is high and PaCO2 is low = respiratory alkalosis 3

• Metabolic disorders: pH and HCO3- move in the same direction

  • If pH is low and HCO3- is low = metabolic acidosis 1
  • If pH is high and HCO3- is high = metabolic alkalosis 1

Step 4: Check for Compensation

• Uncompensated: Only one parameter (PaCO2 or HCO3-) is abnormal 4
• Partially compensated: Both PaCO2 and HCO3- are abnormal, but pH remains outside normal range 4
• Fully compensated: Both PaCO2 and HCO3- are abnormal, but pH has returned to normal range (7.35-7.45) 4

Additional Parameters to Consider

• Base deficit/excess: Sensitive marker for severity of shock and mortality risk, particularly in trauma patients. 1
• Lactate levels: Elevated levels indicate tissue hypoperfusion and shock; serial measurements help predict survival and evaluate response to therapy. 1

Common Clinical Pitfalls to Avoid

• Never rely solely on pulse oximetry when acid-base status and ventilation need assessment, as normal SpO2 can coexist with severe metabolic acidosis or hypercapnia. 3
• Always interpret PaO2 in context of FiO2: A "normal" PaO2 on high supplemental oxygen may actually represent severe hypoxemia. 3
• In COPD patients at risk for CO2 retention, always obtain ABG within 60 minutes of starting oxygen therapy or changing FiO2, as they may develop worsening hypercapnia. 3
• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand. 1, 2, 3
• Use local anesthesia for all ABG specimens except emergencies to minimize patient discomfort. 1, 2, 3