How to determine if an Arterial Blood Gas (ABG) result is compensated?

Determining ABG Compensation Status

To determine if an ABG is compensated, check if the pH has returned to normal range (7.35-7.45) while both the respiratory (PaCO2) and metabolic (HCO3/base excess) components remain abnormal in opposite directions. 1, 2

Systematic Three-Step Approach to ABG Interpretation

Step 1: Evaluate the pH 1, 2

• pH < 7.35 = acidemia
• pH > 7.45 = alkalemia
• pH 7.35-7.45 = normal (suggests either no disorder, fully compensated disorder, or mixed disorder)

Step 2: Identify the Respiratory Component 1, 2

• PaCO2 > 45 mmHg with low pH = respiratory acidosis
• PaCO2 < 35 mmHg with high pH = respiratory alkalosis
• If PaCO2 moves opposite to pH, this indicates respiratory compensation for a metabolic disorder

Step 3: Identify the Metabolic Component 1, 2

• Base excess < -2 or HCO3 < 22 mmol/L = metabolic acidosis
• Base excess > +2 or HCO3 > 26 mmol/L = metabolic alkalosis
• If HCO3/base excess moves opposite to pH, this indicates metabolic compensation for a respiratory disorder

Determining Degree of Compensation

Uncompensated: 3, 4

• pH is abnormal
• Only one system (respiratory OR metabolic) is abnormal
• The other system remains normal
• Example: pH 7.28, PaCO2 55 mmHg, HCO3 24 mmol/L = uncompensated respiratory acidosis

Partially Compensated: 3, 4

• pH remains abnormal (still outside 7.35-7.45)
• Both PaCO2 AND HCO3 are abnormal
• Both systems are moving in opposite directions to correct the pH
• Example: pH 7.32, PaCO2 55 mmHg, HCO3 28 mmol/L = partially compensated respiratory acidosis

Fully Compensated: 3, 4

• pH has normalized (7.35-7.45)
• Both PaCO2 AND HCO3 remain abnormal
• The compensating system has successfully returned pH to normal range
• Example: pH 7.38, PaCO2 55 mmHg, HCO3 32 mmol/L = fully compensated respiratory acidosis
• Critical pitfall: The primary disorder is identified by which side of 7.40 the pH falls (7.35-7.39 suggests primary acidosis; 7.41-7.45 suggests primary alkalosis) 3

Distinguishing Acute vs Chronic Disorders

In chronic respiratory disorders, base excess changes to compensate, whereas in acute disorders it remains initially normal. 3 This distinction is particularly important when:

• Monitoring patients with baseline hypercapnia during oxygen therapy—a rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring reassessment 5, 2
• Evaluating COPD patients, where chronic CO2 retention leads to metabolic compensation with elevated HCO3 2
• Assessing trauma or shock patients, where base excess helps quantify acute metabolic acidosis 3

Common Clinical Pitfalls to Avoid

Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia—always obtain ABG when clinically indicated, not just pulse oximetry. 2

Failing to repeat ABG measurements after oxygen therapy changes in patients at risk for CO2 retention is a critical management error, especially in those with baseline hypercapnia who require monitoring after each flow rate titration. 5, 2