ABG Interpretation Step-by-Step

An arterial blood gas (ABG) measures the pH, partial pressures of oxygen and carbon dioxide, and bicarbonate levels in arterial blood. These values reflect how well the lungs and kidneys are maintaining acid-base balance and oxygenation.

ABGs are ordered for patients with respiratory distress, altered mental status, hemodynamic instability, suspected acid-base disorders, ventilator management, and monitoring of critically ill patients.

Step 1 — Assess pH

• pH < 7.35: Acidosis
• pH 7.35 – 7.45: Normal (may still be compensated)
• pH > 7.45: Alkalosis

The pH tells you the overall acid-base status. Always identify this first before proceeding.

Step 2 — Assess PaCO₂ (Respiratory Component)

• PaCO₂ > 45 mmHg: Respiratory acidosis (hypoventilation — CO₂ retained)
• PaCO₂ 35 – 45 mmHg: Normal respiratory
• PaCO₂ < 35 mmHg: Respiratory alkalosis (hyperventilation — CO₂ blown off)

CO₂ is an acid. High CO₂ = more acid = lower pH. Low CO₂ = less acid = higher pH.

Step 3 — Assess HCO₃¹ (Metabolic Component)

• HCO₃¹ < 22 mEq/L: Metabolic acidosis (bicarbonate depleted)
• HCO₃¹ 22 – 26 mEq/L: Normal metabolic
• HCO₃¹ > 26 mEq/L: Metabolic alkalosis (excess bicarbonate)

HCO₃¹ is a base. Low HCO₃¹ = less base = lower pH. High HCO₃¹ = more base = higher pH.

Step 4 — Determine the Primary Disorder

Match the pH change with either PaCO₂ or HCO₃¹ to identify the primary cause:

• If pH is low and PaCO₂ is high → Respiratory acidosis
• If pH is low and HCO₃¹ is low → Metabolic acidosis
• If pH is high and PaCO₂ is low → Respiratory alkalosis
• If pH is high and HCO₃¹ is high → Metabolic alkalosis

The component that “matches” the pH direction is the primary problem. If both are abnormal, the one that explains the pH is primary.

Step 5 — Evaluate Compensation

Compensation is the body's attempt to restore pH toward normal. The compensating system moves in the same direction as the primary problem:

• Respiratory acidosis: Kidneys retain HCO₃¹ to raise pH (metabolic compensation; takes 2–5 days)
• Respiratory alkalosis: Kidneys excrete HCO₃¹ to lower pH
• Metabolic acidosis: Lungs increase ventilation to blow off CO₂ (minutes to hours)
• Metabolic alkalosis: Lungs decrease ventilation to retain CO₂

Step 6 — Evaluate Oxygenation

• PaO₂ 80 – 100 mmHg: Normal oxygenation
• PaO₂ 60 – 79 mmHg: Mild hypoxemia
• PaO₂ 40 – 59 mmHg: Moderate hypoxemia
• PaO₂ < 40 mmHg: Severe hypoxemia — critical value, notify provider immediately
• SaO₂ < 90%: Hypoxemia; urgent assessment and intervention required

Note: Acid-base interpretation and oxygenation assessment are two separate analyses. A patient can have a normal pH and still be hypoxemic.

• Critical values require immediate notification: pH < 7.20 or > 7.60; PaCO₂ > 60 mmHg; PaO₂ < 50 mmHg; HCO₃¹ < 15 mEq/L
• Compare to prior ABGs when available — trending values often provides more information than a single result
• Assess the patient first. An abnormal ABG must be interpreted alongside respiratory effort, level of consciousness, hemodynamics, and clinical presentation
• Radial artery sampling requires Allen's test prior to arterial puncture to confirm collateral circulation
• Ventilator patients: Notify respiratory therapy and provider of significant ABG changes — vent settings may need adjustment
• Document: Time drawn, patient's current FiO₂ or supplemental oxygen, temperature, and clinical status at time of draw

• Always identify the pH first — this establishes the direction of the primary disorder
• The component that “matches” the pH (both low or both high) is responsible for the primary problem
• Compensation never overcorrects — pH returns toward normal but does not cross to the other side in simple disorders
• Respiratory compensation for metabolic disorders is rapid (minutes to hours); metabolic compensation for respiratory disorders is slow (2–5 days)
• ROME mnemonic: Respiratory Opposite (pH and PaCO₂ move opposite); Metabolic Equal (pH and HCO₃¹ move same direction)
• Kussmaul respirations = deep, rapid breathing = compensating for metabolic acidosis
• A patient with COPD may have a chronically elevated PaCO₂ with compensatory elevated HCO₃¹ — this is their baseline, not an acute disorder