Interpreting arterial blood gas (ABG) could be initially challenging for medical students / House Officers. It is important to master ABG interpretation for management of patients’ acute conditions.

Remember the normal range for ABG parameters

• pH: 7.35-7.45
• PaO2: 80-100 mmHg
• PaCO2: 35-45 mmHg
• HCO3-: 22-26 mmol/L
• Base Excess (BE): -2 to +2
• SaO2: 95-100%

Examples of what base excess (BE) tells us:

• BE +4: Metabolic alkalosis // compensated respiratory acidosis
• BE -4: Metabolic acidosis // compensated respiratory alkalosis

Step 1: Evaluate pH

• Acidaemia: Acidosis is present
• Alkalaemia: Alkalosis is present
• Normal pH: Either no acid-base issue or full compensation

Step 2: Assess respiratory (PaCO2) & metabolic (HCO3-) components

• ↑ PaCO2: Respiratory acidosis
• ↓ PaCO2: Respiratory alkalosis
• ↓ HCO3-: Metabolic acidosis
• ↑ HCO3- : Metabolic alkalosis

Step 3: Check for Compensation – Is the body trying to correct the imbalance?

• Uncompensated: pH abnormal + no compensatory change in PaCO2/HCO3
• Partially compensated: pH still abnormal but compensatory mechanism is active
• Fully compensated: pH normal + PaCO2/HCO3 altered to compensate

Some equations related to compensation:
☘️ Winter's Formula – Expected PaCO2 in metabolic acidosis = (1.5 x HCO3-) + (8±2) 

☘️ Point Seven Plus Twenty Rule – Expected PaCO2 in metabolic alkalosis = (0.7 x HCO3-) + (20±5) 

☘️ 1-2-4-5 Rule – Expected HCO3- changes based on acute vs. chronic respiratory acidosis/alkalosis

Step 4: Evaluate PaO2 for oxygenation status

• Type 1 respiratory failure: PaO2 <60 mmHg + normal/low PaCO2
• Type 2 respiratory failure: hypoxaemia + hypercapnia (PaCO2 >45 mmHg)

Step 5: Final impression

• Once all elements are assessed, define the overall disorder (e.g., partially compensated metabolic acidosis).