Arterial Blood Gas (ABG) interpretation is one of the most intimidating—but essential—topics tested on the NCLEX. Whether you love or dread lab values, ABG questions test your ability to assess a patient’s oxygenation, acid-base balance, and ventilation status.

This guide breaks down ABGs into simple, clear steps you can follow—every time.

What Is an ABG?

An ABG test measures:

• pH – acid or base level in the blood
• PaCO₂ – partial pressure of carbon dioxide (respiratory)
• HCO₃ – bicarbonate (metabolic)
• PaO₂ & SaO₂ – oxygen levels in the blood

These values tell you whether a patient is acidotic or alkalotic and whether the root cause is respiratory or metabolic.

Normal ABG Values You MUST Know

Memorizing these is step one in mastering ABGs.

Step-by-Step ABG Interpretation

Step 1: Look at pH

• pH < 7.35 = Acidosis
• pH > 7.45 = Alkalosis

Step 2: Determine Respiratory or Metabolic

• PaCO₂ abnormal? → respiratory issue
• HCO₃ abnormal? → metabolic issue

Step 3: Match the pH

• If the pH matches the direction of PaCO₂ → Respiratory
• If it matches HCO₃ → Metabolic

Step 4: Check for Compensation

• Uncompensated: pH is abnormal, and only one value is off.
• Partially compensated: all three values are off.
• Fully compensated: pH is normal, but PaCO₂ and HCO₃ are both abnormal.

Common ABG Scenarios on NCLEX

1. Respiratory Acidosis

• Cause: Hypoventilation (e.g., COPD, drug overdose)
• ABG: ↓pH, ↑PaCO₂, normal HCO₃
• Symptoms: Confusion, headache, drowsiness

2. Respiratory Alkalosis

• Cause: Hyperventilation (e.g., anxiety, fever)
• ABG: ↑pH, ↓PaCO₂, normal HCO₃
• Symptoms: Lightheadedness, numbness, tingling

3. Metabolic Acidosis

• Cause: DKA, renal failure, diarrhea
• ABG: ↓pH, normal PaCO₂, ↓HCO₃
• Symptoms: Kussmaul respirations, nausea, fatigue

4. Metabolic Alkalosis

• Cause: Vomiting, diuretics, antacids
• ABG: ↑pH, normal PaCO₂, ↑HCO₃
• Symptoms: Muscle cramps, confusion, slow respirations

NCLEX Tips for ABG Questions

• Always analyze values one at a time—don’t rush.
• Choose answers that address the underlying cause, not just symptoms.
• Look out for compensation clues—especially in chronic conditions like COPD.

Final Thoughts

ABG interpretation doesn’t have to be overwhelming. When you memorize the normal values, follow a logical step-by-step approach, and understand common causes and compensation, you’ll be ready to ace ABG questions on the NCLEX.

Keep practicing with sample questions—and remember: clarity and calm analysis will always lead you to the right answer.