ABG Interpretation Guide: How to Analyze Arterial Blood Gases

Learn a fast, reliable approach to ABG interpretation. Then practice with ABG Practice Quizzes and our Real ABG Analyzer.

1. What Is an ABG?

An arterial blood gas (ABG) measures oxygenation (PaO₂, SaO₂), ventilation (PaCO₂), and acid–base balance (pH, HCO₃⁻). It’s commonly used in the ICU, ED, OR, and during ventilator management.

Samples are usually drawn from the radial artery using a heparinized syringe and analyzed promptly to ensure accuracy.

2. Normal ABG Values (Sea Level, Room Air)

ParameterNormal RangeRepresents
pH7.35 – 7.45Acid–base balance
PaCO₂35 – 45 mmHgRespiratory component
HCO₃⁻22 – 29 mEq/LMetabolic component
PaO₂80 – 100 mmHgOxygenation
SaO₂95 – 100%Oxygen saturation

Values shift with altitude and chronic lung disease; interpret in clinical context.

3. Step-by-Step ABG Interpretation

  1. Assess pH — acidemia (< 7.35) or alkalemia (> 7.45)?
  2. Look at PaCO₂ — high = respiratory acidosis; low = respiratory alkalosis.
  3. Look at HCO₃⁻ — low = metabolic acidosis; high = metabolic alkalosis.
  4. Identify the primary disorder — which component drives the pH change?
  5. Check for compensation — partial vs full, using rules below.
  6. Evaluate oxygenation — PaO₂ in the context of FiO₂ and clinical status.

Try it hands-on with the Real ABG Analyzer.

4. Common ABG Patterns

DisorderpHPaCO₂HCO₃⁻Example
Uncompensated Respiratory Acidosis↓↑NormalCOPD exacerbation
Fully Compensated Respiratory AcidosisNormal↑↑Chronic CO₂ retainer
Metabolic Acidosis↓↓↓DKA, lactic acidosis
Metabolic Alkalosis↑↑↑Vomiting, diuretics
Respiratory Alkalosis↑↓↓Anxiety, hypoxemia

Practice these patterns in the ABG Practice Quiz.

5. Compensation Rules (Clinically Useful)

Metabolic Acidosis → Respiratory Compensation

Use Winter’s Formula:

Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

If actual PaCO₂ is higher than expected → superimposed respiratory acidosis. If lower → respiratory alkalosis.

Metabolic Alkalosis → Respiratory Compensation

PaCO₂ rises by about 0.7 mmHg for every 1 mEq/L increase in HCO₃⁻ above 24.

Respiratory Disorders → Renal Compensation

  • Acute Respiratory Acidosis: HCO₃⁻ ↑ ~1 mEq/L per 10 mmHg ↑ PaCO₂
  • Chronic Respiratory Acidosis: HCO₃⁻ ↑ ~4 mEq/L per 10 mmHg ↑ PaCO₂
  • Acute Respiratory Alkalosis: HCO₃⁻ ↓ ~2 mEq/L per 10 mmHg ↓ PaCO₂
  • Chronic Respiratory Alkalosis: HCO₃⁻ ↓ ~5 mEq/L per 10 mmHg ↓ PaCO₂

6. Oxygenation Interpretation

PaO₂ (mmHg)Interpretation
> 100Hyperoxemia (check FiO₂)
80–100Normal
60–79Mild hypoxemia
40–59Moderate hypoxemia
< 40Severe hypoxemia

Go deeper with the A–a Gradient and Oxygenation Index calculators.

7. Mixed Disorders

Mixed acid–base disorders are common. If pH is normal but PaCO₂ and HCO₃⁻ are both abnormal (in opposite directions), suspect a mixed process.

Let the Real ABG Analyzer flag mixed patterns for you.

8. Real‑World ABG Examples

1) 7.30 / 20 / 10 / 92 → Partially compensated metabolic acidosis
2) 7.52 / 29 / 23 / 96 → Uncompensated respiratory alkalosis
3) 7.38 / 55 / 30 / 75 → Fully compensated respiratory acidosis
4) 7.44 / 18 / 14 / 89 → Fully compensated metabolic acidosis
5) 7.46 / 48 / 33 / 91 → Fully compensated metabolic alkalosis

Practice the full library in ABG Practice.

9. Common Clinical Scenarios

ConditionExpected ABG
DKAMetabolic acidosis (↓ pH, ↓ HCO₃⁻, ↓ PaCO₂ via compensation)
COPD (stable)Chronic respiratory acidosis with full metabolic compensation
HyperventilationRespiratory alkalosis
SepsisMixed metabolic acidosis ± respiratory alkalosis
Vomiting / NG suctionMetabolic alkalosis

10. Troubleshooting & Pitfalls

  • Always interpret ABGs with FiO₂ and the clinical picture.
  • Temperature correction can shift reported values.
  • Venous blood gases (VBG) are not interchangeable with ABGs; use VBG for trends when appropriate.

Practice & Mastery