Arterial vs Venous Blood Gas
Compare arterial blood gases (ABG) and venous blood gases (VBG)to understand when each is useful, how their values differ, and when oxygenation requires a true ABG.
Want to apply this in practice? Try the ABG Calculator or practice venous interpretation with VBGenius.
Key Differences
| Parameter | Arterial (ABG) | Venous (VBG) |
|---|---|---|
| Sampling Site | Radial, femoral, or brachial artery | Peripheral or central vein |
| pH | 7.35–7.45 | Usually about 0.03–0.05 lower |
| PaCO₂ / PvCO₂ | 35–45 mmHg | Usually about 4–6 mmHg higher |
| HCO₃⁻ | 22–29 mEq/L | Often similar, usually within 1–2 mEq/L |
| Oxygenation | PaO₂ 80–100 mmHg | PvO₂ 35–45 mmHg and not reliable for oxygenation decisions |
When to Use VBG Instead of ABG
- Evaluating acid-base status when oxygenation data is not the main question, such as DKA or some COPD evaluations.
- Monitoring pH and CO₂ trends over time in stable patients.
- When arterial access is difficult, painful, or unnecessary.
When ABG is Preferred
- Assessing oxygenation or ventilation in critically ill patients.
- Evaluating response to oxygen therapy or mechanical ventilation.
- Diagnosing significant hypoxemia or severe acid-base disturbance.
Quick Reference
VBG and ABG values often correlate reasonably well for pH and HCO₃⁻, and they can be close enough for trend monitoring in selected patients. But for oxygenation, ABG remains the standard because venous oxygen values do not replace PaO₂ or SaO₂ interpretation.
Common Questions
Can a VBG replace an ABG?
A VBG can sometimes help with acid-base assessment and trend monitoring, but it does not replace an ABG when you need accurate oxygenation data.
Is VBG accurate for pH and CO₂?
VBG pH and CO₂ can track reasonably well with arterial values in many stable patients, but the values are not identical and must be interpreted in context.
Why is ABG better for oxygenation?
ABG directly measures arterial oxygen tension and is used to assess true oxygenation, while venous samples reflect tissue oxygen extraction and cannot substitute for PaO₂.
Related Resources
Last updated: 2026