What is the difference between Arterial Blood Gas (ABG) and Venous Blood Gas (VBG)?

Difference Between Arterial Blood Gas (ABG) and Venous Blood Gas (VBG)

ABG samples arterial blood (typically from the radial artery) and provides the gold standard for assessing oxygenation, ventilation, and acid-base status, while VBG samples venous blood (peripheral or central) and can reliably assess pH and CO₂ but cannot accurately measure oxygenation. 1

Source and Sampling Location

• ABG is obtained from arterial blood, most commonly from the radial artery after performing an Allen's test to confirm dual blood supply to the hand 1
• VBG is obtained from venous blood, either peripheral (cubital or dorsal hand veins) or central (via central venous catheter) 2, 3
• Arterial sampling is more technically difficult, more painful, and carries risks including arterial injury, thrombosis, hematoma, and arterial occlusion 3
• Venous sampling is less invasive, less painful, easier to obtain, and associated with fewer complications 4, 5

Clinical Parameters and Accuracy

pH Measurement

• Mean difference between VBG and ABG pH is approximately 0.03-0.05 units (venous pH is slightly lower) 2, 5
• VBG pH shows very high correlation with ABG pH (Pearson correlation 0.94) and excellent clinical agreement 5
• In hemodynamically stable patients, this difference remains consistent and clinically acceptable 2
• Conversion formula: Arterial pH = Venous pH + 0.05 units 2

PCO₂ Measurement

• Mean difference between VBG and ABG PCO₂ is approximately 4-6.5 mm Hg (venous PCO₂ is higher) 2, 5
• VBG PCO₂ shows strong correlation with ABG PCO₂ (Pearson correlation 0.93) 5
• VBG can be used as an effective screening tool for arterial hypercapnia 2
• Conversion formula: Arterial PCO₂ = Venous PCO₂ - 5 mm Hg 2

PO₂ and Oxygenation Assessment

• VBG cannot accurately measure arterial oxygenation - there is no reliable correlation between venous and arterial PO₂ 5, 3
• Pulse oximetry (SpO₂) combined with VBG provides accurate oxygenation information as predicted by the standard oxygen-hemoglobin dissociation curve 5
• ABG remains essential when precise PaO₂ measurement is required 1

When to Use ABG vs VBG

ABG is Mandatory in:

• All critically ill patients requiring assessment of oxygenation 6, 1
• Patients in shock, on vasopressor therapy, or with severe peripheral edema 7
• Hemodynamically unstable patients or those with circulatory failure (where the pH/PCO₂ difference between arterial and venous blood increases 4-fold) 2
• Peri-arrest situations or after return of spontaneous circulation 1
• Patients on ECMO (samples should come from right radial arterial line to represent cerebral perfusion) 6
• Carbon monoxide poisoning (pulse oximetry readings may be falsely normal) 1
• Major trauma, sepsis, anaphylaxis 1
• Cardiogenic shock requiring precise acid-base assessment 6

VBG Plus Pulse Oximetry is Acceptable in:

• Hemodynamically stable patients without severe acid-base disturbances 2
• Respiratory distress syndrome, neonatal sepsis, renal failure, pneumonia, diabetic ketoacidosis, and status epilepticus 8
• Screening for hypercapnia 2
• When arterial access is difficult or unavailable 4
• Undifferentiated critically ill patients in ED and ICU settings where pH and PCO₂ assessment is the primary goal 5

VBG Should NOT Replace ABG in:

• Neonatal seizure, shock, congestive heart failure, and congenital heart disease 8
• Any situation requiring precise oxygenation assessment without pulse oximetry 3
• Patients with hypotension where agreement may be less reliable 3

Technical Considerations

• For ABG: Local anesthesia should be used except in emergencies; Allen's test must be performed before radial artery puncture 1
• For VBG: Central venous samples show better correlation with ABG than peripheral venous samples in critically ill patients 2
• Samples from arterial sites are most similar to laboratory plasma or blood gas analyzer values 7
• When using VBG, avoid contamination from IV fluid infusing through multilumen catheters 7

Critical Pitfall

A normal SpO₂ does not rule out significant acid-base disturbances or hypercapnia - pulse oximetry will appear normal in patients with normal PO₂ but abnormal pH or PCO₂ 6, 1. This is why VBG analysis remains necessary even when pulse oximetry is normal, particularly in patients at risk for hypercapnic respiratory failure or metabolic acidosis.