How do you differentiate between arterial blood gas (ABG) and venous blood gas (VBG)?

Differentiating Between ABG and VBG

ABG samples arterial blood and provides the gold standard for assessing oxygenation, ventilation, and acid-base status, while VBG samples venous blood and can reliably assess pH and CO₂ but cannot accurately measure oxygenation. 1

Key Clinical Differences

What Each Test Measures Accurately

ABG provides:

• Precise arterial oxygenation (PaO₂) - the critical parameter that VBG cannot measure 1
• Gold standard assessment of ventilation (PaCO₂) 1
• Accurate acid-base status (pH) 1

VBG can reliably assess:

• pH (strong correlation with ABG: r² = 0.70-0.87) 2, 3
• PCO₂ (strong correlation with ABG: r² = 0.75-0.84) 2, 3
• Base excess and bicarbonate levels 2, 4
• Lactate, electrolytes (sodium, potassium, chloride) 4

VBG cannot accurately measure:

• Arterial oxygenation (PO₂) - correlation is poor (r² = 0.29) with mean differences of 53.6 mmHg 2
• Oxygen saturation - correlation is inadequate (r² = 0.31) 2

When ABG is Mandatory

Critical care situations requiring ABG: 1, 5

• All critically ill patients requiring precise oxygenation assessment
• Patients in any form of shock (cardiogenic, septic, hypovolemic, obstructive)
• Patients on vasopressor therapy
• Patients with severe peripheral edema
• Patients on ECMO (sample from right radial arterial line) 1

Respiratory indications for ABG: 5, 6

• Unexpected fall in oxygen saturation below 94% on room air or oxygen
• Deteriorating oxygen saturation (fall ≥3%) in patients with chronic hypoxemia
• Within 60 minutes of starting oxygen therapy in COPD or patients at risk for hypercapnic respiratory failure
• After oxygen titration to confirm adequate oxygenation without precipitating respiratory acidosis

Metabolic indications for ABG: 5, 6

• Suspected diabetic ketoacidosis
• Metabolic acidosis due to renal failure
• Respiratory muscle weakness with suspected hypercapnia

When VBG May Be Acceptable

VBG can substitute for ABG when: 2, 4

• Oxygenation assessment is not required
• Only acid-base status and ventilation need evaluation
• Patient has adequate peripheral perfusion (not in shock states)
• pH and PCO₂ are the primary clinical concerns

Important limitations of VBG substitution:

• Only 72-80% of paired pH and base excess values fall within clinically acceptable thresholds 3
• The 95% limits of agreement are unacceptably wide (pH: -0.10 to 0.11 units; BE: -4.4 to 3.9 units) 3
• VBG cannot substitute ABG in mechanically ventilated patients despite good correlation 2

Critical Clinical Pitfall

A normal SpO₂ does not rule out significant acid-base disturbances or hypercapnia. 1, 6 Pulse oximetry will appear normal in patients with normal PO₂ but abnormal pH or PCO₂, and in patients with low blood oxygen content due to anemia. 6 This is why ABG remains essential even when oxygen saturation appears adequate.

Technical Considerations for Obtaining Samples

For ABG collection: 1, 5

• Local anesthesia should be used except in emergencies
• Allen's test must be performed before radial artery puncture to ensure dual blood supply to the hand
• Samples from arterial sites are most similar to laboratory values

For VBG collection: 1

• Avoid contamination from IV fluid infusing through multilumen catheters
• Sample from cubital or dorsal hand veins
• Collect within 10 minutes of ABG if comparing values 2

Mathematical Conversion Limitations

While mathematical arterialisation methods (v-TAC) exist to convert VBG to estimated ABG values, these show validity only for pH and pCO₂, not for pO₂. 7 The overestimation of pO₂ from arterialized VBG makes this approach unreliable for oxygenation assessment. 7