When do you order Venous Blood Gas (VBG) over Arterial Blood Gas (ABG)?

When to Order VBG Over ABG

Venous blood gas (VBG) testing should be used instead of arterial blood gas (ABG) for most clinical scenarios requiring assessment of acid-base status and ventilation, reserving ABG only for cases requiring precise oxygen measurement or when managing critically ill patients with shock, respiratory failure, or on vasopressors. 1

Appropriate Clinical Scenarios for VBG

Recommended for VBG:

• Assessment of acid-base status in stable patients
• Evaluation of ventilation (pCO2) in hemodynamically stable patients
• Screening for hypercapnia in COPD exacerbations 2
• Diabetic ketoacidosis monitoring 3
• Renal failure acid-base assessment 3
• Stable pneumonia patients 3
• Status epilepticus patients 3

VBG values correlate well with ABG for pH and pCO2 in stable patients, with mean differences of only 0.03 units for pH and 4-6.5 mmHg for pCO2 4. This makes VBG an excellent, less painful alternative for most clinical scenarios.

When ABG is Necessary

Reserve ABG for:

• Precise oxygenation assessment (PaO2) 5
• Patients in shock or on vasopressors 6, 1
• Severe peripheral edema 6
• Patients on prolonged insulin infusions 6
• Hypoxemic respiratory failure requiring ventilatory support 6
• Congestive heart failure with hemodynamic instability 3
• North-South syndrome in ECMO patients 6
• Severe hypoxemia (SpO2 <90%) 6

Clinical Decision Algorithm

1. Is precise oxygenation measurement critical?

   • Yes → ABG
   • No → Continue to next question

2. Is the patient hemodynamically unstable or on vasopressors?

   • Yes → ABG
   • No → Continue to next question

3. Does the patient have severe peripheral edema?

   • Yes → ABG
   • No → Continue to next question

4. Is SpO2 <90% with respiratory distress?

   • Yes → ABG
   • No → VBG is appropriate

Interpretation Considerations

When using VBG instead of ABG, apply these adjustment formulas 4:

• Arterial pH ≈ venous pH + 0.05 units
• Arterial pCO2 ≈ venous pCO2 - 5 mmHg

For oxygenation assessment when using VBG, pulse oximetry (SpO2) should be used in conjunction with the VBG to provide information about oxygenation status 7, 5.

Important Caveats

• VBG has 100% sensitivity for detecting arterial hypercarbia when using a screening cutoff of 45 mmHg, making it excellent for screening purposes 2
• While VBG pH and pCO2 correlate well with ABG values, there is insufficient agreement between venous and arterial CO2 for VBG to completely replace ABG in determining the precise degree of hypercarbia 2
• In patients with circulatory failure, the difference between central venous and arterial pH/pCO2 can be 4-fold greater than in stable patients, reducing reliability 4
• For patients requiring home oxygen therapy, ABG should be performed before and after 1 hour on the intended therapeutic flow rate of oxygen 6

By following these guidelines, clinicians can reduce unnecessary painful arterial punctures while still obtaining clinically relevant information for most patients requiring blood gas analysis.