What tests should be performed before drawing an arterial blood gas (ABG)?

Tests to Perform Before Drawing an Arterial Blood Gas (ABG)

Before drawing an arterial blood gas (ABG), a modified Allen test should be performed to assess collateral circulation in the hand. 1

Pre-ABG Assessment

Patient Selection and Evaluation

• Determine if arterial sampling is truly necessary or if venous blood gas would suffice
  • ABG is the gold standard for oxygenation assessment, particularly in patients with:
    • Severe hypoxemia (SpO₂ <90%)
    • Patients requiring ventilatory support 1
  • Venous blood gas is acceptable for:
    • Monitoring metabolic disorders (diabetic ketoacidosis, renal failure)
    • Following patients with known acid-base disturbances
    • Evaluating ventilation in stable patients without severe hypoxemia 1

Contraindications to Check

• Assess for contraindications to arterial puncture:
  • Patients on antithrombotic medication (increased risk of major complications, OR 1.31) 2
  • Severe peripheral vascular disease
  • Local infection at puncture site
  • Arteriovenous fistula in the limb

Required Pre-ABG Tests

1. Modified Allen Test:

   • Compress both radial and ulnar arteries
   • Ask patient to clench and unclench fist until hand blanches
   • Release ulnar artery pressure
   • Normal result: Hand flushes pink within 5-10 seconds, indicating adequate collateral circulation
   • Abnormal result: Hand remains pale, suggesting inadequate collateral circulation (contraindication to radial artery puncture)

2. Coagulation Status Assessment:

   • Review patient's medication history for anticoagulants or antiplatelets
   • Consider checking INR/PT/PTT in patients with known or suspected coagulation disorders
   • Note that patients on antithrombotic medication have 31% higher risk of major complications 2

3. Oxygen Therapy Documentation:

   • Document FiO₂ (fraction of inspired oxygen) or oxygen flow rate
   • This is critical for proper interpretation of PaO₂ values 3
   • Without knowing FiO₂, oxygenation status cannot be accurately assessed

Equipment Preparation

• Ensure proper equipment is ready:
  • Pre-heparinized syringe (to prevent clotting) 4
  • Ice slurry (if analysis will be delayed >10 minutes)
  • Appropriate patient identification labels
  • Sterile gloves and antiseptic solution

Common Pitfalls to Avoid

• Using glucose-containing solutions as arterial line flush fluids (can contaminate samples) 1
• Inadequate discard volume when sampling from arterial lines (leading to contamination) 1
• Failing to document FiO₂ or oxygen therapy settings 3
• Relying on finger-stick capillary samples in patients with poor perfusion 1
• Not performing the modified Allen test before radial artery puncture
• Failing to apply adequate pressure after arterial puncture (can lead to hematoma formation)

Special Considerations

• For patients with indwelling arterial catheters, closed sampling systems are preferred to minimize contamination and blood wastage 1
• If using open sampling systems, discard at least 3-5 times the dead space volume before sampling 1
• The overall major complication rate from arterial punctures is approximately 0.14%, with higher risk in patients on antithrombotic medications 2