Arterial Puncture and Modified Allen Test
Key Points
- Radial artery is preferred for arterial blood sampling, and collateral circulation must be screened first.
- Modified Allen test is positive when hand flushes in 5-15 seconds after ulnar release.
- Pre-heparinized syringes and minimal air exposure are essential for accurate ABG results.
Equipment
- Pre-heparinized arterial blood gas syringe and needle
- PPE and skin-preparation supplies
- Timing aid for modified Allen test interpretation
- Post-puncture pressure materials and specimen transport setup
Procedure Steps
- Confirm need for arterial sampling and select radial artery as first-choice site when appropriate.
- Perform modified Allen test before radial puncture.
- Ask patient to clench fist (or close hand manually if needed).
- Occlude both radial and ulnar arteries until hand blanches.
- Release ulnar pressure only and observe reperfusion.
- Interpret test: positive if flush returns in 5-15 seconds; do not puncture radial artery if negative.
- If positive, proceed with arterial puncture using pre-heparinized syringe and minimize air exposure.
- Apply prolonged direct pressure after sampling and monitor bleeding, especially with anticoagulant use.
- Transport specimen promptly and ensure correct mixing/handling to avoid sampling errors.
Common Errors
- Skipping modified Allen test before radial puncture → unsafe sampling in poor collateral flow.
- Inadequate post-puncture pressure → hematoma or persistent bleeding risk.
- Air contamination or delayed transport → inaccurate ABG interpretation.
- Needle redirection at unsafe angles → increased nerve/tissue injury risk.
Related
- ventilator-parameter-adjustment-principles - ABG interpretation guides oxygenation and ventilation setting adjustments.
- respiratory-failure - Arterial blood gases support diagnosis and progression monitoring in acute deterioration.