Reference — Clinical Skills
ABG Sampling Sites & the Allen Test
Where to draw an arterial blood gas and how to confirm it is safe — the sampling sites compared and the modified Allen test, step by step, for verifying ulnar collateral flow before a radial puncture.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The radial artery is the preferred ABG site because it is superficial, easily compressed, and has collateral perfusion from the ulnar artery. Confirm that collateral flow with the modified Allen test before a radial puncture.
Sampling Sites Compared
| Site | Advantages | Disadvantages |
|---|---|---|
| Radial | Superficial, collateral ulnar flow, easily compressed — the preferred site | Smaller vessel, technically harder |
| Brachial | Larger and palpable | Deeper, near the median nerve, poorer collateral flow, harder to compress |
| Femoral | Large and accessible in arrest or shock | Poor collateral flow, deep, higher bleeding/infection risk, hard to compress — reserve as last resort |
| Dorsalis pedis | Alternative peripheral site | Small, prone to spasm |
| Indwelling arterial line | Repeated sampling without re-puncture | Requires line care; clear the deadspace/flush before sampling |
Modified Allen Test
Perform the modified Allen test before every radial puncture to confirm adequate ulnar collateral circulation.
- Have the patient elevate the hand and clench the fist.
- Occlude BOTH the radial and ulnar arteries with firm pressure.
- Have the patient open the hand — it appears blanched/pale.
- Release pressure over the ULNAR artery only.
- Color should return within about 5–15 seconds, confirming adequate ulnar collateral flow (proceed with the radial puncture). Delayed or absent return indicates inadequate collateral flow — choose another site.
Inadequate collateral flow. If color does not return within 15 seconds after ulnar release, do not proceed with a radial puncture. Select an alternative site such as the brachial or femoral artery.
Clinical Notes
- Preferred site. The radial artery is preferred specifically because of its collateral circulation from the ulnar artery.
- Post-puncture compression. Apply firm direct pressure for at least 5 minutes after the puncture (longer if the patient is anticoagulated).
- Femoral as last resort. The femoral artery is reserved as a last resort because of its poor collateral flow and elevated bleeding risk.
Related Resources
Sources
- Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC Clinical Practice Guideline: Blood gas analysis and hemoximetry: 2013. Respir Care. 2013;58(10):1694-1703.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Analysis and monitoring of gas exchange.