Situation: A 66-year-old man with chronic obstructive pulmonary disease (COPD) is admitted to the medical ward with increasing breathlessness and purulent sputum for 3 days. He has smoked for 40 years. His clinic record contains an arterial blood gas (ABG) result taken 3 months ago when he was stable, and the physician orders a repeat ABG on admission. Before the radial artery puncture, the nurse performs a modified Allen test. The nurse has compressed both the radial and ulnar arteries, and the client has clenched and then opened his hand, which is now pale. What should the nurse do NEXT?

The modified Allen test checks whether the ulnar artery alone can supply the hand if the radial artery is injured or clotted by the puncture. With the radial artery still compressed, the nurse releases the ulnar artery; prompt return of color to the palm shows adequate collateral flow. If color does not return, that radial artery is not punctured and the finding is reported.

Clinical context
A modified Allen test is performed before a radial artery puncture to confirm that the ulnar artery can maintain adequate perfusion to the hand if the radial artery becomes occluded or thrombosed after the procedure. In a patient with COPD who needs serial arterial blood gas monitoring, protecting hand perfusion is a practical safety step before repeated arterial sampling.

Step-by-step procedure
The examiner first compresses both the radial and ulnar arteries at the wrist. The client then clenches the hand several times and opens it; the palm appears pale because arterial inflow has been temporarily stopped and venous blood has been squeezed out. At this point, the radial artery remains compressed while pressure on the ulnar artery is released. Return of normal color to the palm within approximately 5 to 15 seconds indicates that the ulnar artery can supply the hand through the palmar arch even if the radial artery is compromised. If the palm remains pale or flushing is delayed, collateral flow is considered inadequate, and that radial artery should not be used for puncture.

Why the other options are incorrect
Releasing both arteries at the same time does not isolate ulnar collateral flow, because blood can return through the radial artery and mask a deficient ulnar supply. Releasing only the radial artery while keeping the ulnar compressed tests radial patency, not the safety of radial puncture. Checking the radial pulse after releasing the ulnar artery also fails to evaluate whether the ulnar artery can perfuse the palm independently.

Evidence considerations
A systematic review and metanalysis evaluated the modified Allen test against Doppler ultrasound for detecting hand circulation deficits and for predicting ischemic events after arterial puncture ^[1]. The review included studies on interobserver agreement and on outcomes after abnormal Allen test results. A narrative review further noted that neither the Allen test nor plethysmography-based testing reliably predicts hand ischemia, and that multiple studies question the utility of routine collateral testing before radial procedures ^[2]. Key point! Although the predictive value of the modified Allen test is debated, the procedural sequence remains the same: release the ulnar artery while the radial artery is still compressed, and observe for palm reperfusion. Watch out! An abnormal result does not absolutely predict ischemia, but it is a signal to avoid that radial site and to report the finding for alternative site selection.

Step	Action	Expected finding
1	Compress both radial and ulnar arteries	Hand becomes pale after clenching and opening
2	Keep radial artery compressed; release ulnar artery only	Color returns within 5–15 seconds if collateral flow is adequate
3	Interpret delayed or absent color return	Do not puncture that radial artery; report the finding