Is the Modified Allen's Test a Useful Tool for Evaluating the Vascular Dominance of the Forearm?

Background: The modified Allen's test (MAT) is commonly used to evaluate the collateral blood supply from the ulnar artery before harvesting a radial or ulnar forearm free flap. We hypothesized that the outcome of the MAT would be inversely proportional to the vessel's diameter. Therefore, we investigated the correlation between MAT results and arterial diameter to determine whether the MAT could serve as a decisive factor for evaluation of distal forearm circulation.

Methods: Patients who underwent forearm based free flap were identified by retrospective chart review between September 2012 and April 2023. The diameter of the radial and ulnar arteries was measured by computed tomography angiography (CTA). Correlation between capillary refill time measured during the Allen test and vessel size was analyzed.

Results: A total of 25 patients met the inclusion criteria. The radial artery was significantly larger than the ulnar artery on both sides. However, correlation analysis showed no association between the diameters of the radial and ulnar arteries and the MAT results.

Conclusion: The MAT does not adequately represent the diameters of the radial or ulnar artery; it merely indicates the presence of collateral circulation between them. Preoperative CTA or ultrasonography would be recommended to identify forearm vascularity precisely.