OCRFFF ap harvest candidates have ulnar arteries that sufciently supply the entire hand, including the thumb, via the palmar arch vessels. Clinically, this may be evaluated by the Allen test. Dr. Edgar V Allen rst described the “Allen’s test” in 1929 while studying three thromboangiitis obliterans patients [20]. Here, the patient is directed to make a st with the radial artery simultaneously occluded by the examiner for 1min, followed by release of the artery and observing for return of rubor in the hand while the patient performs active extension and exion of the digits. The original technique has been since adjusted and is now termed the “modi­ed Allen’s test” [21]. Now, the patient is directed to make a st for 30s while occlusive pressure is applied over both the radial and ulnar arteries. The patient is then instructed to open the st, and the ulnar artery is selectively released. If suf­cient collateral circulation is present, there should be a normal return of color to the hand within about 3–12 s. If skin rubor does not occur, this raises concern for incomplete palmar arch connections and is deemed a positive modied Allen’s test. As the modied Allen’s test is subjective in nature, it can be prone to false positive or negative results because of observer biases or anatomical variations. Therefore, various objective noninvasive tests have since been added to the modied Allen’s test to improve its reliability, including plethysmography, Doppler ultrasound, duplex ultrasonography, and pulse oximetry. There is no general consensus on the single best test for assessing collateral circulation of the hand, and the selection of a specic noninvasive test versus clinical diagnosis alone depends upon the prefer­ence of the investigating surgeon.