A 75-year-old man was sent to the noninvasive vascular laboratory for evaluation of patency of his palmar arch prior to possible harvesting of his radial artery (RA) for use as an autograft in a coronary artery bypass procedure. His saphenous veins and one of his internal mammary arteries had been previously used.

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  A dual system consisting of the superficial and deep palmar arches formed by the radial and ulnar arteries (UAs) supplies blood flow to the hand in most people.

  
  
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  Rarely, there is a third artery, the median artery supplying the hand.

  
  
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  A patent UA and a physiologically complete palmar arch network are necessary for safe harvesting of the RA.

  
  
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  The UA commonly forms the superficial palmar arch in conjunction with the superficial palmar branch of the RA. This arch is complete in 84.4% of people.¹

  
  
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  The deep palmar arch is most often formed by an anastomosis between the deep palmar branch of the UA and the dorsal RA.

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  Edgar V. Allen first described the Allen test in1929 when reporting three patients with thromboangiitis obliterans.²

  
  
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  His original description of making a fist with the RA occluded for 1 minute followed by extending the fingers and watching for return of color was later modified and is now called the modified Allen test (MAT).³

  
  
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  The correct technique involves the patient making a fist for 30 seconds while pressure is applied over both the radial and ulnar arteries to occlude them. The patient is then asked to open the fist and the UA is selectively released with the RA still occluded.

  
  
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  Hyperextension of the hand or wide separation of the digits is avoided to avoid a falsely positive abnormal test. In most normal hands it takes 3 to 12 seconds for the normal color to return if sufficient collateral to the hand is present.⁴

  
  
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  The MAT is considered positive or abnormal if color does not return in this time interval.

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  The RA is the third most common conduit utilized for grafting of coronary artery occlusive disease. Cardiac surgeons need precise information of adequate collateral circulation to the hand to avoid harvesting the RA if this would result in hand ischemia with devastating consequences.

  
  
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  Furthermore, if prolonged RA catheterization or larger bore lines are inserted with a greater risk of thrombosis, demonstration of a physiologically complete palmar arch is necessary.

  
  
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  The specificity of the MAT is in the 97% range with a 6-second cutoff.⁵

  
  
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  The predictive value of a positive (abnormal) MAT is such that it does not necessarily mean hand ischemia will result if the RA is harvested.

  
  
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  A negative (normal) MAT safely allows RA harvest, although the cutoff point is somewhat controversial and can range between 3 and 12 seconds.

  
  
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  Because of the lower sensitivity if a MAT is performed and is abnormal, an additional noninvasive test such as a dynamic duplex scan of the palmar arch may be necessary.

  
  
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  However, it must be remembered that there is not a perfect correlation between the MAT and Doppler or duplex evaluation.

  
  
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  It is estimated that between 10% and 27.7% of the patients have nonharvestable radial arteries.⁵