For doing CABG, Allen test is done to test which vessel

. Allen's test is done to find out the patency of radial and ulnar aeries.Both the aeries are compressed near the wrist and allowed to blanch completely in one minute (In the mean time patient closes and opens the first several times for fuher venous outflow).palm appears pale and white .One of the aeries is released and colour of hand is noted.Normally hand will became pink and flushed in no time; whereas in obstruction,the area will still remain pale.Other aretery is also released and looked for changes in hand .Often test has to be repeated to get proper information. Refer page no 173 of SRB's manual of surgey 5 th edition.