but another study in today 's journal of the american medical association found a test for epstein-barr virus `` not helpful '' in determining the cause of chronic tiredness , said authors led by dr. walter hellinger of the mayo clinic 's facility in jacksonville , fla. they found no significant differences between 30 fatigued patients who tested high for levels of a substance indicating the presence of epstein-barr virus and 30 fatigued subjects who the test revealed had low levels of the same substance .