Thank you for your inquiry.
Based upon the description of the rash, unfortunately there is no test that will aid you in determining whether the rash was due to the amoxicillin, and if so, what, if any, immunologic mechanism was responsible for the rash. In this instance, if you wished to give amoxicillin again, you would have to decide along with the parents whether or not the risk/benefit ratio favored the administration of this drug because oral challenge would be the only way to determine whether or not he would react again.
Thank you again for your inquiry and we hope this response is helpful to you.
Phil Lieberman, M.D.