Medical Research University Ranking

Hello people, So I have been trying to find a ranking list for medical universities according to research. For example I found one in a website called USNews. I just do not know how trustworthy these lists are. For example how is it calculated? I guess a logical way to make these calculations would be citations per paper and average impact factor. Do you guys know any reliable source? Or maybe an official list? P.S. I would prefer if it was an international list and not just USA list.... Medical Research University Ranking
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums