Affiliation: | 1.Departamento de Gastroenterología, Hospital de Especialidades,Centro Médico Nacional Siglo XXI, IMSS,Mexico, DF,Mexico;2.Gastrointestinal Unit, Massachusetts General Hospital,Harvard Medical School,Boston,USA;3.Department of Medicine,John Hopkins University School of Medicine,Baltimore,USA;4.MGH Biostatistics Center,Harvard Medical School,Boston,USA |