This guideline is intended to assist transfusion services with the identification of antibodies in patients with a reactive pretransfusion antibody detection test. Its major sections address 1) routine testing and interpretation guidelines, 2) additional guidance and testing, and 3) unusual antibody identification situations. Instructive case studies accompany each section, and more guidance is included in several appendices. Although this resource can be used as a training tool, those readers who are already familiar with related material will derive the most benefit. The scope and depth of the content will appeal to facilities and technologists who work with a single antibody identification panel using the same method employed for antibody detection as well as laboratories that use multiple panels and special testing methods.